GRAPHIC EQUALISER - worldradiohistory.com

January 7940p

GRAPHIC EQUALISER

iff GRAPHIC EQUALISER

Vari-Wiper ProjectKeep your windscreen clean

Viewdata SystemsCoupling your TV to the phone

Touch Switch ProjectFingertip control

Valdemar Poulsen

Slave Flash TriggerProject for the photographer

Basic ProgrammingHow it's done

Pinball WizardsA mere bagatelle

Into ElectronicsThe early days of tape recording How transistors work

aillbito_ internationalProduction of the new catalogue has been held up for a few weeks since we havejust been appointed as distributors for two of the most exciting ranges of radiocomponents products yet The Micrometals range of iron dust torroids cores andformers, and the OKI range of VLSI for digital frequency displays for receivers.We apologize for any inconvenience, but these two ranges are really worth the wait,and include some products you will find hard to believe, like the MSM5523 IC, anIC with less than ten external components that gives AM frequency readout to lkHzfrom LW to 39.999MHz, FM frequency readout in 100kHz steps (all usual IFoffsets programmable by diodes), a 24 hour format clock with 12 hour display,independent on and off timers, time signals on the hours, stopwatch facility anda sleep timer. This costs £14 with its timebase crystal, and makes all that has gonebefore an expensive and time wasting excercise. Rather like the way the IntersilICM7216 has revolutionized the instrument counter market. (See the OSTS ad.)And those of you familiar with Amidon and IG dust torroids, favoured.in manynew RF designs, will be pleased to know Ambit will be stocking a broad range ofthe Micrometals types for applications from EMI filters to RF PA stages.OKI frequency counter ICs: details in cat2 A brief summary of some of our range of ICs:MSM5523 for CA LEDs with RHDP such

as FNI13507 E14 me xtalMSM5525 for 3% digit LCD AM/FM with

direct segment drive. no clockOf times Ell incxtal

Other types for fluorescent displays etc OAOther new semiconductor additions:KB4437 pilot cancel rripx decoder 4.35KB4438 muting stereo prearnp 2.22HA1370 supercedes TDA2020 2.99TDA1090 HiFi AM/FM 3.35TDA1220 low cast AM/FM 1.45PRICES DOWN ON VMOS: as expected, thisnew technology in power transistors is gettingcheaper 120v comp pairs /100W for E10.00Price reduction on CA3189E ....now f2.20New varicaps) to add to the biggest rangeKV1211 2.9v bias to tune MW, like the

KV1210, but a double diode £1 75New pilot tone filters from TOKO2088LR series, individual per channel with a26/38kHz version for pilot cancel decoderapplications. Flat to 15kHz £0.90New crystel fitter for amateur NBFMTOYO 1OM4B1 with over 9048 adjacent ch.rejection for 2m NBFM 10 7MHz E14

New ceramic IF filters for 455kHx... .........CFM455H 6kHz/6r18, 15kHz max /60k18ideal for MC3357 etc 110

T0A1062/1.95; TDA1083/1.95; HA1197/E1.40CA3123E/E1.40, T8A651/E1.81, CA3089/194HA1137/E2.20; MC1310/E2.20; HA1196/E3.95K84424/E2.75, KB4423/E2.53606000/0.75K84412/E2.55; K1344131E2.75; K1344171E2.55MC1495L/E6.86'; MC1496P/E1.25LM381N/E1.81; LM1303/E0.99; ULN2283B/0.00; LM380N/E1; T8A810AS/E1.09TCA940E/E1.80, TIDA2002/E1.95;ICL8038CC/E4.50'; NE566/E2.50H; NE567/0.50'; NE5608/0.50; NE561B/E3.50,NE562B/E3.50'. NE565A/£2.50'SSE THE OSTS ADVERT FOR CMOS/TTLREGULATORS, OPTO DISPLAYS, and othertypes of linear devices.

ome transistors for RF specifically)13F256LB/0.34, 40822/0.43'; 40823/05140673/0.55' ; BF900/961/0.80'; BF960/1 60'13E224/0.22; BF274/0.18; 8E195/0.18:8E240/0.22; 8E241/0.22; BF362/0.70;BF479/0%; BF6796/0.70; BFY90/0.913PIN and other Varicep diodes:BA102/0.30; BA121/0.30; ITT210/0.308131048/0,40; MVAM2/(1.48; MVAM115/f1.05; MVAM125/1.05, KV1210/£2.75BA479/0.35; TDA1D61/0 95: BA182/0.21METER MADE low coat panel meters :

3 x 930 series with blanks and dry transfersheet of wales and ledgends for £12.5

lit last, DIY Hi Fi which looks as if it isn't.That's not to say it doesn't look like HiFi just that it doesn't look like the usual sort ofthing you have come to associate with DIY HiFi. The Mk3 outstrips and outperforms allBritish made HiFi tuners, and most imported ones too. Certainly at the price, there isn'tone near it. But more than that, it looks superb A small pic here would be an insult,so send an SAE for details on the kit that looks as if isn't. It's something else

* Exceptionally high performance - exceptionally straightforward assembly* Baseboard and plug-in construction. Future circuit developments will readily

plug in, to keep the Mk Ill at the forefront of technical achievement* Various options and module lineups possible to enable an installment approach

_ to the system

and now previewing the matching 60W/channel VMOS amplifier:

* Matching both the style and design concepts of the Mk!!! HiFi FM tuner* Hitachi VMOS power lets characterized especially for HiFi applications* Power output readily multiplied by the addition of further MOSFETs* VU meters on the preanip not simply dancing according to vol level* Backed with the usual Ambit expertise and technical capacity in audio

The PW Dorchester-LW,MW,SW,IL FM stereo tuner

In much the same way as we have swepbaway the 'old technology' in frequency/timercounters - with the OKI and Intersil single IC counters, we now offer a single IC "All Band"radio tuner. Don't confuse this one chip radio with things like the ZN414 - for this is agenuine superhet receiver with a mechanical AM IF filter, and ceramic IF filters for FM.The AM section employs a balanced input mixer section, covering all broadcast bands - plusa BFO and MOSFET product decetor for SSB/CW - though at this price, the tuner is notintended as a "communications receiver" although we know of many lesser designs thatmake that claim. The AM sensitivity is nevertheless better than 5uV, and FM sensitivityis 1.2uV for 303 S/N. As a multiband broadcast superhet receiver, it is a unique constructorproject that fulfills the requests we very frequently get for a general coverage circuit thatisn't over complicated. The set has CA3089E FM performance, with mute etc., and a PLLstereo decoder with full pilot tone filtering.The tuner board with "on board" PCB mounted switching, all components etc : £33.00The case/cabinet with PSU, meter and mechanics etc £25.00An SAE for full details please. See the feature article in Practical Wireless (Dec/Jan)

TERMS etc) CWO please, VAT on Ambit Items is generally 12'6%, except where marked 1'1.Catalogue part 1:45p, part 2 50p all Inclusive. Postage 25p per order, carriage on tuner kitsED Phone Brentwood 102771 216029/227050 9arn-7pm. Callers welcome inc. Saturdays .

2 Gresham Road, Brentwood, ESSEX.

oars Since AMBIT introduced the "One Stop Technology Shop" to our service, we have been pleased to see just how many users ofelectronic components appreciate our guarantee to supply goods only from BS9000 approved sources. More than ever, professionaland amateur electronics engineers cannot afford to waste time on anything less than perfect pedigree products.

004000 cmos400040014002400640074008400940104011401240134016401740184019402040214022402340244025402640274028402940304031403240334034403540364037403840394040404140424043404440454046404740484049405040514052405340544055

452245274528452945304531453245344536453845394541454345494553455445564557455845594560456145624566456845694572458045814582458345844585

149p157p1029141p90p

141p1259614p380p1509110p141p174p399p440p153p

77p3864117p388p218p65p

,530p159p28143034

25p600p3194164p84p63p100p

PRICESmicromarket SLASHED TTL6800 series 8216 1.95 2114 £10

88227284

4.783.50 2708 110.55

8751 6.258255 5.40MEMORIES2102 EL 702112 E3402513 E7.540027 E5 78

6800P 6.506870P E66850P 2.756810P E46852 3.658080 series8080 6.308212 2.30

DeoeloumentMEK6800 E220TK80 C306AMI, Signet icsTI,Harris etc. OA

Voltage BegsNEW LOW PRICES7800 series UC T0220 package 1 A all 95p7900 series UC TO220 package 1A all El78MUC series TO220 package YrA all 90p78LCP series T092 100mA all 35p1200 up to 3A/adjustable V&A 195p78MGT2C Ramp adjustable volts 175p79MGT2C Kamp adjustable colts 175p723C precision controller 65p

MAINS FILTERS FOR NOISE/RF I etc1 amp in IEC connector £4.835 amp in 'wire in' case E3.87NE550A 73p

LIfEARSnoeconSemtBIMOSCA3130E 84pCA3130T 904CA3140E 35PCA31407 724CA31SOE 904CA3t60T 999Op amps

1250 LM30141-1 -674

382P LM301 AN 3041039 LM3O8H 1214

57p LM308N 97p120p 4520 1094 LM318F1 279p135p 4521 2364 LM318N 224P NE531 N 1059

TERMS CWO pse., VAT to be dded at 8% (inland), pp 25p per orde . When ordering from theOSTS and Ambit - a single combined remittance and pp charge is suffi lent. Account details OA.

OPTO 7 seg display

17p174174

109418p130p58p58p17p17p55p52p80pB0460p93p82p90P17p76p17p

180p55p72p

100p58p

250p100p145p2000120p250p100p105p250o

83p90P859B5480e

150p130p99pGap55055p65065p65p

4059406040634066406740684069407040714072407340154076407740784081408240854086408940934094409640974098409941604161416241634174417541944501450245034506450745084510451145124513451445154516451745184519

56341154109p53p

400p25970920P20420p20420p90920p20p20p20482p82p

150p509

190p10543724110p122p90p90p90P909

104495p95923p9Ip69p51p55p

2489994

149P380

205p260P300p

LM339N 66pLM348N 18%LM3900N &Op709HC io5 64p709PC dil 36p7tOHC toy 65p713?C dil 59p723CN 65n741CH toy 66p741CN Bdil 27p747CN 70p748CN 36pNE5317 12C,

0.43" High Efficiency HP:5082- 7650 red CA5082 7653 red CC5082- 7060 yellow CA5082 7663 yellow CC 233P5082 7670 green CA508? 7673 green CC

0.3" Standard HP5082 7730 red CA11745082-7740 red CC0.5" Fairchild

NO500 red CC 150pENDS(/' red CA 150p

2 Gresham Road, Brentwood, EMIL

740074017402740374047405740674077408740974107411741274137414741574167417742074217422742374257426742774287430743274337437743874407441744274437444744574467407144874497450745174537454

:Standard gine LP SchottkySEE THEAMBIT AD TOO/

'N"LSN''LSN' 'N"LSN' 'LSN'

13774553

7460 1735 24 74126 57 44 74185

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18 7473 32 74138 7419338 7474 27 38 74139

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20 74 7481 86715 24 7480 48 74144 312

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30 52 7483A 74148 109 7424951 130 7484 97 99 74251

24 7485 104 99 74151 64 84 74253 10530 40 74153 64 54 74257 tog

74154 9630 7489 205 74258 15316 24 7490 33 90 74155 54 110 74259 42029 24 7491 76 110 74156 80 110 74260 15324 14 7492 38 78 74157 67 55 74261 35327 7493 32 99 74158

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74273 1242736 27 74954 65 99 74160 82 130 74275 31227 29 7496 58 120 74161 91 78 74279

74162 92 130 774422080335 32 7497 18517 24 74100 119 74163 92 78

74164 104 7429374295

25 24 74104 6340 32 74105 62 74t65 105

7429840 24 74107 32 3833 24 74109 63 38 74167 20

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7411174110 685414200 74326115

70 99 7,4141113.38 74172 625

2 BB 74170 230 200

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94 74118 83 74175 87 110

4382 74119 119 74176 75 77436665

56 99 74120 115 74177 7899 74121 25 74180 85 7744336687

17 74122 46 74181 165 350 7437317 24 74123 48

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909390

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13037

140139133180150

9290

140110249

MISCEL LENYNE555 30p5655.436 187 0084

ICM7217 950p1CM7208 14954ICL7106CP -LCD DVM IC

9554LCD DVM KIT

,2480p12° 3% digit LCD9095 display 11504

1CL7107 LEDDVM kit 2065pICM7216 - 8 digit10M113 DFM/timer 09.82lfor LED C.Cathl

d:er7A1 d9Le

SCALAR

C'y° MI100H'

51-1900C 472B0P0p

11C900-neC 140%618 w -dividey 100 or 10

for 120/60MH/4509

The ICL7216B PI is still the cheapest way to make a full 8 dig't/ 10MH3 frequency counter/timer,and with 10 external components a display - it is also one oft e simplest. For E19.82, it lakes alot of beating. The mains Sifters have been extended now to in lode a Same IEC version at E5.10,and with the amount of electronic noise on the average supply (next door's fridge, for instance) itis a really worthwhile addition to any sensitive equipment. LPSN TTL now includes many moreof latest types, all of course are absolutely prime first quality types. And don t forget our rangeof OPTO displays includes Hewlett Package high efficiency 0.43" types in all colours renownedas the finest quality in the market. For other types of component discrete LEDs, radio and audiodevices, tuner modules, kits etc., see our other advertisement for more details - or send for theAMBIT catalogue system. Part one 145p/ includes details of our background 'standard' items, andthe new part two includes all the latest introductions and developments, plus a rundown on OSTS.

Vol. 1. No. 3January, 1979

Daedalus coverdrawing Gavin Roberts

Monitor 4What's happening in the world of electronics

Graphic Equaliser Project

Add quality to your audio system

Valdemar PoulsenHow the tape recorder was invented

Touch Switch Project

Let your fingers do the work

7

14

17

BASIC Programming 21

Learn a new language - it's easy!

Vari-Wiper Project 27 Starship Daedalus

Slow, slow, quick, quick, slow

HE Book Service 30Read on

A look at the next HE 31Some of February's features

Market place 32Real bargains in time keepers

Short Circuits 26, 33, 37, 70Circuit to set you thinking

Viewdata 34

What you get by adding a TV to the phone

Reaching for the stars

38

Into Electronics Part 3 46Getting to understand transistors

Breadboard Report 54A look at the recent amateur electronics show

Pinball WizardsA mere bagatelle

57

Flash Trigger Project 59

Freeze the action

SolderGetting it together

63

Kit Review 67"An inexpensive audio sig genny

73

75

Good EvansOur instant jargon language course

Colour CodesSort out your components

Hobby Electronics25-27 Oxford Street, London W1R 1 RF01-434 1781Published by Modmags Ltd,Distributed by Argus Distribution Ltd,Printed by Q.B. Ltd, ColchesterHobby Electronics is normally publishedon the second Friday of the month priorto the cover date

Editor: Halvor Moorshead

Editorial Staff: Phil Cohen B Sc, SteveRamsahadeo, Paul Edwards, Pete Howells

Advertising: Mark Strathern, David Sinfield,Joy Cheshire.

Admin Staff: Margaret Hewitt, Bren Good-win, Kim Hamlin, Val Tregidgo, Tim Salmon.

Copyright. All material in this publication issubject to world-wide, copyright protection.Permission to reproduce printed circuit boardpatterns commercially or marketing kits ofthe projects must be sought from the Editor.All reasonable care is taken in the preparationof the magazine to ensure accuracy butModmags cannot be held responsible for itlegally.

MonitorRaspberries To EMI!

A fruity tale this. I assure you we are notwinding you up. Right? Read on .. . . A teamof engineers has been disguising accelerationtransducers, accelerometers - which convertforce into electrical energy - as Raspberries.And hanging them out on bushes.

The number of comments one could makeat this point is truly staggering, so one willsay nothing at all.

As anyone ?) knows picking raspberries isa sticky process. When ripe they are soft andfragile and liable to squash flat at the drop ofa basket. Furthermore they hide beneathleaves upon brittle and.lethal canes.

Automation is called for.Automation has arrived.Only problem is the prototypes smashed

the berries to a pulp in no time flat. Slimpickings, and this is where our men in whitecoats with the fake raspberries come in.Hang these little fakers out with the realberries and if the machitie doesn't smashthem to bits too, you know exactly whatforces are being produced at the crucialparts, and can adust your machinery accor-dingly. Clever eh?

EMI produce the transducers, called En-tran, and the Scottish Institute of Agricultu-ral Engineering are the loonies . . . er engin-eers who produced the model raspberries andwent out hanging them full moon?)

The actual accelerometers are only 3.6mmsquare, and give out 1 mV for every 'g' ofacceleration they are subjected to. The falsefruits are wired up to both magnetic andpen -recorders to give full details of the fall ofthe raspberries. I didn't believe it first timeeither ...)

So You Think You're Nelson?

One of the more interesting games to appearfor kiddies (i.e. the entire human race-notethe box photo is not an adult!) is this Com-puter Battleship offering.

The basic game is played in a mannersimilar to the old paper and pencil game -both players have a fleet cunningly (theyhope) distributed across a board upon whichthe opposition keep dropping bombs. Theidea is - of course - to annihilate the enemywhile remaining untouched (hah!) yourself.

The machine does all the paperwork foryou, and adds little touches like defeaningexplosions if you actually HIT anything.Based upon a TMS 1000 microprocessor,Random Access Memory (RAM) is used tostore the board as seen by the players, andkeep track of 'shots' etc.

It is designed in Britain too (oh for a fewDreadnoughts now eh Callaghan?) and listsaround £29, from patriotic toy shops every-where as they say

GEE, FELLAS, WON'T ONE OFYOU LEND ME A PEN?

Our unintentional contributor this month landed an order for the $6.9 million Novawas Pye Electronics Ltd, who have just Scotia mobile radio system. The above

photo shows the contract being signed.

4 Hobby Electronics, January 1979

News' from the Electronics World

Scope For Cost Savings?

A new range of oscilloscopes is on the mar-ket, specifically aimed at the home construc-tor. Calscope is from the House of Scopex asit were, themselves a well-known tracemaking company. The Calscope 6 is designedto give good performance at a reasonableprice. It would make a good instrument foranyone starting to build up a workbench athome.

Sensitivity to inputs can be switched from50mV to 50V - which means that this is thesignal required to produce a trace one inchlong on the screen. Bandwidth is quoted at6MHz - the scope responds to AC between 0and 6 MHz, and the time base and triggeringfacilities are claimed as outstanding in thisprice range.

The Calscope 6 should cost around £160 -it can be bought from Maplin and Marshallsamongst other people.

Oracle Of The Present

This 1110p of sup ti 11 can be

pro dusk in real time u sing

a pa tan tipe machine.

...04111111111.111116...

Tests will begin next year on the use ofOracle - the ITV's between the lines infor-mation service - to provide programmesubtitles for deaf viewers. The service couldbe utilised with a Palantype set-up to give'real-time' i.e. instant, subtitles while news orsports programmes are being transmitted.

The Palantype produces the shorthandEnglish - mispelt but readable - via akeyboard and processor. Such a system isnow in use in the Houses of Parliament byJack Ashley the deaf MP. Live transmissionswill be attempted next year.The photo shows a typical caption pro-

duced by the Palantype/Oracle system.

Technically Speaking

Computer Speech has come a long way sincethe Daleks first clanked their way across theTV screen. In the USA add-on devices forhome systems (peripherals) can bepurchased to make the small system talk in areasonable - at least recognisable manner.

Microspeech is the first such unit to bereleased in the UK as far as we are aware. Asyou can see from the photo it is not a massivesystem at all. The program for the boardconverts typed in phonetic speech from thekeyboard into sets of data which is thentransmitted to the synthesiser.

roil OA it Lao

There are nine parameters controlled bythe unit which make possible manipulationof the frequency, amplitude and resonance ofthe final sound, and in this manner malespeech can be reasonably imitated.

Microspeech also has an external inputwhich enables it to produce 'talking instru-ment' effects from guitars and the like. Thedata coming out is converted back to audioform by an 8 -bit digital -to -analogue (DAC)convertor using nine sample and holdcircuits which can 'store' information forwell over a minute.

Relatively little memory is used up by thesystem, and any system using BASIC is itslanguage can be adopted to operate withMicrospeech. Costronics, 13 Pield HeathAvenue, Hillingdon, Middx.

Projected IndexNow this is one of those ideas thatsomeone should have thought of long ago,and now that someone has the rest of us musthang our heads in shame and wonder why wedidn't ....

A very clever man called M. L. Scaife hascompiled a complete index of all electronicprojects appearing in all the revelentmagazines from 1972-1977. Next year a

second listing will appear which will bring

the index completely up to the minute.Some 2500 projects are listed, all with brief

description where applicable, a list of howmany components are used in each - andwhat they are - method of constructionPCB. Veroboard etc, and source. Our sistermagazine ETI is naturally included as are allETI Specials.

Subjects are sensibly grouped together tomake browing easy and the listings clearlyand precisely done.

This truly amazing piece of work costsonly £1.50 a copy from the patient M. L.Scarfe at: Central Library, NorthumberlandSquare, North Shields, Tyne and Wear.Recommended in the strongest possibleterms to all who ever intend to build a projectagain.

DIGESTThe Christmas laser light show was switchedon in London's Oxford Street last month,despite attempts by the Westminster Councilto ban it for safety reasons.

1111111111111111111French viewdata engineers have protestedon 'technical grounds' against the Britishviewdata character set (which includes £ and$ signs). This is against a background ofprospective international agreement onstandardisation for viewdata symbols.

1111111111111111111

Researchers at London's university Collegeare working on the interpretation of theoutput from acoustic microscopes. Thesesystems use acoustic signals at 3MHz to finda 'map' of the viscosity of plant and animalcells.

1111111111111111111The Science Research Council is puttingup £1.6M to set up IC manufacturing facilities'at the Rutherford Laboratory and four uni-versities.

1111111111111111111

At a meeting of the Optical Society ofAmerica recently, it was suggested that alaser -carrying satellite could provide de-tailed information on worldwide wind veloc-ities every 12 hours.

ERRATA: -Confession time again. Below are the errorswhich crept into our projects in our last issue.We apologise to our readers for these andwill wear our sackcloth and ashes withabashed dignity.

LED DICE: The pinout given for the LED's isnot, unfortunately, universal we discover.When buying LEDs check with the supplieras to polarity. Also a 6V or 12V rated tarta-lum capacitor can be employed for C4, in-stead of the 35V type specified with a con-siderable saving in cost!

LIGHT BEAM TELEPHONE: The battery, B1on the circuit diagram, should be a 12V type.SHORT CIRCUITS: The item entitled One -Transistor Amplifier in particular. Referringto the circuit, the resistor from the functionof RI and R2 has a value of 100k, and thecurrent flowing through it is 10 uA.

Hobby Electronics, January 1979 5

STEVENSONElectronic Components

LOUDSPEAKERS56mm dia. 8ohms 70p64mm dia 8 ohms 75p64mm dia. 64 ohms 75p70mm dia. 8 ohms 100P70mm dia. 80 ohms 110p

PLUGS AND SOCKETS1mm plugs and sockets suitable for lowvoltage circuits. Available in red or black,Plugs 6p each. Sockets 7p each.

4mm plugs and sockets. Available in black,blue, green, brown, red, white and yellow.Plugs 11p each. Sockets 12p each.

Jack plugs and sockets

unscreened plug screened plug2.5mm 9p 13p3.5mm 9p 14pStandard 16p 30pStereo 23p 36p

Din plugs and sockets

plug

2 pin speaker 7p3 pin 11p5 pin 180° 11p5 pin 240° 13p

CABLES

socket7p9p

10p10p

socket7p8p

15p18p

Connecting wireAvailable in packs of 8 metres (one metre of each colour)or packs of forty metres (five metres each colour).

single standardEight Metre pack 16p 16pForty Metre pack 76p 70p

Screened CableSingle screened 8ptwin individually screened 11p

ALUMINIUM BOXESBoxes complete with lid and screws.

AL1AL2AL3AL4AL5AL6

Ribbon cable10 way- 58p metre20 way 100p metre

Length width height3 2 1

4 3 11/2

4 3 2

6 4 2

6 4 38 6 2

48p58p65p70p85p

116p

We now have an express telephone order service. Weguarantee that all orders received before 5pm. are ship-ped first class on that day. Contact our Sales Office

now! Telephone: 01-464 2951/5770.

Quantity discounts on any mix TTL,CMOS, 74LS and Linear circuits:25+ 10%. 100+ 15%. Prices VAT inc.Please add 30p for carriage. Allprices valid to 30th April 1979.Official orders welcome.

BARCLAYCARD ANDACCESS WELCOME.

AC127AC128AC176AD161AD162BC107BC108BC109BC147BC148BC149BC158BC177BC178BC179BC182BC182LBC184BC184LBC212BC212LBC214BC214BC477BC478BC479BC548BCY70

17p16p18p38p38p

8p8p8p7p7p8p9p

14p14p14p10p10p10p10p10p10p10p10p19p19p19p10p14p

TORS

BCY71BCY72BD131BD132BD135BD13960140BF244BBF Y50BF Y51BF Y52MJ2955MPSA06MPSA56TIP29CTIP30CTIP31CTIP32CZTX107ZTX 108

14p14p35p35p38p35p3 5p36p15p15p15p98p20p20p60p70p65p80p14p14p

ZT X109 14pZTX300 16P2N697 12p3N1302 38p2N2905 2202N2907 22p2N3053 18p2N3055 50p2N3442 135p2N3702 8p2N3704 8p2N3705 9p2N3706 9p2N3707 9p2N3708 8p2N3819 22p2N3904 Sp2N3905 8p2N3906 8p2N4058 12p2N5457 32p2N5458 30p2N5459 32p2N5777 50p

DIODES1N914 4p 1N4148 3p1N4001 4p 1N5401 13p1N4002 4p 1N5402 15P1N4004 5p 1N5404 16p1N4006 6p 1N5406 18pBZY88 series 2V7 to 33V 8p each.

LINEARA SELECTION ONLY!DETAILS IN CATALOGUE.

709 25p LM324 50p NE556 60P741 22p LM339 50p NE565 120P747 50p LM380 75p NE567 170p748 30p LM382 120p SN76003 200pCA3046 55p LM1830 150p SN76013 140pCA3080 70p LM3900 50p SN76023 140pCA3130 90p LM3909 60p SN76033 200pCA3140 70p MC1496 60p TBA800 70pLM301AN 28p MC1458 35p TDA1022 650pLM318N 125p NE555 25p ZN414 75p

000LEDs

RedGreenYellowClips

0.125in 0.2in.

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DL704 0.3 in CCDL707 0.3 in CAFND500 0.5 in CC

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LS266 40pLS283 60pLS290 55pLS365 45pLS366 45pLS367 45pLS368 45pLS386 35pLS670 180p

RESISTORS

0.25W0.5W

Carbon film resistors.High stability, low noise 5%.

E12 series. 4.7ohms to 10M. Any mix

each

1p1.5p

100+

0.9p1.2p

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Special development packs consisting of 10 of eachvalue from 4.7 ohms to 1 Megohm (650 res.)0.5W £7.50. 0.25W £5.70

CAPACITORS

HERE ARE JUSTA FEW OF THE

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TANTALUM BEAD each

0.1, 0.15, 0.22, 0.33, 0.47, 0.68,1 & 2.2u F @35V 9p4.7, 6.8, 10uF @ 25V 13p22 @ 16V, 47 @ 6V, 100 @ 3V 16p

MYLAR FILM0.001, 0.01, 0.022, 0.033, 0.047 3p0.068, 0.1 4p

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Elec.Comp"orve

cnentsMail orders to: STEVENSON (Dept ET)

236 High St, Bromley, Kent BR11PQ, England

GraphicEqualiserAnother project for those interested in audio work, this will make anexcellent companion to the HE Mixer in last month's issue.

A GRAPHIC WHAT, you ask? A graphic equaliser is acomplex form of tone control. It can be used to smoothout the frequency response of a Hi-Fi, or as a guitareffects unit. In fact, it will prove useful in any audioapplication.

FREQUENCY RESPONSEIn order to explain how the equaliser works, first a quickexplanation of the term 'frequency response'.

Say we take a circuit (any circuit) and set it up likethis:

SIGNALGENERATOR

METE

CIRCUITUNDERTEST

The ratio of the reading of meter 1 to the reading ofmeter 2 is called the response of the circuit. If thefrequency of the signal used is varied, the responsevaries - because the circuit behaves differently whenfed with different frequencies. A graph of responseagainst frequency is called - you guessed it - thefrequency response.

The frequency response of a typical amplifier lookssomething like this:

RESPONSE rF REQUE NC Y

The central section is fairly 'flat' but when it comes to thevery high or very low frequencies it loses power - the

Hobby Electronics, January 1979

reading on meter 2 drops and so the response becomesless.

Once the signal from the amplifier has been passed tothe speaker (which has its own frequency response aswell), the response of the system overall may look likethis:

R

F

This will be further modified by the response of the roomthat the Hi-Fi is in - even your curtains have a responsecurve! By the time the signal finally reaches your ears theoverall response will be fairly well mangled.

An equaliser is a device for correcting (equalising) thefrequency response of a system.

7

IRONING OUT THE BUMPSSay, for instance, that the frequency response lookedlike this when it reached you (rather exaggerated,perhaps!):

R

F

-14 SYSTEM 1-

and we would of course like it to look like this:

_R IDEALSYSTEM

F

If we have a device (called an equaliser) which has aresponse like this (the opposite to the one we wish tocorrect):

Rl EQUALISER)

F

The board. Notedele sockets and the electrolytic capacitororientation.

and we put it in series with the system, the overallresponse would be the sum of the two responses:

R

F

EOL14LISER

In this way we can take any system, be it a

microphone, a telephone line or a Hi-Fi system, and 'ironout' the variations in its response.

There are two ways of finding the right equaliserresponse - directly or indirectly. The indirect methodinvolves measuring the system curve and then designingan equaliser to fit it. This is fine if you are prepared to doall the sums and build a complete new unit for everyapplication you run across.

The direct method is to build a device which has avariable response, connect it to the system and then varythe equaliser curve to give the desired effect.

The way this is usually done is to build a unit whichwill split the incoming signal into a number of frequencybands and then remix these in the desired ratios. Thiswill give the device a number of plateaux on its responsecurve, all of which can be moved up or down indepen-dently of each other to give an approximation to thedesired shape.

An equaliser of this type is called a graphic equaliser ifthe controls which determine the positions of theplateaux are of the 'slider' type. The positions of the


Graphic Equaliser

4

control knobs will then look like the frequency responsegraph of the equaliser.

The amount by which the HE Equaliser can vary thelevel of a particular frequency band is about 8 dB boostor cut.

The dB is a measure of voltage ratio - it measures theresponse in terms of the output divided by the input.

8 dB should prove sufficient for most applications.

CONSTRUCTIONThe equaliser should be fairly easy to build - the onlydifficult part being the front panel.

Take care to mount all the electrolytic capacitors, theICs and the LED the right way round and you should beokay. We suggest you use IC sockets to be on the safeside - these solder directly onto the board and the ICs

' RESISTORS (all 1/4W, 5% tolerance)R1, 2R3, 4, 8, 9, 13, 14, 18, 19R5, 6, 10, 11, 15, 16, 20, 21R7, 12, 17, 22, 23R24, 25

plug into them.Don't forget the wire links on the PCB, by the way!We stuck the batteries on with double -sided adhesive

pads (useful things!). The PCB will mount into the casewe've specified with the aid of some plastic 'spacers' toincrease the distance between the front panel and theboard,

After constructing the board, use the positions of thepotentiometers, the LED and the switch to mark out thefront panel.

The holes for the LED and switch can be drilled simplyenough. The holes for the potentiometers can be fairlysloppy as we're using stick -on bezels which go onto thefront panel and have cut-outs the right shape for theshafts to run in. Make sure there are no burrs sticking outof the front panel.

Parts List47k18k1M47k4k7

CAPACITORS (all polyester unless otherwisespecified)

C1C2C3C4C5C6C7C8C9C10, 11

Ou 1

33n3n38n2820p2n2220p470p47p220u 16V electrolytic

SEMICONDUCTORSIC1, 2, 3LED1

POTENTIOMETERSRV1RV2, 3, 4, 5MISCELLANEOUSOne panel -mounting switch (single pole, doublethrow); Two sockets (see text); Three 8 -pin ICsockets; Vero case; PCB; 2 off PP3 batteries withclips.

All of the components in this project should beavailable from decent stockists. The slider poten-tiometers we used ar RS stereo types - we've onlyused one channel of each. The box is from the Verorange - in fact it's the same one we used for themixer last month.

1458any red LED

10k logarithmic slider100k linear slider

Approximate cost: £20

+9V FROM BATTERIES

LONG LEADTHIS SIDE

- 9V FROM BATTERIES

How to to position thecomponents on the PCB.

Hobby ElectronicsLK r WIRE LINK

SK 2

0/P 1GND.

SK 1

I /P GND.

9

INPUT

B1PP 3

B2PP3

+9V

-9V

C1

SKI Oul

SW1 iv, LED 1

220p

C11

220p

RI47K

ICI TO IC3ARE 1458

R247K

6 ICla5

7

RV1

10KLOG

R3

18K

R51M

R818K

R10

C233n

RV2 100Klin

R418K

3n3 R61M

RV3 100Klin

R918K

C5820p R111M

1M

C62n2 // //

I I

IC2a

R13 RV4 100K R1418K lin 18K

--NAAA--^AA+9V TOICI TO 1C3 R15

R24 PIN 84K7

R254K7

9V TOICI TO IC3PIN4

C7 R16IM -220p

R1818K

R201M

C8470p u

RV5 1001< R19

lin 18K

A V\AA-C947p

An internal view showing how all the main components gotogether.

10

R211M

R747K

R1247K

R1747K

R2247KV.V

1C3a--

R2347K

IC1b

/1 /1

SK2

OUTPUT

The circuit diagram.


Graphic Equaliser

How it WorksThe input to the unit is decoupled (to removeDC) by Cl and fed into IC la, which acts as a`buffer' - it can be driven from a source with avery small current capability, which would beincapable of providing enough input otherwise.The output of IC la is sufficiently powerful,however, to drive the rest of the circuit.

The output from IC la is fed (via RV1, whichcontrols the overall volume) to the four filterstages (ICs 2b, 2a, 3b, and 3a). These eachrespond to a particular frequency band and theiroutput levels are adjustable by means of RVs, 2,3, 4 and 5. The outputs from these filters aresummed by IC lb, which acts as a virtual earthmixer - the "-" input is held at zero volts byvirtue of the feedback through R23 and so theoutput of the unit is the inverted sum of thevoltages at the outputs of the filter ICs.

The individual filters work as follows: thefeedback effects will cause the output to beequal to the input times (-Zf/Z,), where Zf isthe impedance from the output to the "-" inputand Zir, is the impedance from the input to the"-" input.

This is the same situation as in the buffer -ICla. In its case, Z,= 47k and Zf = 47k. Thus the

output is -1 times the input (i.e. the signal willbe 'inverted' - it will sound the same, though).

In the filters, if the variable resistor is atmid -position, with an equal resistance betweenthe wiper and either end, then Z, = Zf. Thuseach filter will pass all frequencieswith output = -1 x input when the slider is inmid -position.

When the slider is at the left-hand end on thecircuit diagram, however, the impedances of thecapacitors will cause the gain of the filter(gain = output/input) to vary with frequency insuch a way as to increase the gain in a particularfrequency band.

Similarly, moving the slider to the other endof the potentiometer will cause the same band offrequencies to be attenuated.

Thus, by moving the slider from one end tothe other, the response of the filter to its parti-cular frequency band can be changed. As theoutput is the sum of all the filters' outputs, theoverall frequency response of the unit willfollow the shape the sliders make on the frontpanel - pushing one of them up will boost thatparticular frequency band.

We used phono sockets for the input and output butthere's no reason why any sockets suitable for audiosignals shouldn't be used if there's room.

Having built the unit, you're now ready to use it - buthow?


The back of the front panel, showing the method of connectingthe batteries -with the positive terminal of one connected tothe negative terminal of the other. The slightly oversizedpotentiometer holes can also be seen.

11

The

prin

ted

circ

uit b

oard

foil

patte

rn -

as

view

ed fr

om th

eop

posi

te s

ide

of th

e bo

ard

to th

eon

eth

eco

mpo

nent

sar

em

ount

ed o

n.

EF

FE

CT

S U

NIT

The

inpu

t to

the

equa

liser

sho

uld

be o

f a fa

irly

high

leve

l-

a m

icro

phon

e or

gui

tar

will

not

do.

The

out

put o

f the

HE

Mix

er w

ould

be

idea

l, ho

wev

er!

Any

aud

io o

utpu

t fro

m a

dev

ice

whi

ch e

ither

a)

requ

ires

batte

ries

or b

) pl

ugs

into

the

mai

ns s

houl

d be

of

a hi

gh e

noua

h le

vel.

The

out

put f

rom

the

unit

shou

ld g

o w

here

ver

the

inpu

t was

goi

ng b

efor

e yo

u bu

ilt th

e un

it! T

his

coul

d be

aP

A a

mpl

ifier

or

your

Hi-F

i or

tape

rec

orde

r.T

he s

ort o

f effe

cts

you

can

get f

rom

this

uni

t are

: ate

leph

one

line

(with

the

500H

z sl

ider

up

and

the

rest

dow

n), a

sho

ut fr

om a

long

way

off

(with

the

8kH

z sl

ider

up a

nd th

e re

st d

own)

, or

just

a s

impl

e ba

ss b

oost

(w

ithth

e sl

ider

s fo

rmin

g a

diag

onal

up

at th

e le

ft).

Of c

ours

e, b

y tr

ying

the

unit

your

self,

you

can

ada

pt it

to n

ew a

pplic

atio

ns o

r us

e it

in c

onju

nctio

n w

ith o

ther

effe

cts

units

to p

rovi

de a

ver

satil

e ad

ditio

n to

you

r ef

fect

seq

uipm

ent.

HI -

F/ S

MO

OT

HE

RN

atur

ally

, if y

our

Hi-F

i is

ster

eo, y

ou'll

nee

d tw

o of

thes

eun

its, b

ut th

at s

houl

dn't

prov

e to

o m

uch

of a

stu

mbl

ing

bloc

k to

a h

arde

ned

Hi-F

i per

fect

ioni

st!

The

uni

t sho

uld

go b

etw

een

your

pre

-am

plifi

er a

ndpo

wer

am

plifi

er. T

he s

impl

est w

ay to

adj

ust i

t is

by e

ar,

alth

ough

it's

not

the

mos

t acc

urat

e m

etho

d. Y

ou c

anre

duce

that

ann

oyin

g 'b

oom

ines

s' y

our

spea

kers

hav

eal

way

s ha

d, o

r bo

ost t

he b

ass

and

treb

le a

nd c

ut th

em

iddl

e fr

om th

e si

gnal

from

you

r ta

pe r

ecor

der.

If yo

u w

ant t

o do

it p

rope

rly, h

owev

er, y

ou w

ill n

eed

asi

gnal

gen

erat

or (

see

the

kit r

evie

w in

this

issu

e!),

a g

ood

mic

roph

one

and

an o

scill

osco

pe, w

ith th

e eq

ualis

erco

nnec

ted

as d

escr

ibed

abo

ve. A

ssum

ing

the

mic

roph

one'

s re

spon

se is

abs

olut

ely

'flat

' (i.e

. the

sam

eat

all

freq

uenc

ies)

, you

sho

uld

be a

ble

to m

easu

re th

eou

tput

of y

our

Hi-F

i at a

par

ticul

ar fr

eque

ncy

by m

ea-

surin

g th

e si

gnal

leve

l with

the

osci

llosc

ope.

Mer

ely

adju

st th

e eq

ualis

er u

ntil

the

syst

em's

res

pons

e to

all

freq

uenc

ies

is th

e sa

me.

Mak

e su

re th

e am

plifi

er's

tone

cont

rols

are

in m

id -

posi

tion.

Thi

s so

unds

sim

ple

enou

gh -

but

rem

embe

r th

at th

ero

om's

res

pons

e w

ill c

hang

e if

you

mov

e th

e so

fa o

rop

en th

e cu

rtai

ns -

so

first

adj

ust t

hese

to th

eir

norm

alpo

sitio

n. A

lso

rem

embe

r th

e ne

ighb

ours

!H

E

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0011 .14 4099 1.30 84016 .05 BSY21 .20 210304 /0 253707 .00 258 MN 1.80 for complete list of carnponenls


ValdemarPoulsenAlthough he receives rare credit, Poulsen did much to bring about the taperecorder. We take a look at his experiments carried out at the turn of thecentury.

ONE OF THE ODDITIES in the history of science is thenames that we remember in connection with variousinventions. Practically everyone, for example, associatestelevision with Baird, despite the fact that Baird's systemwas out of date before it was built and had no connectionwith the TV system which we use. Anyone heard ofCampbell Swinton, of Zwrykin, or Blumlein? These weresome, just a few, of the people who really made themodern TV system, yet only a few professionals knowthese names.

Valdemar Poulsen is another of these less well knownnames which deserve to be better known. What heinvented was the tape recorder, a device which is in usepretty extensively all over the world, yet for many yearshis work was unknown, his invention a curiosity that wasalmost forgotten. Poulsen, like many 19th centuryinventors, was fascinated by electromagnetism, a topicwhich was well understood by the second half of thecentury in theory, if not in practice. The basic principleswhich Poulsen made use of were:

1. A current passed through a coil of wire causes thecoil to become a magnet. If the current is an altern-ating current, the magnetism also is alternating.

CONTACTS

14

a.

/CARBGRAN?jt4LEs

TIME

b.

CARBONM IC

SIGNAL

d.

2. Some materials, called hard magnetic materials,can be magnetised by being held near a magnet, andwill remain magnetised when the magnet is takenaway.3. When a magnet is moved past a coil of wire, avoltage is inducted in the wire.

All of these principles had been known in the 1840's,but there's always a gap between discovering a principleand making use of it. Before anyone could think abouttape-recording, a method of converting sound -wavesinto electrical signals had to be invented, and this wasachieved by Alexander Graham Bell in 1876; his inven-tion was the telephone. Bell's telephone used a carbonmicrophone, which makes use of grains of rough carbonheld between a conducting plate and a thin metaldiaphragm. The two metal plates linked by the carbongrains form part of a circuit in which most of theresistance is the resistance of the carbon grains. Whenthese grains are compressed by pressing the diaphragm,the resistance of the carbon decreases, and more currentcan flow. When the diaphragm is pulled out, the grainsloosen, the resistance increases, and the current de -

TIME

STIRRUP

HEADCARRIE

Fig. 1. (left) Carbon -granule microphone(a) Cross-section. (b) Connection incircuit. (c) Sound wave, (d) Correspon-ding current wave.

Fig. 2. (above) Telegraphone diagram -simplified.


creases. The effect of a sound wave on the diaphragm isalternately to compress and pull out the diaphragm, sothat the sound wave is converted into a wave of current(Fig.1). The invention of the carbon -granule microphonewas the last link in the chain which was needed beforePoulsen could devise a working magnetic recorder. Thecarbon granule microphone is nowadays almost amuseum -piece and is being replaced even for telephoneuse by modern electret types.

EARLY DAYSWe know very little of what Poulsen was aiming at, orabout his early experiments. What we do know withcertainty is that he filed a patent in 1898 for a devicewhich seems like an automatic telephone answeringmachine - well ahead of its time. Almost every featureof tape recording is present in this machine - except forthe tape. The technology for manufacturing tape took alot longer to develop and Poulsen used steel wire, amaterial which was still used in dictaphones until quiterecently.

Poulsen called his invention the Telegraphone, andFig.2 shows a diagram taken from the original patent.Let's take a look at how he claimed it worked. Unlike themodern machine in which the tape is reeled past therecording or replay heads, Poulsen's machine used ahead which was moved along the wire. This was done bywinding the steel wire onto a drum, and revolving the

D

WIRE ONDRUM

Fig 3. Detail of head con-struction of Telegraphone.

head around the drum. The head was held on a stirrup -shaped arm, which revolved around the drum, and thehead assembly was free to slide up and down one arm ofthe stirrup. Why such a peculiar arrangement? Well,Poulsen seems to have realised that to get enoughrecorded signal on steel wire he would have to use thickwire and that made it impossible to wind the wire fromone reel to another. The drum idea looks very similar inmechanical detail to another wonder of the age, Edison'sPhonograph, the priginal wax -cylinder gramophonewhich was by then appearing in homes all over thewestern world.

The mechanical action of the Telephone is ratheringenious (Fig.3.). When the stirrup starts to revolve, theweight A tries to keep moving in a straight line (theaction that used to be called centrifugal force untilsomeone realised that there is no such force), so that theweight flies outwards, stretching the spring B. As theweight A flies out, the pawl on the end of the hinged armC pushes the arm D, which in turn presses therecording /replay head, E, against the drum. The headappears to have consisted of a rod of soft iron with aradius machined at the end to match the radius of therecording wire, so that the head was a good fit againstthe wire. A winding on the soft -iron rod constituted therecording /replay coil, and the wire connections to the

Poulsen's original tape recorder built in 1900.

head were led through the hollow stirrup to a shieldedcontainer above the drum in which one wire was earthedto the frame, and the other made a connection through aslip -ring and brush to the lead -out cable. The drum couldbe turned out at a fairly constant speed by means of aclockwork motor.

FOR THE RECORDThe use of the machine was straightforward. For recor-ding, a carbon microphone, with energising battery, was

British Patent No. 8761of 1899, Poulsen's or-iginal drawing.

connected to the recording head. The drum was allowedto rotate, so that the head contacted the wire. As therecording was made, the screw action of the wirewinding pulled the head assembly steadily higher up thestirrup. At the end of the recording, the machine wasswitched off, whereupon the head was pulled back fromthe drum by the spring B, and the whole assembly thenfell down to the bottom of the stirrup again - a most


Valdemar PoulsenB

REMAININGMAGNETISMIN TAPE

A

SATURATION

SATURATION

B

NO RECORDEDSIGNALS

+1

CURRENT THROUGHRECORDING HE ADCOIL

Fig. 4. Graph of magnetism retained by tape plotted againstrecording head current.

effectively simple type of rewind operation.For replay, a telephone earpiece was connected in

place of the microphone, and the machine was startedagain. The head would engage the wire again at the startof the recording and the listener would hear the recordedvoice.

It was a simple device, like the Phonograph, and likeso many other simple devices, it had several problemsand faults. Surprisingly enough, though, it got roundsome problems which were to perplex later designers,but the shortcomings of the machine led to it becomingalmost forgotten, although steel -wire recordings weremade and used in small numbers.

PROBLEMSThe main shortcoming was the volume of signal onreplay. Echson's Phonograph used purely mechanicalmethods, with the fluctuations of the recorded groovevibrating a diaphragm, and the sound produced by thisvibration being considerably 'amplified' by the use of ahorn. The replay signals from the Telegraphone weredecidedly feeble, and could only just be heard using asensitive telephone receiver, so that there was no marketfor the Telegraphone as a method of family entertain-ment. As a telephone recording machine, it was notsufficiently developed, nor was there any demand forsuch a machine at the time, when a clerk could beemployed to answer telephones for a couple of quid a

Poulsen's early work led to the tape recorder. This one was oneof the first production models, the 'Magnetophon' made byAEG in Germany in 1935; the tape was made by BASF.

16

REMAININGMAGNETISMI N TAPE

A

STEADY D.C.CURRENT ( BIAS )

D

RANGE OFMAGNETISMON TAPE

MAX SIGNAL CURRENTPEAK TO PEAK

Fig. 5. Use of steady bias current to avoid distortion.

week. Poulsen's invention looked set to disappear frommemory.

The principle was rescued by two other devotees ofthe magnetic recording principle, Blattner and Stille. Inthe early part of this century, the Blattnerphone was anacceptable office dictating machine, selling steadily, andusing reels of steel wire. The invention of thermionicvalves, making it possible for the first time to amplify theweak replay signals provided at last the missing link forwhich magnetic recording was waiting. Amplification,on recording and also when replaying, overcame manyof the disadvantages of the early machines. Blattnermachines, using steel tape, were, in fact used by theBBC for programme recording in the 30's, and wereabandoned only when the onset of war made it impos-sible to import the special steel tape from Sweden. Allthat was needed for Stille to construct the first 'modern'tape recorder in the 30's was the invention of coatedpaper (later plastic) tape.

BIASOddly enough, Valdemar Poulsen had solved one of thegreat problems of any magnetic recorder. Fig. 4 showswhat that is - it's a graph of the retained magnetism ofthe tape plotted against the current through the recor-ding head. The shape of this graph is the problem.Recording is fine so long as the current in the recordinghead causes the magnetism of the tape to trace out thestraight-line part of the curve. At low recording currents,there's no magnetic signal recorded on the tape or wireat all, and at high recording currents, the tape or wiresaturates - it's as magnetised as it can be, and changesof current through the head produce no effect. Decem-ber's HE article on Bias goes into this in detail.

Poulsen had solved both of these problems by using acarbon microphone. The fact that a carbon microphonepasses a steady current even when no voice signals arebeing converted means that the current through therecording head is never zero. That, in turn, means thatPoulsen never had to worry about the section of thegraph marked AB(Fig.5). He, had, in fact, invented biasand it may be that he was the first to use bias in anyshape or form. Also, because he used no amplification,the output from the microphone was never enough tomagnetise the material to the saturated region, CD, ofthe recording graph.

Later workers had to grapple with both of theseproblems - the modern solution to the bias problem is,the use of AC bias. Modern, you say? AC bias wasinvented in 1918 . but that's another story, anotherinventor. HE


Touch SwitchFor turning on equipment sensitive to vibration - photographic enlargers forexample - or even just for a doorbell, this circuit is a boon

IF YOU'VE EVER got involved in repairing a transistorradio, you will probably know the trick of dabbing yourfinger at the input to the audio stage to see if you canhear the hum and/or increase in noise level to prove thatthe audio stage is working.

It is also possible to make use of this effect to switch acircuit positively. Switching by touch has become quitecommon in the last two or three years since the ICmanufacturers have introduced devices specifically toperform this function. TV sets and lifts are amongst thecommon users (though some TV sets make use of yourfinger bridging two terminals and operate differently).

Touch switches may seem highly extravagant butindustrially they are far more reliable, than mechanicaltypes. In TV sets, touch switches have the great advan-tage of enabling the preset pots which control thevaricap diodes in the timer to be put at the back of theset.

RESISTORS, All 1/4W, 5%R1 56kR2 100kR3 10MR4 15kR5 15kR6 3k3R7 100k

Above: the completed PCB. In this version we did not use arelay connecting a resistor and the LED in series across theoutput.

Parts List

CAPACITORSC1 100n polyesterC2 100uF 25 V electrolyticC3 2u2 25 V electrolyticC4 100n ployester

SEMICONDUCTORSIC1, IC2 741 Op AmpD1, D2 1N4148 silicon diode

MISCELLANEOUSPCB as pattern, Miniature relay (500 ohm or morecoil and change -over contacts).

Approximate Cost: £4

On the right is the component overlay forthe PCB. Be sure to position thepolarised components C2, C3, ICY, IC2,D1 and D2 the right way round. If you'rebuying a relay get one to fit the holes asshown otherwise extra drilling and sol-dering may be needed.

EXTERNALCONNECTION


RLA1

+COIL

I

fit

i I-I I

a

TO R8 TO LED K

17

Touch Switch

How it WorksWhen a finger is placed on the touch contact, alow level of mains hum is introduced to IC I viathe DC blocking capacitor C1 and R2. The gainof IC2 is arranged at a very high level and thehum is thus enormous amplified and switchesthe output of IC 1 to about 7 V peak -to -peak. Thesignal passes through C2 which blocks the DCto diode Dl.

When the signal is swung positive, D I con-ducts and charges up C3. When the voltage atthe non -inverting terminal of IC2 (pin 3) ex-ceeds the voltage at the inverting terminal(Pin 2)the output of IC2 goes high and switches therelay.

IC2 acts as a voltage comparator. When thefinger is removed, R7 discharges C3 and theoutput of IC2 goes low and the relay switchesoff.

D3 and C4 are included to protect IC2 fromthe very high voltages caused by abruptly tur-ning RL1 off.

If used to switch mains equipment, great careshould be taken to isolate the switched mainsfrom the rest of the circuit. R2 and C I give only amargin of safety and are no substitute for takingcare. The power for the circuit itself should onlybe derived from batteries.

R2100k

Cl100n

TOUCHSWITCH

Above is the circuit diagram, showing, on the right, theconnections for either an LED (as in the photo on page 17), or arelay.

On the right is the copper foil pattern for the PCB we used,printed actual size.

CONSTRUCTIONWe have designed our PCB so that the touch contract ison the board itself but have also allowed a terminal fortaking the touch contact to somewhere more remote.

Small relays don't normally come in standard typesand for that reason we have only left a general areashowing the coil contacts, though these may have to bechanged. We have not even attempted to show theswitch contacts but there is plenty of room to run thecontracts to terminals sited between the supply connec-tions.

Apart from the obvious precautions to ensure that theICs, diodes and the two electrolytic capacitors (C2, C3)are connected the right way around, construction iseasy.

For safety reasons, we strongly recommend that thecircuit is battery operated. If it is used to switch a relay,the contacts of the relay can be used to switch a mainssupply on.

12-18V

NOTE:IC1,2 ARE 741D1,2 ARE 1N4148LED1 IS TIL209

OUTPUT

R81k

LED1 NC

0---ANO

RELAY


illithnoprevious knowledge youcanbuild this radio

only£23ZFREE 100 page Instruction BookFREE BOOK Hamlyn's 150 page 'Electronics'

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Electronic horn.

PRICE

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Yes. In one Kit, 100 projects ... and it's as simple to changefrom one to the other as it is to build the first, no extra tools are required.

This electronic Kit is educational for the learner while useful to the more advancedstudent. It's ingenious, it's safe and it's fun! The circuitry is varied by your positionalchanges of the plug-in blocks on the basic matrix. All blocks are marked with electronicsymbols. It's as easy as 'Painting by Numbers', and you're learning all the time.

You must have one of these Kits! Just £23.55 is great value for 100 projects, andthe knowledge you gain is priceless.

TECHNICAL SPECIFICATION Shock resistant ABS case. Audio Output: 500 mW (from internal

output module and loudspeaker). Size: 260mm x 200mm x 50mm. Batteries: 4 x HP7 (Not supplied).

Accessories provided: Crystal Earpiece. Aerial & Connecting Wire (ferrite rod

aerial built in). Clips & Test Rods.

REMINDERThe DOFtAM Electronic Hobbies

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Order now.

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Send the coupon immediately,Your kit will be delivered

within 25 days.If you are not completelysatisfied you may returnyour purchase within 30

days for a full refund.Doram Electronics Ltd

PO Box TR8. Leeds L512 2UF

GAn BectrocornponentsGroup Company

OFFER CLOSES

14th February 1979

To Doram Electronics Ltd PO Box TR8 LS 12 2UFPlease send 60-551 Kit(s) at £24.50 ea. (£23.55+95p p&p)Please send a Hobbies Catalogue

I enclose cheque/PO for £I am paying via National Giro Centre Bootle Lancs to your AccountNumber 00/643/7257

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ATALLOWS US TO SELL SUCH FANTC PRIC!

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COMPLETE UNITInc.assernbly instructions e%

C5.00 £6991 -rf;,9THREE FOR £13.50 THREE FOR £20

Send cheque. P.O./ M.O. for the correctamount which includes VAT and P&Por pay by Access/Barclaycard. Sendname/card number lit applicable) andaddress to:

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BUILD -IT -YOURSELFTEST GEAR KITBASIC SERVICINGINSTRUMENTS WITH

EASY STAGE BY STAGE BUILDINGINSTRUCTIONS - IDEAL FOR THE AMATEUR

MULTI RANGE TEST METERA general purpose meter covering all usualranges of A.C. and D.C. volts current andresistance measurements

AUDIO SIGNALGENERATORNew design covering 10Hz to 10K Hzand vari.able output. Distortion less than0.010/0 Ideal for HIFI Testing.

OSCILLOSCOPEA basic 3" general purpose cathode rayoscilloscope for simple testing andservicing work. Sensitivity 0.3 volts/cm

SEND NOW FOR FREE DETAILS ---To LERNAKITS, P.O. Box 156, Jersey.

Name

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19

SPECIALSTOP PROJECTSBook 1 + 2: £2.50 + 25p P&P.Master mixer, 100W guitar amp., low power laser, printmeter, transistor tester, mixerpreamp., logic probe, Ni-Cad charger, loudhailer, 'scope calibrator, electronic ignition, cartheft alarm, turn indicator canveller, brake light warning, LM3800 circuits, temperaturealarm, aerial matcher, UHF -TV preamp., metal locator, four input mixer, IC power supply,rumble filter, IC tester, ignition timing light, SOW stereo amp. and many more.

Book 3: SOLD OUT!

Book 4: £1.00 + 25p P&P.Sweet sixteen stereo amp., waa-waa, audio level meter, expander/compressor, car theftalarm, headlamp reminder, dual -tracking power supply, audio millivoltmeter,Temperature meter, intruder alarm, touch switch, push-button dimmer, exposure meter,photo timer, electronic dice, high -power beacon, electronic one-armed bandit ...

Book 5: £1.00 + 25p P&P.5W stereo amp., stage mixer, disco mixer, touch organ, audio limiter, infra -red intruderalarm, model train controller, reaction tester, headphone radio, STD timer, double dice,general purpose power supply, logic tester, power meter, digital voltmeter, universaltimer, breakdown beacon, heart rate monitor, IB metal locator, temperature meter ...

Book 6: E1.00 + 25p P&P.Graphic equaliser, 50/100W amp. modules, active crossover, flash trigger, "Star and Dot"game, burglar alarm, pink noise generator, sweep oscillator, marker generator,audio-visual metronome, LED dice, skeet game, lie detector, disco light show ...

Graphic Equaliser Marker Generator

Power Amplifier Modules ET Sound

CCTV Camera Neathdlotte Adaptor

IED Dice Sound light flask Trigger

Expander -Compressor.

TOPpall

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Pink Noise Generator GU Monitor

Sweep Oscillator Stereo Simulator

ELECTRONICS TOMORROWComprised entirely of new material, theedition covers such diverse topics as StarWars and Hi-Fi! The magazine containsprojects for everyone - none of whichhave appeared in ETI - and a look at thefuture of MPUs, audio, calculators andvideo. How can you not read it?

75p + 25p P&P.

ETI CIRCUITSBooks 1 & 2.Each volume contains over 150 circuits,mainly drawn from the best of our Tech -Tips. The circuits are indexed for rapidselection and an additional section isincluded which gives transistor specs, andplenty of other useful data. Sales of thispublication have been phenomenal -hardly surprising when the circuits costunder 1p each!

£1.50 + 25p P&P each.

electronicstomorrmv 75P

ghgaiL' mi

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TRANSDUCERS INMEASUREMENT ANDCONTROL

This book is rather an unusual reprintfrom the pages of ETI. The series appeareda couple of years ago in the magazine, andwas so highly thought of by the Universityof New England that they have re-published the series splendidly for use as astandard textbook.Sydenham, M.E., Ph.D., M.Inst.M.C.,F.I.I.C.A., this publication covers practi-cally every type of transducer and dealswith equipment and techniques not cov-ered in any other book. Enquiries fromeducational authorities, universities andcolleges for bulk supply of this publicationare welcomed: these should be addressedto H. W. Moorshead, Editor.

£3.00 + 25p P&P.

TRANSDUCERS INMEASUREMENTAND CONTROL

by PETER H SYDENHAM

MOM THE PUBLISHERS OFELICTSIONICS 10051 INISPIYABONAL

En

CIRCUITS

No2£1.50

ELECTRONICS - IT'S EASYBooks 1, 2 & 3.

Our successful beginners series came to anend some time ago now, and the wholeseries is available from us in reprint form.The three books between them contain allthe information presented in the series(sometimes in more detail!) and togetherform an excellent starting point for any-one interested in learning the art ofelectronics.

£1.20 + 25p P&P each.

ComputersDisplay SystemsDigital instrument..Transmission linksStapes di RecordersCentro! of Power

ORDER FROM

SpecialsModmags Ltd25-27 Oxford StreetLondon W1R IRF

Postage and packing also refersto overseas. Send remittance inSterling only.

Please mark the back of yourcheque pr PO with your nameand address.

from ETI20 Hobby Electronics, January 1979

BasicProgrammingWhat exactly is involved in computer programming? The easyanswer is to say: "Try it and see", but if you don't have access to acomputer this can be a trifle difficult. Instead, Jim Perry takes youthrough the whole thing bit by bit.

THE ENGLISH LANGUAGE contains about 650 thou-sand words but most people only know a tiny percentage(the average is about 12,000). Even more surprisingly,in conversation we only use a couple of thousand or sowords. By learning as few as 200 words in any foreignlanguage, you can make yourself be understood in mostsituations - but obviously the words need to becarefully chosen. The main difference between a com-puter langugage and any other is that you must obey thegrammar rules - otherwise the computer will notunderstand. English people will smile, but understand, ifa foreigner asked "Cinema is you know where?"; badgrammar is not a total barrier to communication.

Computer languages are based around a very smallvocabulary, usually less than 100 words - but if a wordis misspelled, or the grammar not obeyed, the computerwill either reject the instruction or do something totallydifferent. There are dozens of different computer langu-ages in use , the most common on small computersbeing called BASIC. The name is an acronym for Begin-ners All-purpose Symbolic Instruction Code and it wasdeveloped by researchers at Dartmouth College in theUSA in 1964. BASIC may not be the best computerlanguage, but it can be learned in a few hours and isreasonably powerful (we use the term powerful inrelation to computer langugages to mean versatile).

Just like any other langugage BASIC has several,slightly different, 'dialects'. Different computer special-ists have developed their own versions of the originalDartmouth BASIC, usually adding extra words and usingslightly different grammar rules. As an introduction tothe wonderful world of BASIC this article will deal withthe dialect spoken (and understood) by Commodore'sPET computer.

PARLEZ VOUS BASIC?

To communicate with a computer you need to speak (orrather type) in a Inaguage it can understand. Theinstructions that you give it form a 'program', which thecomputer will execute when instructed to do so. Theinstructions (or program) are divided into seperate lines


(sentences), with each line having an indentificationnumber allocated to it.

When the computer is told to RUN the program, itstarts at the line with the lowest number and works up insequence. To help in modifying programs, the linenumbers are usually allocated in multiples of ten - asthe computer will only accept whole line numbers, andyou may need to insert extra lines as the program isdeveloped.

The line number also tells the computer that you areentering a program, any instructions without a pre-ceding line number will be forgotten after they havebeen executed. At the end of each program line the'return' key is pressed, this tells the computer that youhave finished the line. The return key is the most usedkey on every computer, without it every line would haveto be the same length so that the computer would knowwhen a line was finished.

WHAT GOES IN . .

So how can you tell the computer to perform a simple,repetitive, calculation? Suppose you need to calculatethe square of any number, a simple enough task! Firstyou need to tell the computer what number you want tosquare so type in:

10 INPUT X (then press RETURN key)The 10 is the line number, the word INPUT is a

BASIC instruction meaning 'Ask the outside for anumber, call it X, and then go to the next instruction line- but remember the number X'.

20 A= X*X (RETURN)An equals sing is a BASIC instruction, as is the' which

is the same as a multiplication sign. X times X is the sameas X squared.

30 PRINT AThe PRINT instruction is telling the computer to

display the value of A on the screen, as A is X times X(line 20); the number displayed will be the answer wewant.

To start the program the instruction RUN is typed intothe computer, followed by the return key. The com-puters response will be a question mark, which is its way

21

of asking for an input. If you type in a 4 and then pressreturn the answer 16 will appear instantly, followed by.READY.

What has happened is that the computer has startedat line 10, asked for a number a number, multiplied it byitself (squaring it), and has assigned the result to a letterA, then moved to the next line. After printing the value ofA the computer has found no further instructions and sosays that it is READY for more instructions. The screenwill look like this:

10 INPUT X20 A=VX30 PRINT ARUN?416READY

This will be followed by a flashing white square on thenext line, this is the CURSOR which shows the positionof the next position on the screen, anything typed willmove the cursor to the next position. If you want to RUNthe program again you have to type in the commandRUN again, and the program will repeat. To keep theprogram running an extra line can be added.

40 GOTO 10GOTO instructs the computer to jump back to line 10,

it is an unconditional instruction - the computer willchange the operation sequence to give:

RUN?416

With line 40 the computer will continue to square anynumber typed into it, if you had told the computer toGOTO a line that had not been allocated it would object- any instruction must be valid.

On a simple program like ours anybody would beable to read it and inderstand what was happening.However as soon as programs get any longer it can bedifficult deciphering them after a period of time, to helpthe programmer a BASIC instruction REM is used. REMis short for REMark and is used like this:

0 REM A PROGRAM TO SQUARE A NUMBER5 REM ASK FOR A NUMBER10 INPUT X15 REM SQUARE THE NUMBER AND CALL IT A20 A=X*X25 REM PRINT THE RESULT30 PRINT A35 REM GO BACK AND DO IT AGAIN40 GOTO 10

It is very rare that a program would be as well REMedas this example, but REM statements should be usedwhenever a program goes off from the normal sequence.Another way of squaring a number is as follows:

20 GOSUB 100100 A=VX110 RETURN

All the previous lines are the same, but in thisexample the actual squaring operation has been madeinto a 'subroutine'. The GOSUB instruction is one of themost useful commands in BASIC, without it you wouldhave to include certain sections of program many times.With every GOSUB there must be a RETURN ( not thesame as the return on the keyboard), the RETURNmakes the computer go back to the line after it went off

22

to the subroutine. As with GOTO, any GOSUB musthave an allocated line number that exists. If you had toldthe computer to GOSUB 99 it would have replied'UNDEFINED STATEMENT', the modern way of saying'DOES NOT COMPUTE'.

To demonstrate how precisely you need to tell thecomputer what to do, line 40 could be missed out(deleted). The computer response would be:

RUN?416RETURN WITHOUT GOSUB IN 110

The computer does not like to find RETURNS on theirown, without line 40 the computer has 'fallen throughthe bottom' of the program.

Both X and A are variables, and can be manipulated inany way that numbers can be on a sophisticated calcu-lator. Standard functions that are available in BASIC(ardtan) and even exponentiation. By using the ex-ponentiation function (written as XTY) line 100 could bewritten:

100 A =XTY11 INPUT Y

Line 0 should also be changed to indicate the newnature of the program, as should line 15. The programwill now run as follows (if line 40 is END):

RUN?4?364READY

If the original line 40 is put back in we now get:40 GOTO 10RUN?3?29

We now have a much more useful program that canfind any power of any number, within the limits of thecomputers highest number and lowest number. On mostBASICs the largest number is 1.70141'104\38 which israther large, and should be enough for most programs!

By using more sophisticated PRINT instructions theanswer can be made clearer:

30 PRINT A;"IS-;.X;-/r;YThe semicolon instructs the computer to continue

printing on the same line, all text enclosed in quotemarks will be printed exactly as shown. All variables willhave their current value printed.

RUN?4?216 IS 4T2

In the program example we have used A, X and Y asvariables but we can use any combination of letters andnumbers as a variable. The computer however only looksat the first two characters and the first one must be aletter. Also any variable cannot contain a BASIC com-mand word.

For example, instead of A we can use OFFENCE but


Programmingcannot use OFFEND as the BASIC command END is in it.Another common word that cannot be used is TO, whichis also a BASIC command word. All the BASIC reservedwords are listed below:ABS, AND, ASC, ATN, CHR S, CLOSE, CLR, CONT,COS, DATA, DEF, DIM, END, EXP, FN, FOR, FRE, GET,GOSUB, GOTO, IF, INPUT, INT, LET, LIST, LEFTS,LEN, LOAD, LOG, M1DS, NEW, NEXT, NOT, ON, OPEN,OR, PEEK, POS, POKE, PRINT, PRINT READ, REM,RESTORE, RETURN, RIGHTS, RND, RUN, SAVE, SGN,SIN, SPC(, SQR, STEP, STOP, STR $, SYS, TAB(, TAN,THEN, TO, USR, VAL, VERIFY, WAIT.

Now suppose we want to find the square, cube andfourth power of a number, we could use our existingprogram - but would have to keep entering the valuesfor X and Y (2, 3 or 4). A BASIC command DATA can beused to make the job simpler:

10 DATA 2,3,420 INPUT X30 READ Y40 PRINT XTY; "IS;X;"+";Y50 GOTO 30

Each time that Y is read it takes the next item of DATAas its value, when run this program would act as follows:

RUN?24 IS 2+28 IS 2+31 6 IS 2T4OUT OF DATA ERROR IN 30READY

Whoops! Not quite what was wanted! As soon as theDATA has been read the computer has no more to use

and assumes that you have made a mistake - which youhave!

To reuse the DATA another statement is used, theRESTORE command. When a program needs to reusethe DATA, the RESTORE makes it start at the first itemagain. But if RESTORE is used with the program as it isshown where will it go? The answer is simple, nowhere!With the GOTO in line 50 it cannot come after (it wouldnever get reached), and if placed before 50 the answerwill always be the same:

RUN?24 IS 2T24 IS 2T2

Etc.The program will only stop if the BREAK key is

pressed. A way round the problem is to check the valueof the DATA, IF the DATA is 4 THEN RESTORE:

10 DATA 2,3,420 INPUT X30 READ Y40 PRINT X+Y;"1S"; X; "T";Y50 IF Y=4 THEN 7060 TOTO 3070 RESTORE80 GOTO 20

Now the computer will print the 2nd, 3rd and 4thpowers of X, then ask fora new value for X. The IF, THENis a conditional statement and will be ignored if thecondition is not met, a GOTO is not needed at the end ofthe statement - BASIC assumes you want to GOTO theline number after THEN. You can place a GOSUB orother command after THEN if you want to. DATA can beplaced on any line in a program, for example


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10 DATA 211 DATA 3,4

Will give exactly the same result as the previoussingle line. But

10 DATA 211 DATA 4100 DATA 3

will produce the answers out of order, with the 2nd, 4thand then 3rd power being printed.

The equals sign in the IF THEN statement can also beone of the following <, >, <>, = <, = >, = <, = >.Two variables can also be compared using the IF THENstatement (remember you can use a variable instead of anumber in programs).

As well as the standard greater than, less than andequals (with the combinations shown above) logicalcomparisons can be made with AND, OR and NOTstatements.

IF A= 1 AND B=2 THEN PRINT "A IS HALF OFB"

FOR OUR NEXT TRICK

Suppose we want a table of squares for each numberfrom 1 to 10, we could use the following program:

10 PRINT 1;14,220 PRINT 2;24,230 PRINT 3:34,2

This method would work but it's very inefficient, byusing the IF THEN statement the program becomes a lotsmaller:

10 N =120 PRINT N Nt230 N =N +140 IF N< =10 THEN 20

This program will produce exactly the same result asthe previous one. But what is happening in line 30?Mathematically N = N +1 is nonsense, however in thisstatement the equals sign is a shorthand for "replace thevalue of the left side with that of the right side". Thestatement replaces N with the value N +1 every time it isexecuted. As N starts off with a set value of 1 it changesto 2,3,4,5,6,7,8,9 and 10 as the program operates. Butwhen the square of 10 has been printed N becomes 11and the condition in line 40 is not met - 11 is not lessthan or equal to 10. The program cannot go back to line20 and as no more program exists, it will stop.

The technique used in the program above is called'looping' or 'iteration', and is used extensively inprogramming. In fact it is such a useful technique that aspecial BASIC command exists to do it.

10 FOR N =1 TO 1020 PRINT N;N4,230 NEXT N

The output of this three line program is the same asthe previous ten and four line programs. At line 10 N isset at 1, line 20 prints the value of N and the square of Nand line 30 adds 1 to the value of N. This continues untilN is 10. The variable following FOR is the same as thevariable after NEXT, but both could be any valid variable- as long as they are the same.

If only every second square was wanted, a STEPinstruction can be added to line 10:

10 FOR N =1 TO 10 STEP 2Now the computer will add 2 to the value of N instead

of 1, and only every second value will be printed. We can


Programmingeven get the results printed in reverse order by reversingthe FOR conditions and stepping a minus number:

10 FOR N=10 TO 1 STEP -2If no STEP is given the BASIC will add 1 to the

variable each time. A loop can be placed inside anotherloop, as long as it is all inside. If two loops overlap thenthe results are predictable, the computer will say -NEXTWITHOUT FOR" when it has finished the first loop itfinds. Putting one loop in another is called 'nesting'.

STRINGS AND THINGSSo far all the manipulation has been with numericvariables, but BASIC can also cope with alphanumericvariables, All sets of alphanumeric are called 'strings'and denoted by a dollar sign at their end.

10 INPUT A$20 PRINT ASRUN?BASICBASICREADYAll strings can be up to 255 characters in length.

The strings can be manipulated in many ways, LEN(XS)gives the total number of characters in XS. LEFT $ (XS, I)gives the firstIcharacters of the string XS, RIG NTS (XS,I)gives the lasticharacters of XS. The command MID $ isused in a similar way to extract characters from themiddle of the string.

10 INPUT XS20 FOR N =1 TO LEN(X$)

HF TTL OSCILLATOR

A squarewave oscillator withcomplementary outputs and with afrequency range of 20 Hz to 10MHz can be made from one IC, a7413 which is a TTL dual Schmitttrigger. The oscillator is always selfstarting and runs from a 5 V sup-ply, current drain 20 to 30 mA.The 7413 is a Schmitt trigger withhysterysis levels (at its input) of+0.9 V and +1.7 V. That is, whenthe input level exceeds +1.7 V theoutput jumps to a low condition(+0.2 V). When the input voltageis lowered it needs to fall below+0.9 V before the output jumpsback to a high condition (+3.4 V).

When the Schmitt trigger isconnected up as shown in the dia-gram the device will oscillate.Imagine the output is high. C1 ischarged up via R1. When the vol-tage on C1 reaches +1.7 V, theoutput falls to +0.2 V. C1 is nowdischarged via R1 in parellel withR2 (D1 is now forward biased) untilthe voltage on C1 reaches +0.9 V.Then the output jumps to a highstate and the process repeats itself.The second Schmitt trigger merelyinverts the squarewave output. The

30 PRINT LEFTS(XS,N)40 NEXT NRUN?BASICB

BABASBASIBASICREADY

Strings can also be added together after they havebeen taken apart, the addition sign " can adddifferent strings - this is called "concateration".

10 INPUT AS20 BS=AS+AS30 PRINT B$RUN?REPEATREPEATREPEATREADY

If all this seems like a maze of jargon then there is onlyone surefire answer, get hold of a computer and trywriting a program! All the BASIC instructions are givenhere with short explanations. Most small computersystems come with BASIC manuals and some areexcellent (the Tandy one in particular). A very good bookfor beginners is Illustrating BASIC by Donald Alcock,available from the HE bookservice for £2.20 (includingpostage) or most good bookshops; it is published byCambridge University Press.

Computers are not anything to be afraid of, they areas easy to use as a motorcar - you just have to learn todrive them. HE

Short Circuit

IF APPLICABLE

frequency of operation is given bythe formula:

Fox = 2 X 10'Cl

where Fox is in Hz and C1 is inFarads.

therefore for:10 MHz, C1 =220 pF (poly-

styrene or ceramic)1 MHz, C1=2200 pF (poly-

styrene)100 KHz, C1 =22 nF (polyester)

10 KHz, C1 =220 nF (polyesteror electrolytic)

1 KHz, C1 =2.2 uF (electrolytic)100 Hz, C1=22 uF (electrolytic)

20 Hz, C1=100 uF (electrolytic)

47n CERAMIC

10MHz

r--

1MHz -

100kHz

+ 5 VPIN 14

LOCATE CLOSE TO IC

7413

V,11/7

+0V9

COMPLEMENTARY OUTPUT

/\/\/\PIN 1, 2, 4, 5

Os TIME

VIN 6

3V4P

+ OV2

WAVEFORM DEGRADATION WITH FREQUENCY

---1

1111k TIME


VariwiperThis pulsed windscreen wiping circuit canwith most types of modern wiper motors.

WHEN OPERATING IN heavy rain windscreen wipersoften have difficulty providing adequate visability.However, during light rain or mist all that is necessary isan occasional sweep of the blades at intervals of a fewseconds.

Turning them on and off repeatedly takes the driver'sconcentration off the road, and his hands off the wheel,increasing the risk of an accident. Alternatively, if thewipers are kept working all the time in such conditionsthe blades tend to scrape on dry glass, wearing out therubber inserts, your nerves, and worst still, the screenitself.

The answer is obvious; have the wipers operateintermittently at a duration which can be varied to suitthe conditions.

Figure 1 shows the circuit of a modern wiper assem-bly. Dynamic braking is achieved by applying a shortacross the armature, by a cam -actuated change -overswitch synchronised with the wiper blades. When thewipers are switched off, the change -over switch shortsout the motor armature via the main wiper ON /OFFswitch.

,12\/

SWITCHPARKING

WIPERMOTOR

ONp WIPER ON/OFFSWITCH

Fig. 1. Circuit of modern wipermotor assembly. Dynamicbraking is achieved by applyinga short across the armature.

The circuit of Fig. 2 is suitable for use with negativeearth cars fitted with permanent magnet motors. Someearly model cars are fitted with wound field coil motorsand are not suitable for use with this circuit (more aboutthem later).

Some types of permanent magnet wiper motors,especially those on British cars, have a fifth wire ex-tended to the wiper switch. These motors are designedto operate independently of an earth to allow for their useon either positive or negative earth vehicles. The circuitof Fig. 2 can also be used with these motors providedthey are fitted to a negative earth car. However, somemore cars have wiper motors which are reversed in theparking sequence to lower the blades below the bottomof the windscreen when not in use. The Vari-Wiper unitdescribed cannot be used with these wipers.

Before installing the Vari-Wiper unit make sure thatyou have one of the types of permanent magnet wipermotors described. If necessary remove the cover of themotor and identify the wire to the centre contact of thecam -operated switch.


be used on cars fitted

FEME

A 001

NORMAL WIPER OPERATIONConventional operation of the wipers is obtained byusing the vehicle wiper switch in the normal way. Figure2 shows the sliding contacts of this switch in the correctposition for each function. Note that in the OFF positionthe switch shorts lead B to lead C. In the LOW positionthe short is removed and an earth is extended to B, whilein the HIGH position the earth is removed from B andextended to A. For single speed wipers slide contact Awill be omitted.

DELAYED OPERATIONWhen delayed operation is required, the upper switch isleft in the OFF position and the timing circuit energisedby operating SW1 which is part of the switch /potentiometer RV1 .

After a time which is set by the position of RV1(0.5-25 secs.) the relay contacts RL1 (1) change over,removing the short circuit from the motor armaturebefore energising the motor by extending an earth viathe now closed relay contacts.

As the motor gathers speed the associated cam -operated switch changes over, removing power from thetiming circuit (causing the relay to drop out), and

27

extending an earth to the wiper motor via the wiperswitch contacts B and C, the now de -energised relaycontacts, and the cam -activated switch.

The wipers continue their sweep across tne screen,but on their return the cam -operated switch cuts in justbefore the end of the sweep. This removes power fromthe wiper motor and places a short across the armature.The motor is thus dynamically braked and remainsstationary until the next relay closure from the timingcircuit. When this arrives the sequence is repeated.

WOUND FIELD COIL MOTORSBecause wound field motors do not use dynamicbraking, the Vari-Wiper can be made without a relay.Figure 3 shows the simplified Vari-Wiper circuit and itsconnections to either a positive or negative earth vehicle.The same printed circuit is used for both arrangements.Operation is similar to the previously described unit,having an earth extended through the SCR to start themotor.

CONSTRUCTIONAssemble and solder all components on the printedcircuit board as shown in Fig. 5. Do not bend the lugs ofthe SCR too close to its case and ensure all semicon-ductors are the right way round.

To connect the unit to the wiper motor circuit, theexisting lead from the centre pole of the wiper motorchange -over switch to the wiper ON /OFF switch (shownin dotted lines in Fig. 2), should be broken at points Xand Y and these leads taken to the normally closedcontacts on the relay. Ensure that point X goes to thefixed contact and point Y to the moving one.

The potentiometer should be connected to the unitwith just enough wire to allow the printed circuit to bemounted in a convenient position under the dash. Thepotentiometer can be mounted through a 10 mm holedrilled in the facia panel or by attaching it to a bracketmounted in a convenient place. HE

Cl22p16VTAG

+12V

R5330R

RLA (1)

OFF

LOW

HIGH

Parts ListRelay Output Unit

RESISTORS ALL 1/4W 5%

R1 10kR2 470RR3 47RR4 1kR5 330R

POTENTIOMETERRV1 1M switch pot

CAPACITORC1 22u 16V electrolytic

SEMICONDUCTORSD1 1N4001Q1 2N2646 unijunctionSCR1 C106D (400V, 4A)

MISCELLANEOUSRL1 Miniature printed

circuit boardmounting heavyduty 12V relay

PCB Variwiper 2Nylon terminal strip

SCR Output UnitAll components identical, except:R5 deletedD1/2 1N4001SCR2 C106D (400V, 4A)RL1 deletedPCB Variwiper 1

Approximate Cost: £6

PARKINGSWITCH

! 5TH WIREEXTENDED TO

1I SWITCH ON1 SOME MOTORS

±WIPER SWITCH

Fig. 2. The Vari-Wiper circuit using relay output for use withpermanent magnet motors.

28

A -12V

iro2-7_ 1N4001

C122p22,J

16VTAG

R347R

B1

Q1

R2470R

PARKING

ArSCR2C106D1

±WIPER SWITCHPOT.SWITCH

0-0

Fig. 3. Simplified Vari-wiper for use with woundfield coil motors with positive earth.


Variwiper+12V

Fig. 4. The simplified circuitfor negative earth vehicles. --AL D1

1N4001Both this and Fig. 3 usethe same PCB.

Cl22p

T16VTAG

SCR2r-C106D1 A

OFF

LOW

HIGH

PARKINGSWITCH_.

I-

1

WIPER SWITCH7.7

0781..IIF7Jailar-°T,E

0

11111.1117:0 <

NEG. EARTH POS. EARTH

Dl40001ewe /WWI..

elSWITCH

I I I1.41sees

RL1

d/feieeses

,les®40°1 e ®EARTH

How it Works

+12V

N.C.

EARTH

N.C.

EARTH

-12V

PARKINGSWITCH

The timing circuit is energized by operatingswitch SWI, which is part of switch/potentiometer RV 1. This switch applies powerto the unijunction /SCR circuit via the still -closed parking switch contacts.

Capacitor CI charges via RV1 and RI, at arate determined by the setting of RV1, until theunijunction 'fires', producing a positive goingpulse which triggers the SCR into conduction.Resistor R4 ensures that the SCR latches on,thus energizing relay RL1.

Relay contacts RL1 (1) now_ changeover,removing the short circuit from the motorarmature before energizing the motor by ex-tending an earth via the now -closed relay con-tacts.

As the motor gathers speed, the associated

+12V

'Y'

Fig. 5. Component overlays.Note that the same PCB isused for both earth polarities.

cam -actuated switch changes over, removingpower from the timing circuit (causing the relayto drop out) and extending an earth to the wipermotor via wiper switch contacts B and C, thenow de -energized relay contacts, and the cam -actuated switch.

The wipers continue their sweep across thescreen, but on their return the cam -actuatedswitch cuts in just before the end of the sweep.This removes power from the wiper motor andplaces a short circuit across the armature.

Operation of the unit is similar except themotor, which does not require dynamic braking,can be driven directly from the SCR, saving thecost of a relay. Note that either D I or D2 becomeredundant depending on the polarity of thevehicle.


HobbyElectronicsBook ServiceBUILD -IT BOOK OF OPTOELECTRONIC PROJECTS byCharles Adams £4.75

A 48 project learn -by -building guide to the super -practical world of optoelectronicswith thoroughly readable instructions on how to create everything from an LED circuitmonitor to an electronic stopwatch.

HOW TO READ ELECTRONIC CIRCUIT DIAGRAMS by BobBrown and Paul £4.00

This book is for the beginner who wants just enough practical knowledge ofelectronics to pursue a hobby.

EASY SPEAKER PROJECTS by Len Buckwalter ... £3.75Explains how to make speaker systems ranging from simple shelf board andceiling -mounted enclosures to more advanced bass -reflex and omnidirectional ones.

ELECTRONIC GAMES AND TOYS YOU CAN BUILD by LenBuckwalter £3.75

A 'just -for -fun' book. No complex electronics theory anywhere - just step-by-stepinstructions. construction photos. and detailed wiring -construction diagrams forbuilding 15 fascinating electronic games and toys.

MICROPHONES: HOW THEY WORK & HOW TO USE THEM byMartin Clifford £4.75

Takes all the mystery out of microphones. shows you how to record almost anythingwith professional quality, clearly explains how to get different sound effects andbetter stereo. and fully describes mike positioning techniques.

BASIC ELECTRONICS COURSE by Norman H. Crowhurst£5.50

A self -study text for the novice, hobbyist. student - a perfect reference and brush -upguide for technicians and others who want to advance their knowledge of electronics.

HOW TO TEST ALMOST EVERYTHING ELECTRONIC by JackDarr £3.50

Practically every testing technique for almost every kind of electronic equipment islaid out for you in easy, highly readable style.

ELECTRONICS FOR THE AMATEUR by Louis M. Dezettel£6.55

Written especially for the radio amateur, discusses radio -wave propagation as it

applies to amateur band frequencies, reception and transmission as they pertain tothe equipment 'hams' use, and the special field of antennas and how to feed them.

ELECTRICAL SOLDERING (2nd edition) by Louis M. Dezettel£4.45

Teaches the hows and whys of electrical soldering. Tells about solder alloys andfluxes. Discusses the various types of soldering devices. Then concentrates on theprinciples of good soldering and acquiring the needed skill.

BUILD -IT BOOK OF HOME ELECTRONICS by Graf andWhalen £3.45

Put electronics to work for you in your home. The authors present thirteen interestingand fun projects that use the latest solid-state devices and ICs.

BUILD -IT BOOK OF SAFETY ELECTRONICS by Graf andWhalen £3.45

Contains ideas for building several novel projects to protect toddlers from invadingthe medicine chest to a motel room from a burglar: and to scare a car thief and more.Thirteen tested safety projects are detailed.

30

BUILD -IT BOOK OF FUN AND GAMES by Graf and Whalen£3.75

All the projects in this hook are designed for fun and game applications. The projectsinclude a sight'n'sound metronome. an electronic football game and stereo balancemeter, a sound -activated switch and a metal detector.

IMPROVING YOUR HI-FI £3.50This book is aimed at the novice who has just purchased his first audio set-up or isabout to do so. It begins with details of how to connect the units together to obtain topperformance and explains the meaning and proper use of the many control knobs andswitches found on modern hi-fi gear.

GUIDE TO PRINTED CIRCUITS by Gordon J. King . £3.50The Text is angled especially at the keen amateur constructor. the experimenter. theservice technician and apprentice.

ELECTRONIC TEST EQUIPMENT by Harry T. Kitchen £5.00This book is designed to provide interest for almost everyone involved withelectronics. The six chapters explore the more general groups of instruments suchas test meters of various types. audio and r.f. signal generators, attenuators andoscilloscopes. Each chapter explains the principles and requirements of particulartypes of test equipment. including typical circuitry. then discusses the choice, careand use of the equipment.

FUN WITH ELECTRONICS by McEntee £3.40An exciting show -tell -and -build -it book that teaches electronics with plenty of fun inthe process.

AUDIO AMPLIFIERS FOR THE HOME CONSTRUCTOR by IanSinclair £2.60

Describes in seven chapters the working principles of transistor amplifiers andoutlines the steps and circuit design using a modern simplified transistor theory.

INTRODUCING ELECTRONIC SYSTEMS by Ian Sinclair£1.80

This book by author -lecturer Ian Sinclair is intended to provide a basic insight intowhat makes electronics 'tick'.

INTRODUCING AMATEUR ELECTRONICS by Ian Sinclair£1.55

This is the book for the complete novice of any age, in which author Ian Sinclairassumes no previous knowledge of the subject by the reader.

ABC'S OF FET'S by Rufus P. Turner £3.05Describes the field effect transistor )FET) in simple language. Deals in detail with thrprinciples of FET operation and stresses the applications of FETs in practical circuits.

ABC'S OF ELECTRONICS (2nd edition) by Farl J. Waters£4.15

An introduction to the ever-expanding field of electronics. Covers the principles ofelectricity. functions of atoms and electrons. magnetic forces and their relationshipto electronics.

Note that all prices include postage and packing. Please make cheques. etc, payable to HobbyElectronics Book Service (in sterling only please) and send to:

Hobby Electronics Book Service,P.O. Box 79,Maidenhead, Berks.


Hobby 494Electronics 4Short Wave Receiver

Back in the bad -old -days when there were just valves.and they were expensive, you thought hard beforeadding another stage of amplification: first you tried tobe clever. Our SW radio next month uses just twosemiconductors yet will give surprisingly good perfor-mance over the range 5.5-25 MHz if used with areasonable aerial.

11111111111111111111111111111111111111111

Instant Circuit Layout

(1. ,*

Do you have trouble getting your brain to translate acircuit into a practical layout? If so you're in goodcompany but next month join the elite by overcomingthis. We give you practical advice on how to lay outcomponents from practically any circuit diagram.

111111111111111111111111111.1111111111111111

Scratch /rumble FilterAn add-on circuit to couple to an existing audiosystem, this project enables you to select the cut-offfrequency of the system at both ends of the spectrum.

Radioactivity

Although most people shudder when they think ofradioactivity, there's a lot more to it than fallout fromnuclear bombs. Radioactivity is widely used inmedicine and in industry and our article describessome of the uses and traces the history of its develop-ment.

III IIIIIIIIIIIIIIIIIIIII11111111111111111

Video Tape Recorders

The age of the Video Cassette Recorder has arrivedand soon they'll be common. However, they are themost complex, sophisticated, pieces of engineeringthat have ever crossed the doormat in reasonablenumbers. Next month we explain how they work andtake a look at the different systems being offered.

milimummummummimm0 ST Rules, OK?Got the hang of Ohms Law? No problem but the worldisn t yours yet - have you ever found a problem itcan't cope with? The chances are that you have.However, there are two other approaches to help yousolve the nasty ones: Superposition and Thevenin'sTheorem. They may sound complex but in reality theymake life simpler.

Sine/Square WaveGenerator

An essential part of anybody's test gear and ourproject next month enables you to add one to yourworkbench at low cost

11111111111111IIIIIIIIIIIIIIIIIIIIIIIIIIProjects Using The CA3130

L

We publish one chapter from R. A. Penfolds '50Circuits using the CA3130' (brought out by Babani)and mighty interesting they are too. The projectsinclude an electronic organ, metronome, alarm andlatching circuits.

Holograms

Today they are only a curiousity, shown as exhibitsand as special effects but much work has been goingon behind the scenes. Today's Holograms really make'you wonder if you can believe your eyes.

The February issue will be on sale on January 12thThe items mentioned here are those planned for the next issue but circumstances may affect the actual content.


Hobby Electronics

MarketplaceC

I)44,14"tt

Size: 100mm x 130mm x 60mm.

Over 10% of Electronics Today International's readers have purchased adigital alarm clock from offers in that magazine - the offer is nowextended to Hobby Electronics readers This is a first rate brandedproduct at a price we don't think can be beaten.

The Hanimex HC -1100 is designed for mains operation only(240V/ 50Hz) with a 12 hour display, AM / PM and Alarm Set indicatorsincorporated in the large display. A switch on the top controls aDim /Bright display function.

Setting up both the time and as buttons areprovided for both fast and slow setting and there's no problem aboutknocking these accidentally as a 'locking switch is provided under theclock. A 9 -minute 'snooze' switch is located at the top.

An example of this digital alarm clock can beseen and examined at our Oxford Street offices.

£895(Inclusive of VAT and Postage)

To:DIGITAL Alarm Offer,Hobby Electronics,25-27 Oxford Street,London W1R 1RF.

Please find enclosed my cheque/P.O. for£8.95 (payable to Hobby Electronics) for myDigital Alarm Clock.

Name

Address

Please allow 14 days for delivery.

We feel we've got to tell you carefully about this offer which we'reintroducing for the first time. Why? Because our price is so enormouslylower than anywhere else you may suspect the quality.

The exact same watch is currently being offered by another magazineas a special at £24.95 - some of the discounters are selling it at£29 95 the price to HE readers for exactly the same watch is Et 2.95.

The display is LCD and shows the seconds as well as the hours - andminutes - press a button and you'll get the date and the day of theweek.

Press another button for a couple of seconds and you have a highlyaccurate stopwatch with hundredths of a second displayed and givingthe time up to an hour. There is a lap time facility as well - and of coursea back light.

Our Chrono comes complete with a high grade adjustable metal strapand is fully guaranteed.

An example of this LCD Chronograph can beseen and examined at our Oxford Street offices.

£12095(Inclusive of VAT and Postage)

To:LCD Chrono Offer,Hobby Electronics,25-27 Oxford Street, London W1R 1RF. 1

Please find enclosed my cheque/ P.0 for£12.95 (payable to Hobby Electronics) for myLCD Chronograph.

Name

Address

Please allow 14 days for delivery.


Short CircuitA SINGLE OP AMP OSCILLATOR

An op amp can be made to oscillategenerating a square -wave output.The circuit is a Schmitt trigger andintegrator all rolled into one.

To understand the operationimagine the output is high; C1 ischarged up via R3. The voltage atpoint A is +0.9 V due to the resis-tor divider network R1, R2. Whenthe voltage at B exceeds this vol-tage, the output of the op amp flipsinto its negative (low) state, C1 istherefore discharged by R3. Whenthe voltage on Cl reaches -0.9 V,the reverse process occurs and theop amp output flips back to itshigh state. Thus the circuitoscillates producing a squarewave going from +10V to -10 V.

The frequency of operation canbe obtained from the voltagechanges on C1. This is the trin-cated section of an exponentialcharge /discharge curve, but weshall ignore this and assume thatthe curve is linear (which it almostis).

The frequency can be obtainedfrom the formula F=I/AVxC Hzwhere I is the charging current

SCMITTTRIGGER

(approximately 1000A), AV is thecharge accross Cl (3.6 V) and C isthe capacitance measured in.Farads.

Therefore F =10-4/ 3.6 x CHz. Thus, if C1=1 00nF,F=270 Hz; if C1 is lOnF,F=270 Hz and if Cl is 1nF,F=27 000 Hz.

R1100k

INTEGRATOR

t -10V

-10y

DUAL POWER SUPPLY OPERATION

EXPONENTIAL CHARGE

+10V

o'+0V9

OV9

10V

VOLTAGE AT POINT B SINGLE SUPPLY OPERATION

POWER SUPPLY WITHIC REGULATORA regulated power supply used torequire a fairly complex circuit butthe introduction of special ICs hasmade matters much simpler.

The chart shows the variousvoltage requirements for four typesof voltage regulator. The last twonumbers of this range of IC indicatethe voltage so the 7812 will give1 2V. Care has been taken that thevoltage ratings of the regulator arenot exceeded as this can blow upthe IC.

The secondary voltage from thetransformer (it must be an isolatingtransformer but most are) is full -wave rectified by the diodes and.then smoothed by capacitorC i . The voltage here is unregu-lated and has AC ripple superim-posed upon it. The IC removes thisand gives an almost ripple free.stable DC voltage. C2 and C3 mustbe sited close to the regulator toprevent any loss of performancedue to high frequency instability.Note that the capacitor and com-mon lead from the IC should bewired to the same point, this helpsto reduce instability and currenthum problems that can occur dueto poor l,ayout.

The 78XX series of regulatorswhich come in a T0220 packagegenerally can supply 500 mA of

0 0L FUSE

045

N

'PRIMARY240V SECONDARY

V SCREEN

TO ANY EXPOSED METALWORK

REQUIREDOUTPUTVOLTAGE

-DID--

1N4003

REGULATOR78XX

UNREGULATED RAIL

COMMON NODE

10220 PACKAGE

IN

COMMONOUT

78XX

500 mA MAXIMUM

V eg I t dOUTPUT

C3220n TANTALUM

BOLT ONTO A HEAT SINK

OV

REGULATORMETAL TAB IS INTERNALLY CONNECTED TO COMMON PIN17BXX REGULATORS ONLY)

TRANS-FORMERSECONDARYVs

CAPACITORVOLTAGECl

TYPICALMAXIMUMINPUT VOLTAGETO REGULATOR

5V12V15V24V

7805781278157824

current. Therefore he maximumpower dissipated in them is prob-ably going to be 500 mA times thevoltage difference between theinput and output terminals. This

8 TO 9 V RMS12 TO 15 V RMS15 TO 16 V RMS20 V RMS

16 TO 25 V25 TO 35 V35 TO 63 V35 TO 63 V

might mean that the regulator hasto dissipate 5 W of heat. Thereforean adequate heatsink must beused. The 78XX series is currentlimited which means that if the

25V30V30V38V

maximum current is exceeded, theoutput voltage drops towards OV.The regulator is thus short-circuitprotected as long as proper heatsinking is provided.


ViewdataPrestel, the British Post Office's viewdata service,available during early 1979. Angus Robertson takes abackground and its future implications.MOST READERS will by now have come across teletext,the information service transmitted by both BBC and ITVand called Ceefax and Oracle respectively. These ser-vices have been widely promoted through both the TVprogramme magazines and directly on -air - teletextreceivers can be seen at most respectable rental andtelevision showrooms. Basically, a special receiver isused which decodes and displays the digital teletextsignals which are transmitted on four of the total 625lines comprising each television channel. Typicalteletext services include news, weather, sport, and suchtopical subjects - however the maximum number ofpages is only a couple of hundred, otherwise thetransmission cycle becomes rather extended and thewaiting time for pages becomes unacceptable. Viewdatauses the same transmission format as teletext, butinstead of information being transmitted on the limitedspare lines of a TV signal, telephone lines are used.

In Britain there are some 23 022 000 telephonesconnected to 14 862 000 lines (allowing for PBXs andextensions) and the capital cost of these, togetherwith cabling to exchanges, is £1 548 100 000. Theaverage number of daily calls per line is only 3.36, andobviously if subscribers can be persuaded to make morecalls each day, this investment will be better utilised,thus effectively keeping phone charges down. Thespeaking clock is one such 'traffic generator' receivingaround 400 000 000 calls each year - the Post Officehope that their Prestel viewdata service will be another.

The works. This shot shows the innards of a Mullard Teletextunit. The size could be reduced even further to fit inside a TVset.

34

becomes publiclylook at viewdata's

Viewdata systems can appear in many different forms andguises - you don't have to use a domestic TV set. In fact,there's no reason why the system shouldn't be as an accessoryto the telephone rather than the TV.

HOW IT'S DONEInformation is stored at the viewdata centre in much thesame way as teletext, using a small computer and bulkdisc storage. However, with teletext each page isaccessed and transmitted every 30 seconds or so, butviewdata pages are only transmitted when a viewermakes a specific request - which makes it a consider-ably more efficient system and able to handle manymillions of pages rather than teletext's few hundred.More importantly, since a telephone line must beestablished to the viewdata computer to receive infor-mation, the computer can then accept other instructionsfrom the home terminal making it totally interactive - ineffect it means a commercial computer terminal in onesliving room or office. However, unlike a commercialcomputer terminal, the protocol required to com-municate with it is very simple and can be understoodeven by children after only a few minutes instruction.

Perhaps I should make the rather important point thatunlike the free teletext service, charges are made foraccessing viewdata pages and these can accumulaterapidly while one browses through the system. One otherimportant difference is that while teletext information isoriginated by the BBC and ITV, the Post Office is onlyoffering its Prestel viewdata service as a 'medium' inmuch the same way as the telephone system. In otherwords, private companies and organisations can rent atelephone, and then transmit whatever information theyso wish - the Post Office will originate no pages itself,other than indexing, although certain PO departmentsmay rent pages themselves for various services such asdirectories, dialing codes, postal charges or whatever.


a wide network of computers or lines, the PO will havean initial advantage since many different organisationswill be using those same computers and lines.

It might be interesting to look at the charges beingmade for the PO Prestel viewdata service. Users willeither buy or rent a viewdata receiver from their local TVshop and this will be plugged into a normal PO jacksocket as used for extension telephones. Actual receivercost is difficult to estimate but will probably initially be.about £700 or £18 per month. To contact Prestel,pressing a button automatically dials a special telephonenumber. The computer answers automatically with arequest for the receiver's 'user number', which is storedelectronically. When this has been received by thecomputer, it offers the first indexing page and chargingcommences.

Apart from the local phone call which works out at221/2p per hour in the evening and at weekends, there isa 2p per minute charge while connected to the com-puter. Some pages will be free such as indexing andadvertisements, while others will carry a charge which isprominently displayed in the top right corner of thescreen. This varies between 1/2p and about 10p althoughhigher charges are possible for valuable information.Most information works out at around 1/2p to 2p, withonly topical news items and such at an extra cost. Thischarge per page is paid directly to the companies thatsupplied the information, less a 5% handling charge forthe PO. In addition, information providers (as they are

Obviously the normal laws of libel and decency apply,but pages will not be vetted as such by the P0. However,subject matter should be within a specific sphere in orderthat indexing can be provided.

This might appear rather complex, but in practice isvery simple and the system is designed to allow rapidaccess of information without recourse to directories.Basically, each page provides prompts leading towardother pages in the form of a maximum of 10 choices forfurther information which are then simply accessed bypressing a single digit on the receiver keypad. This newpage should in turn provide further prompts until therequired information is reached - the final page has aprompt leading back to the route's beginning so one canstart again.

In the initial stages at least, Prestel will be primarilyproviding 'hard' information such as classified adver-tisements, houses and cars for sale, stockmarket results,entertainment guides, holiday and travel timetables,hobbies and pastimes, cars, education, agriculture andfarming, and employment. In addition to informationopenly available to all users of Prestel, closed usergroups can also be created. This is of particular interestto companies and organisations that need to supplyrapidly changing information to a large number ofoutlets widely scattered around the country (or world),such as travel agents, banks, building societies, chains -of shops and wholesalers, service departments and soon. In this case, only preselected users will be permittedaccess to certain pages - these users might for instancehave to pay an additional service charge or simplybelong to a particular group of companies.

When Prestel is eventually fully operational, theinteractive capability will allow information to be passedback from user to the original information provider. Forinstance, if one has been examining airline reservations,a booking could be completed through the Prestelsystem and (potentially) could also be paid for using acredit card (whose number Prestel could convenientlyalso store). Similarly, one could complete a sales trans-action and the system could be on-line to bankingcomputers enabling one to examine up-to-the-minutestatements and make transactions directly without paperof any form. Prestel can also be used for tasks such ascalculating mortages (which requires considerable efforton most pocket calculators). One of the first Prestel pagesdemonstrated almost three years ago was one of inter-active games although these are not being initiallyprovided on the public service. The Maze Games pro-vided mazes of seven different complexities and one hasto move a marker around using four digit keys -virtually impossible with the most complex!

TOMORROW THE WORLDMaybe here I should say that eventually there might bemany viewdata services operating both in Britain andaround the world - the PO Prestel service is just the firstand the PO certainly does not have a monopoly overusers of its telephone lines. Mullard ResearchLaboratories have been privately demonstrating theirown viewdata system that uses a radically differentindexing system. Eventually, this might be made avail-able to the public. Since one of the principle economicnecessities of viewdata is that it should be availablecountrywide for a local telephone charge, thus requiring


termed), pay additional charges for having their infor-mation stored on Prestel in the first place. The 'A' rateapplies to regularly updated information and comprisesa £4 000 a year service charge with a £4 per framecharge on a one year contract reducing to £2 400 a yearand £2.40 per frame on a five year contract. The 'B' rateapplies to more archival information that is onlyoccasionally updated for which an annual charge of£1 000 is levied with a £1 per trame storage charge -however, 1/2p is deducted from each frame access tocover the increased cost of bulk storage. This secondrate is appropriate for encyclopedias which mightrequire about 200 000 frames storage and for which agreater waiting time than normal one or two secondsmight be acceptable, so that frames could be retrievedfrom tape or some such system.

IMPLEMENTATIONSo how is viewdata actually implemented? The frameformat is identical to teletext, 24 rows of 40 characterswith seven colours. However, the top line is reserved forpage and charging data and the name of the informationsupplier, while the bottom line is used for computermessages. Information between receiver and computeruses the PO DATEL 600 service which provides a 1 200

One of the many possible applications for a viewdata typesystem is the fast dissemination of quickly -changing informa-tion. This could be financial, advertising or weather informa-tion, although some applications won't even be thought of untilthe system is in common use!

baud (bit /sec) frequency shift modulation circuit bet-ween computer and receiver (1 20 words/ sec) and a 75baud link in the reverse direction for keypad commands.

Although early receivers used standard PO modems,this would have been totally uneconomic for a publicservice and so the PO has relaxed one of its moststringent rules on the supply of equipment connected tothe public switched telephone to allow private industryto develop and sell direct to consumers a vastly sim-plified modem built into the receiver itself. This changeis only part of a broader attitude that the PO has beentaking during the past couple of years which should see

36

One of the many possible configurations for a Viewdataterminal. This one would be suitable for someone with thehome computing bug!

other items of telephone equipment and new servicesbecoming available over the next few years (and which Ishall be writing about shortly in Hobby Electronics).

The computers used for Prestel are GEC 4080 typesusing 70Mbyte discs (storing about 70 000 pages each)and these operate in pairs at each Prestel computercentre, each with separate memory, but capable ofaccessing the other. The computer centres will have alarge number of 'ports' which comprise telephone lineswitching answering, modem and a line buffer with a1 Kbyte memory which temporarily stores a page trans-ferred from disc while it is being transmitted to thereceiver. It is hoped that each computer will supportsome 700 simultaneous users engaged only in informa-tion retrieval, while this would have to be reduced for

The system which made it all possible - the widespread use ofCeefax and Oracle systems makes the connection to the phonelines a feasible proposition. It's simply a matter of arranging forthe system to get its information from the phone rather thanthe top few lines of the TV picture.


Viewdataservices requiring more processing time such as gamesand calculations.

Message services will also become available so thatusers can send written messages to each other - Prestelis even going to be linked to the international telexnetwork so that users will be able to send messages toany telex users worldwide - and vice versa of course(useful for small businesses that cannot justify the£500 -odd rental for a telex). Although initially Prestelwill operate from a central London computer centre, inthe early years of the project this particular centre willbecome the national update computer which informa-tion providers will access to change their information,while further computer centres in the north and south ofLondon will provide the local Prestel service. Other areasgetting computer centres (with twin computers) includeGlasgow/Edinburgh, Manchester/Liverpool andBirmingham - further centres will be added as demandis generated. Projections vary, but around 1 000 000Prestel viewers are anticipated by 1985 although inearly years business and commercial users will farout number private consumers.

There might be a few stumbling blocks to cross, butviewdata is here to stay. When the various interactiveand message services commence operation, demand

should increase considerably. I'm even thinking ofbecoming a customer myself! HE

With Viewdata, it will be possible for industrial users to sendinformation to the system for 'publication' via their ownterminal.

Short CircuitRIAA STEREO PREAMPLIFIER

There are two types of recordplayer pickups, ceramic and mag-netic. The first type is the cheapestand generally gives a large outputvoltage (0.5 V). This type of pickupdoes not usually require anyfrequency response correction, butthe sound quality produced is notas good as that which can beachieved with a magnetic pickup.Records are cut with a frequencyresponse such that when they arereplayed with a magnetic pickupand a preamplifier with a RIAAequalisation (Recording IndustryAssociation of America) the repro-duced sound will be as similar tothe original as possible.

The disc is cut at constantamplitude, except from 500 Hz to2120 Hz where it is cut at constantvelocity. When this disc is replayedwith a magnetic pickup, therelative output voltage rises withfrequency, this being due tothe fact that the magneticallygenerated voltage is proportionalto the velocity of the stylus as itmoves sideways in the groove. Torestore the original sound quality, apreamplifier with a frequency res-ponse that gives decreasing outputwith increasing frequency is req-uired. This response curve isknown as the RIAA equalisation

and it is tailored accurately tofit the cutting and replay pro-cesses. The signal level from amagnetic pickup is low, generally20 mV pp and so a low noise pre-amplifier is needed.

The circuit shows a realisation ofthis requirement. The low noiseamplifier is the LM381 made byNational Semiconductors. A DCbias control is included RV1, 2),and the feedback componentsgenerate the RIAA curve. Usescreened cable for the wiring to thepickup, keep the circuit away fromtransformers (and the pickup andits wiring) and connect all theearths shown in the circuit diagramtogether, near to the IC.

PLAYBACK RESPONSE

(RIAA EQUALISATION)

TO MAGNETIC PICKUP(LEFT CHANNEL)

R147k

R V1470k

BIAS: SET PIN 7TO +4V5

TO MAGNETIC PICKUP(RIGHT CHANNEL)

R747k

ClluO,

9V

R2150R

C210u

R42k2

C44u7

R31k0

R51M

C53n3

R6100k

C61n0

+30

+20

+10

R82k2

R9

C9 R10z 0 R V2 4u7 100k

(.7 10470k

BIAS: SET PIN 8 1--"/W-20

TO +4V5 R11 C101k0 3n3

-30 -IF100Hz 1kHz 10kHz 100kHz10Hz C11

1n0FREQUENCY

C31u0

LEFT OUTPUT

C81u0

RIGHT OUTPUT


(lb

37

ProjectDaedalusEarlier this year the British Interplanetary Society published a report on'Project Daedalus'. It was nearly 200 pages long and took five years towrite. It's packed with detailed calculations for the design of an interstellarcraft. These show that the capability is almost within our grasp. Phil Cohenanalyses the results of the report.

The DAEDALUS PROJECT is the brainchild of the BritishInterplanetary Society, founded in 1933 with the aim ofadvancing the space industry in the UK.

The 'study group' which worked on the projectconsisted of a small number of professional scientistsand engineers from establishments such as the UKAtomic Energy Authority; British Aircraft Corporation,RAF and City University, London. The work was carriedout in their spare time over a period of five years andculminated in the publication of a JBIS (Journal of theBritish Interplanetary Society) report nearly two hundredpages long which contained a summary of the results ofthe study!

The name Daedalus is from the Greek. In legend,Daedalus (meaning 'cunningly wrought') built for him-self and his son, Icarus, sets of wings. During their flight,Icarus disobeyed his father's instructions and flew tooclose to the Sun. The wax holding his wings togethermelted and he was killed. Daedalus, however, reachedhis destination without mishap!

The Daedalus craft is an unmanned interstellar probewhose purpose is to gain information about nearbystellar systems - and especially to search for planets,which may contain the first alien life we contact.

THE MISSIONThe mission of the probe is to accelerate to about 1 2% ofthe speed of light (which works out at 3.6 x 10' m /s, or20 000 miles per second!) and fly past Barnard's Star(5.91 light years (Ly) away - about 50 years at 20 000miles per second), dropping probes which will collectdata about the star and its (possible) planetary system.This information would then be transmitted back toEarth.

Barnard's Star is not definitely known to haveplanets. Recent observations have shown that the star'waltzes' slightly - suggesting that its 'dancing partner'is a massive planet somewhat similar to Jupiter. This'waltz' is, however, not pronounced enough to provewith any degree of certainty the existence of a planetarysystem.

Why Barnard's Star, then? There are two other stellarsystems closer to us - Proxima Centauri at 4.3 Ly andAlpha Centauri A/B at 4.4 Ly. Alpha Centauri A/B is adouble star and must surely be as interesting as Bar-nard's Star?

The answer is that the Daedalus Project is an attempt,not to design a probe completely, but to provide a designframework for further studies. The design team consid-ered that if it was possible to use a Daedalus -type craft toreach Barnard's Star, it should be possible to get toAlpha or Proxima Centauri also.

One major consideration in deciding on the actual'mission profile' chosen was that it had to yield somesort of results within a human lifetime. This was becauseit was considered unlikely that any state would under-take a longer -term project!

LCETI5

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STA

18 HR

The positions in space of the stars closest to the sun. The figurebeside each star is it's 'interest' ranking. These were arrived atby taking into account the distance, the 'uniqueness' of thestar (how frequently similar types occur) and the probability ofhabitable planets. The obvious choice for a mission would beAlpha Centauri but the probe was designed to reach Barnard'sStar for good measure.


The first twenty years of the project would be spentdesigning, building and fuelling the craft in orbit aroundJupiter (for reasons which will become apparent later).The craft would then accelerate out of the Solar Systemfor 2 years, at which time the first stage would bedropped to save weight. The second stage would thentake over for another 1 .8 years, accelerating the craft toits final, awesome speed.

There would then come a 40 -year wait, with the crafttransmitting only data about the interstellar medium -the dust concentration, for instance, which would beinvaluable for the design of later craft.

At the end of this period the craft would be closeenough to its target to detect the existence of any 'gasgiant' planets similar to Jupiter. At about this point itwould start to disperse its probes. The dispersal has totake place this soon because it takes a lot of power tochange course at that sort of speed and the probes wouldhave to fly past the star itself as well as any planets,which may be at wide orbits. The decisions about whichdirection to send each probe would be taken by the maincomputer on board the craft. As radio waves would takeabout 5 years to reach Earth from the ship by this time(and thus a ten-year wait for a reply!), all decisions wouldhave to be taken by the ship's computer.

As the craft reached the outer limits of the system, theprobes would begin to send back information to it. It

fi

. f.I' r 4,1 .

Rol AlhogUi

\

. :

4' 0- Sob..

Barnard's Star as seen from the Earth. Unfortunately it's notvisible to the naked eye. It appears in the constellation ofOphiuchus.

would relay the information back to Earth, the totaltransmission time being about 3 years - the beginningof the message would still only be half -way home whenthe craft stopped transmitting!


THE CRAFTThe Deadalus probe has a two -stage engine, the firststage being dropped to save weight when its fuel isfinished. These two stages propel the payload of smallerprobes, computers for controlling the mission, com-munications and other equipment.

The first stage weighs over 40 000 tonnes fullyfuelled and is about 150 metres long and 190 metreswide. It consists only of a giant motor and six sphericalfuel tanks. It 'burns' for about two years continuously atthe start of the mission. The designers also took intoaccount which materials would be used for the craft'sconstruction - the materials used for the engine'sreaction chamber, for instance, have to stand tempera-tures from 3°K (3 degrees centrigrade above absolutezero) to 1 600° K. This means using an exotic alloy. Theone chosen was molybdenum with titanium, zirconiumand carbon, internally nitrided - you don't come acrossalloys much more exotic than that!

The fuel tanks are dropped during the course of the'burn" to save weight. As they weigh over 16 tonneseachthis is quite a saving. Remember, the fuel is beingused up also - the first stage itself, without fuel andtanks, weighs only 100 tonnes.

The second stage is almost the same as the first,except that it's about 1 / 10th of the mass and about 1/2the size. It has four fuel tanks which are also disposableand carries the payload bay. This is about 30 metreslong and about 50 metres diameter. It holds (starting at

160

140

130

12

110

100

90

60

8 0 0 0

BUFFER CAPACITOR CCNOITONER

CHEMICAL A PUPAYLOAD- POWER CONDITIONING

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INDUCTION LCORTP -SUPPO

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the front) an erosion shield to protect the craft frominterstellar dust erosion, the eighteen disposable self-propelled probes, the main telescopes, the communica-tions equipment and computers and the 'wardens'.

As the craft will be on its own for some decades, andas only the most optimistic would expect there to be nofailures on board during all this time, it is necessary tohave some form of automatic repair. This is where thewardens come in. Controlled by the main computers,they are multi -purpose self-propelled robots, flexibleenough to perform any repair or replacement necessary(within reason). The ship would also carry a largecomplement of spares - hopefully, the wardenswouldn't have to build anything from scratch.

The ship would be 190 metres long at launch andwould weigh over 54 000 tonnes - that's a lot of massto get moving!

THE PROPULSION SYSTEMThe starship is propelled by a series of very small nuclearfusion explosions, occurring at a rate of 250 per second.

Earlier systems had been proposed by other groupswhich suggested using conventional atomic bombsejected from a craft carrying an immense 'pusher plate'.The momentum from the explosions would be transfer -

A diagram of the second stage. This is similar to the first stagebut carries the payload -a package of smaller probes to bedeployed near the end of the journey.

EROSION SHIELD 064 COO

- -POWERED SUB -PROBE 18 OFFPROBE DECK 411 50 OCO

5m L ST 2 OFF

ASTRONOMY DECK

DUST GE NERATER--- PRESSURE CHAMBER--COMMUNICATIONS DECK

'OMENS. IS PROBES. SPARES

WARDENS N2 N4 TANK02 120SUPPORT STRUCTURE

LH2 TANK 012 000

THRUSTERSPROPELLANT TANK*30 BMMANEOVRE TANK 09500

-BUFFER CAPACITORREFLECTORCCOIMUNICATONS EQUIPMENTACTUATORSPELLET INJECTOR

SUPPORT STRUCTURECOIL 2 1 011200DEPLOYABLE ANTENNA FEED

COL 22 036020

.SEPARATION INTERFACE

- - -nDuCTON LOOP

TANK SUPPORT ANDJETTISON ARM

Section A -A

1.vt 2ioff-Section B -B

4!-- al I 1 I MI rit7.0-003


Project Daedalusred from the pusher plate to the craft at a reasonable ratevia a pneumatic spring system. This type of system wasdropped because of the required size of the vehicle, "thelimitations imposed by the nuclear test -ban treaty andthe difficulty of testing the system."!

The Daedalus propulsion system contains the energyof the explosions in a very strong magnetic field andreleases it between explosions by squirting out theexplosion products at an exhaust velocity of about 107m /s.

The fuel for these explosions is a mixture of isotopesof hydrogen and helium in a solid fuel 'pellet' about 10to 20 mm across, These are stored in fuel tanks at atemperature of 3°K (3°C above absolute zero) to keepthem from melting!

The pellet structure is rather similar to a particularbrand of sweet - a thin hard coating and a honeycombcentre. The coating is not made of chocolate, however,but of a superconducting material. This makes it pos-sible to shoot the pellets into the reaction area bymagnetic means at an acceleration of about 106 g. Thisphenomenal acceleration is necessary so that they cancross the gap between the pellet ejection system and theignition point between the time when the last explosionhas died down and the next one is required to start. Asthis happens 250 times a second, this crossing has to befairly fast.

Once at the ignition point, beams of high-energy.electrons are shot at the pellet. This vaporises the outershell instantaneously, which increases the pressure andtemperature of the centre to the levels required to ignite

Lll

0

0

Injector PowerStorage Capacitor

Magnetic

Field

PropellantStorage

Injection Gun

Forward Field Coil

Fusel Pellet Of 0314e

//(Exhaust

Reaction Chamber

Rear Field CoilRelativistic ElectronBeam Generator

Induction PickoffCoils

The Daedalus propulsion system. This works by controllednuclear fusion explosions. Solid pellets of a hydrogen isotopemixture are fired on by electron beams and a magnetic fieldcontains the blast. See the text for a full description.


EXHAUSTFLOW

IOC

A computer -generated cross-section of the area around thereaction chamber, showing the magnetic field profile. The peakfield generated in the coils would be about 14 Tesla.

The fuel pellet structure. The superconducting shell enablesthe pellet handling system to fire the pellet into the reactionarea, where the boiling of the deuterium coat generates enoughpressure to detonate the 'trigger' pellet. Each fuel pellet is 1 to2 cm in diameter.

DEUTERIUM HONEYCOMBWITH LIQUID 3H

FILLING

I IDEUTERIUM/TRITIU

TRIGGER

SUPERCONDUCTING

SHELL

DEUTERIUM PLUG

DEUTERIUMCOAT

fusion. The ignition is helped by a 'trigger' particle in thecentre.

The expanding plasma is trapped by immenselystrong magnetic fields set up by two coils surroundingthe engine. The coils generate a peak field intensity ofaround 14 Tesla and are cooled by liquid helium flowingthrough the hollow conductors to keep the temperaturedown to 4 K. The field is deformed by the explosion and(hopefully!) contains it and keeps it within the reactionchamber - exactly like the 'magnetic bottle' which can

REACT IONCHAMBER

be used to contain nuclear fusion reactions in terrestrialfusion power generating stations.

One of the reasons for choosing the particular fuelused is that very little of the reaction energy is releasedas neutrons. This means that the engine is relatively'cold' (in radioactive terms only!), as the electrons andprotons from the reaction can be trapped magnetically,whereas neutrons cannot. This lack of neutrons meansthat very little shielding is needed to protect the rest ofthe ship - a weight saving.

Unfortunately, the fuel is rare on Earth and this bringsits own problems. For the entire mission, about 3 x 1010fuel pellets would be required - with a total mass ofaround 50 000 Tonnes. This would consist mainly of30 000 Tonnes of helium -3 and 20 000 Tonnes ofdeuterium. As these are both very rare they have toeither be produced artificially on Earth or 'imported'from elsewhere.

MINING JUPITEROne possible source of suitable fuel - the largest suchsource in the Solar System - is the atmosphere of theplanet Jupiter.

While just getting to Jupiter would be a major featand the prospect of setting up factories around Jupiter toproduce 50 000 tonnes of propellant seems dauntingthe reports points out that once 'mining' had started, thefuel produced could be used to fuel power stationreactors on earth. The mining may well have beenstarted due to economic pressures by the time aDaedalus -type probe is built.

As Jupiter has no solid surface, the factories could notbe built on Jupiter but they could be built in Jupiter.

One possible design for such a factory would be agiant 'hot air balloon' filled with jovian atmosphereheated by the factory's waste energy.

The skin of the balloon would be woven of carbonfibres or a similar material. It would be 200 odd metresin diameter.

The factory would float in Jupiter at a level which had


Project Daedalus

a pressure equal to the pressure of earth's atmosphere.Unfortunately, this would put it in Jupiter's weather,which has been observed to generate 90 m/s winds!Not ideal conditions for a balloon. The verticalatmosphere currents are an unknown quantity but maybe even worse. The report suggests sending anatmospheric probe to study the conditions - this couldprobably be done using present technology.

Another little problem is how to inflate the balloon

Cane,

50 cn

rac tor

araaraianvolt

,,,,, Ilan,Vara ge

dock en(

when it is initially dropped into Jupiter. One solution tothis would be to fuel it with about twelve tonnes of liquidoxygen - which would burn nicely in the hydrogen -richjovian atmosphere.

The factory itself would hang free of the balloon, withit's waste heat directed through a funnel into the neck ofthe balloon. It would weigh over 100 tonnes. About 128complete factories would be used in total. These wouldprobably be unmanned. With a 1 2 -second communica-tions gap between the factory and an orbiting platformthere would probably have to be a fair degree ofautonomy in the factory computer's operation.

The alternative-manning the factories-may, if itproves necessary, be the future equivalent of North SeaOil drilling, with massive wages and long, dangerousshifts. It wouldn't be possible to fish someone out ofJupiter, though, if the factory collapsed!

The conditions encountered in space would provideproblems for the Daedalus probe also, as it sped throughspace at 12% of light speed.

Preliminary design for a vehicle which would float in theatmosphere of Jupiter, allowing it to be 'mined' to provide thehydrogen isotopes required to fuel the Daedalus craft. Thesame fuel would be ideal for the production of 'clean' fusionenergy for use on Earth.


PROBE DEBRIS PROTECTIONThe chunks of rock which plagued Dan Dare and his ilkby ploughing through the walls of spacecraft duringgaps in the plot are not as numerous in interstellar spaceas was once imagined.

In the main, the matter which will be encounteredbetween stars consists of ionised and neutral hydrogenclouds and very fine dust grains.

The average mass of the grains is thought to be about0.1 pg (or 10-16kg). While this is not exactly enormous,the craft will hit quite a large number of them, all at avelocity of 20 000 miles/second!

There are two problems to be countered - theheating effect of the ionised gasses (as large numbers ofhigh energy protons and electrons hit the vehicle) andthe erosion caused by the impact of the dust. Thedesigners predict that the erosion shield of the probe willreach a temperature of 193°K - well within reasonablelimits.

The material used for the shield will probably beboron and the report concludes that a thickness of 9mmof boron will survive dust erosion long enough to protectthe vehicle from the X-rays produced by the impact ofprotons and electrons (and of course the protons andelectrons themselves) during the course of the entirecoast period.

When the probe reaches the Barnard's Star System,however, it will encounter the same problem again, buton an entirely different scale.

The target system, if it's anything like our own, will befull of all manner of junk - ranging from material thesame size as the interstellar dust all the way up toasteroids weighing up to 1012 tonnes! Of course, thelikelihood of meeting something large is small, so tospeak. The designers of the probe took as a targetprotection against a 0.5 tonne object.

To protect the vehicle against half -tonne rocks com-ing towards it at 20 000 miles per second is not asdifficult as it sounds, luckily! The system used is to fly asmall chemically -propelled vehicle about 200 km aheadof the main probe and use it to deploy a smoke cloudabout 100m thick and with a total mass of 6 kg. Whilethis seems rather insubstantial, the calculations showthat anything under 500 kg will be totally vaporised onmeeting this cloud and that the expanding vapor will betoo thin to harm the vehicle when it passes through it,200km behind the smoke cloud and 0.005 secondslater. It seems that you can stop anything if only it'smoving fast enough!

A similar method would be used to protect the smallerprobes which are shot into other parts of the targetsystem. The probe couldn't really be said to fly past theBarnard system - it punches several holes in it and fliesthrough it!

SUBPROBESMost of the information to be gained about the targetsystem will be via the 220 tonnes of smaller probes themother ship will carry. These will be 'launched' somedistance from Barnard's Star and will follow carefully -planned trajectories through the system, transmittinginformation back to the main vehicle.

The probes (1 8 in all) would be designed for specifictasks -3 for terrestrial plants, 5 for stellar physics etc.

1,1

itt /11s1..n

One of the subprobes. These would fly through the targetsystem, sending information to the main craft which wouldrelay it back to Earth.

They would each contain a debris protection systemsimilar to the main ship's. In fact, there would benineteen holes punched in the target systems detritus!There is an important principle in physics which statesthat you can't study anything without changing it -that's certainly true here.

The Daedalus craft would also carry five 'interstellarmedium probes' for finding the shape of variations indust concentrations, for example. These would bespread around the mother craft (three at a time, with twoin reserve) at a distance of 1000 to 10 000 km. Whenthe craft flies through the edge of a cloud of dust, theinformation from all four sources (including the mainvehicle) would give information about the shape of theedge and how the dust varied throughout the cloud.

COMMUNICATIONSNaturally, it will be useful for the probe to be in contactwith the earth at all times - it should be capable ofsending information back and receiving major 'policychange' messages.

During the boost phase, a large plume of plasma(dissociated sub -atomic particles) will trail the vehicle,making microwave communications impossible. For thisreason the probe will carry a communications laser foruse during the early period of the mission. This systemwill have a bandwidth (frequency response) of 20 kHzand a range of one light year (20 000 miles/second overone year). This requires a laser with a peak power of 1.3MW, operating in the infra -red (which the plasma wouldbe transparent to).

When the boost phase finishes, the craft will deploy amicrowave transmitter/receiver which will be mountedin what was previously the reaction chamber, using thechamber to focus the microwaves. This would operate at2.24 - 3.02 GHz and would have a data rate of 864 kbaud (864 000 bits/second). The range would be aboutseven light years - sufficient for the 'Post -encounter'transmissions of data about the interstellar medium onthe far side of the targer system. One thing which had tobe taken into account in designing the system was thattransmissions would be received at a lower frequencydue to the Doppler effect!

Less powerful transmitters and receivers would alsobe required for communication with the disposableprobes and the wardens.


Project DaedalusTHE COMPUTERSPerhaps the one aspect of the Daedalus project whichwill require the greatest extension of present-daycapabilities is the self -repair function.

The concept of the multi -purpose 'warden' robots isall very well but these are, after all, only as intelligent asthe software (computer programs) which control them.

All the way through the report the wardens crop up asa sort of deus ex machina for repair and even improve-ment of the craft.

As was mentioned before, the speed of radio waveslimits the amount of control from the earth to an absoluteminimum. The report shows, by extrapolating data frommilitary and commercial aircraft, that a long flightwithout on -board repair is not feasible.

This means, in effect, that before the project isundertaken there must be a major advance in the state ofwhat is known as 'artificial intellegence'.

The ability of modern computers to deal with pre-dictable repairs - items which will inevitably wear out ina certain way - and such other tasks as will be known inadvance is probably adequate when projected to themission date. However, the complete inability of soft-ware as it stands at present to deal with a) un-predictableevents and failures and b) failures in the software itself isdiscouraging.

Then again, looking back at the advances in com-puting in the last few years . . . who knows?

The memory capacity of the system, calculated from

LTER ORBITGE SEPERATION (2 YRS)

rGE BURN -OUT (3.8 YRS)

the amount of information to be stored during flypast fortransmission back to earth, is about 3 x 1 010 bits. Usingthe latest in high density information storage - themagnetic bubble memory - this would require about200 000 integrated circuits, weighing 11/2 tonnes!

SUMMARYThe Daedalus Project report makes fascinating reading.What is most impressive is the level of detail to whicheverything has been thought out. For instance, thematerial used for the insulators in the electron beamgenerators (part of the engine) was chosen to be Berylia,which has the required mechanical and electrical pro-perties. The mere fact that the designers have gone intosuch detail lends the report much credibility.

The whole thing is written in a clear (if highlytechnical) manner with references to all sources of data- all in all a very impressive work - but what use is it?

The report will have several effects. One will be toswell the ranks of the British Interplanetary Society -not a bad thing. Another may be the serious considera-tion at some time in the future of a Daedalus -type project.It's worth noting that the same society produced afeasibility study on a lunar mission some thirty yearsbefore Appollo 11! HE

Further information on the BIS is available from:The Executive Secretary, The British InterplanetarySociety, 12 Bessborough Gardens, London, SW1V2JJ.


IntoElectronicsby Ian Sinclair Part 3In this part we enter the field of active components: Semiconductors. Firstwe deal with the materials from which diodes and transistors are madefrom, then we move on rapidly to applications.

ONCE UPON A TIME, folks, life looked simple. We hadconductors, which conducted electric current and insu-lators which didn't. Then we discovered why somematerials conduct and others don't, and it's not sosimple as we once thought. As usual it all boils down tothese electrons. The best conductors, metals, have theiratoms arranged so that some electrons are loose, aboutone electron for each atom. These loose electrons cantravel through the material, so that the metal is aconductor. We once thought that this was the only waythat electricity was conducted through a solid material,but it's not.

Around the turn of the century, a physicist called Hall,who had set up an experiment to test the sign of chargeof the current carriers in metals, found that positivecharged particles seemed to carry current also. We nowknow that these positive particles exist only in materialsthat form crystals and that they are really gaps in the

TABLE 1RESISTIVITY AT 20°C

The resistivity figure for a material is a way ofcomparing the resistance, R, of samples ofstandard size. For a wire which is s metres long andA m2 cross-section, the resistivity is iv , unitsohm -metres. The resistivity figures for somematerials of interest are shown below.

Material Resistivity in ohm -metresAluminium 2.7 x 10-8Copper 1.7 x 10"8Iron 10.5 x 10-8Constantan 45 x 10.8

(Constantan is an alloy of copper, nickel andmanaganese used to make wire -wound resistors).

GermaniumSiliconDiamondPerspexQuartz(silicon oxide)

about 10-'about 10410131019

1020

regular arrangement of electrons. These gaps can moveand behave like positively charged particles. We callthem holes. What convinces us that they are not trulyparticles is that they cannot be removed from thematerial as electrons can. Inside a metal, though, thehole is as 'real' as the electron.

SEMICONDUCTORSBetween the good conductors, like metals, and theinsulators like so many non-metals, there are somecurious materials called semiconductors. The differencebetween semiconductors and other materials is not somuch their resistivity (Table 1) but the way in which thevalue of resistivity can be altered. Take a piece of metalwire, measure its resistance at room temperature, thenheat the wire. What happens? The resistance increasesby only a few percent. Try the same measurement withan insulator, and the resistance is too high to measure,both times. Now use a chunk of a pure semiconductormaterial. Whatever value of resistance it had at roomtemperature, it's a darn sight less when you heat it, notjust a few percent but a really big change. A typical resultmight be a change from 4k to 200R for a temperaturerise of 50°C.

That's a very obvious difference, and the otherdifference is even more important. Compare two bits ofwire, one pure copper, the other copper with about 1%zinc. The resistivity values are pretty much the same,

RESISTANCE

TEMPERATURE

RESISTANCE

TEMPERATURE(a) (b)

Fig. 1. Resistance/ temperature graphs (a) for a metal, (b) for apure semicondutor.


because the impurity (zinc) has as much effect as youwould expect from the quantity - 1 %. Try mixing twoinsulators and you still can't measure any resistancecharge.

Semiconductors behave quite differently. Almostimmeasurably small amounts of impurity will cause theresistance of a sample of a semiconductor to dropenormously. The amounts of impurity needed are not1%, or 0.1% but something like one millionth of onepercent to make a huge change in the resistance.Difference No. 2, and more to come.

Pure semiconductors, like germanium and silicon, aremore like insulators than conductors. Adding otherelements to the semiconductors will make them intoconductors and we can even choose how they conduct.Elements such as phosphorus make semiconductorsconduct mainly by electron movement. Elements suchas indium make semiconductors conduct mainly by holemovement. The addition of impurity is called dopingand is rather a delicate operation, because we don'texactly operate with bucket loads! Nevertheless, we cannow make semiconductors which have whatever resis-tivity values we want (within reason). What's moreimportant we can make them N -type (most of thecarriers electrons) or P -type (most of the carriers holes)by doping with the appropriate materials. Incidentallybecause these doped materials have a good supply ofelectrons or holes, heating them makes little moredifference to their resistance value than it does for ametal.

UP THE JUNCTION:We wouldn't think much of the show so far but for onediscovery - one of the discoveries that's changing lifearound us right now. Take one tiny crystal of silicon orgermanium and slice a wafer from it. Now dope it on oneside with indium (making P type) and on the other sidewith phosphorous (making N type). Heat it up in avacuum, so that the atoms of the doping materials canspread into the semiconductor and at some stage in theprocedure, P and the N bits will meet each other.Logically enough, this meeting place is called a junction.We don't make junctions this way now, but the junctionof P and N is the big step forward, because of itsbehaviour in a circuit.

Try to imagine a junction. Fig 3 is a diagram thathelps, showing + and - signs to show which sign ofcarriers can move. If this junction is made part of a circuitwith one wire connected to the P type side of the crystal

P -DOPINGMATERIAL

(a)

SEMICONDUCTOR

N -DOPINGMATERIAL

GAS OF P -DOPINGMATERIAL

N -TYPE SILICON

BELLJAR

Fig. 2. Creating a junction (a) Diffusing solid material into bothsides of a thin crystal, the old-fashioned method. (b) Modernmethod of diffusing the impurity atoms into oppositely dopedmaterial from a hot gas atmosphere.


P N

JUNCTION

DEPLETIONLAYER

1- -

Fig. 3. Junction action. (a) Diagram showing the signs of themobile charges (b) Action under reverse bias, carriers aredrawn away from the junction. (c) With forward bias carrierscross the junction, so that the junction conducts.

and another wire connected to the N -type side, theaction of the junction will depend on which way roundthe circuit is connected. With the N -type side connectedto battery + and the P -type side to battery -, no currentflows. Why not?

Good old-fashioned 1 8th century electrostatics,that's why. The battery + attracts the loose electrons ofthe N -type material away from the junction, and thebattery attracts the loose holes of the P -type materialaway from the junction. Nothing moves right round thecircuit, so no current flows. The junction is depleted -drained of electrons and holes, just a chunk of non-conducting material. This ,connection is called thereverse bias connection.

DIODESNow, think of the battery connected the other wayround. This time it's a very different story. The battery +pulls loose electrons across the junction from the N -typeside. The battery - pulls loose holes across the junctionfrom the P -type side. Everything is moving, it's allhappening, current is flowing; the bias now is forward.The whole arrangement of P and N materials meeting ata junction is called a semiconductor diode.

The P -type part of the diode is called the anode, andthe N -type is called the cathode. The diode is forwardbiased when the anode is more positive than thecathode. A germanium diode will just start to conductwhen the voltage between anode and cathode is about0.15 V, a silicon diode will just start to conduct when thevoltage between anode and cathode is about 0.55 V.

When the bias is in the reverse direction, there are sofew electrons and holes (called the minority carriers) leftin the junction that the diode is almost an insulator. Areverse current of only a few nanoamps (1 nA = 10--9A) istypical.

We use diodes, as described above, in great quant-ities; in power supplies, in radio reception, in com-puting. Details later, folks, but most of the uses fordiodes stem from one thing, the current flows one wayonly. The symbol for a diode shows this direction of

CURRENT.TO-)

ANODE

CATHODE Fig. 4. Diode symbol and terminal names.

current by an arrowhead, using the usual convention ofcurrent flowing from battery positive to negative.

ZENER DIODES.0ne exception is the zener diode, though. This type

47

FORWARDCURRENT 60-mA 50-

FORWARDCURRENTMA SO-

40 - 40-30 - 30-

REVERSEVOLTS

30

20 -

20 10 10 - FORWARDVOLTS

REVERSEVOLTS 1

20 -2 -10 FORWARD

VOLTS-10 I -10-20 -20- 30 -30-40 -40-50 -50-60 -60

REVERSE REVERSECURRENTMA

CURRENTmA,

(a) (b) (L)

Fig. 5. Diode characteristics. (a) Signal or rectifier diode. Notethe difference in the sizes of the voltage and current scales inthe reverse direction. (b) Zener diode. The reverse currentstarts at a definite voltage, and large reverse currents can pass.(c) Zener diode symbol.

of diode (different symbol, too) is always used reverse -biased, but conducting. Reason is that a junction that'sheavily doped on each side will break down and start toconduct if enough voltage is put across it on the reversedirection. How much voltage? Well, that's what is souseful. Suitable design and doping will give a range ofbreakdown voltage from around 3 V to around 200 V.What's more, the voltage is pretty well fixed in any givendiode. Whether you pass 1 mA or 100mA through thezener diode (reverse direction) the voltage across thediode is the amount written on it - the zener voltage.We use them for voltage stabilisation - obtaining avoltage which remains steady despite large changes incurrent or supply voltage.

DOUBLE YOUR JUNCTIONSIn 1948, Brittain, Bardeen and Shockley made one o11the most important discoveries of all time. Theymanaged to make two PN junctions really closetogether: the first transistor. What's so special abouttwo junctions close together? It's like Siamese Twins,that's what, and anything that happens in one affects theother.

Imagine a very thin layer of P -type material as themeat in an N -type sandwich (Fig. 6). By a thin layer wemean just a few hundred atoms thick. Now imagine oneN -type layer connected to battery + and the otherconnected to battery -. From what we know so farthere's no way this arrangement could -conduct current,its like two diodes connected anode to anode. No matterwhich way you look at it, one diode is reverse biased andtherefore can't conduct. This is where the closeness ofthe junctions has an effect. Suppose we start one

COLLECTOR

BASE

EMITTER

Fig. 6. The transistor. (a) Arrangement of junctions. (b)Two -diode circuit which would give the same readings on an

ohmmeter. When two terminals of a transistor are found whichdo not conduct in either direction, the other terminal must bethe base. (c) Symbol of NPN transistor.

48

-BIAS -4

(a)

-BIAS

- N - -

P

-BIAS

+ BIAS

(b)

- BLAS

-P.-- - .

. .N- -

- BIAS

Fig. 7. Transistor effect. (a) With the base -emitter P -N junctionunbiased, both junctions are depleted, no current flows. (b)When the base -emitter junction conducts, both junctions areflooded with carriers (electrons in this case) so that bothconduct.

junction conducting; the one between the P -typematerial and the N -type that is connected to batterynegative. It won't conduct until the voltage across it issomewhere around 0.55 V (assuming a silicon device),but when it does, the junction is flooded with carriers.Because the junctions are so close the other junction alsobecomes flooded with carriers, though it is reversebiased. This turns the second junction into a conductingjunction, despite the reverse bias, so that current flowsbetween the two N -type regions. This current can flowonly for so long as carriers are injected from the junctionthat has been deliberately forward biased.

It's time we put some names to the bits of this device,the bipolar (or junction) transistor. The meat in thesandwich is the part called the base, the other tworegions are the collector and the emitter. The reversebiased junction is the junction between collector andbase and the forward biased junction is the junctionbetween base and emitter.

Look at it all again. With no bias (or reverse bias)between the base and the emitter, no current flowsbetween the collector and the emitter; reason for that is

RI

VR1

(a)

COLLECTORCURRENTlc

a,/

BASE CURRENTS lb

Fig. 8. Current gain. (a) Circuit for measuring base and collectorcurrents. (b) Typical graph of collector current plotted againstbase current.

the reverse bias between the collector and the base.When the base -to -emitter junction is forward biased,though, the carriers enter the collector junction, so that itconducts. As a result, we can change the collector -emitter circuit from insulating to conducting by alteringthe base voltage slightly.

We can do even better than this. When we make thebase -emitter junction conduct, most of the base carriersare swept across the other junction (base -collector). As aresult, much more current flows between the collectorand emitter than between base and emitter; a smallcurrent between base and emitter controls a largecurrent between collector and emitter. Controls? Yes,controls, because the ratio:


into Electronicscollector-emitter current is fairly constant.base-emitter current

A typical value is 100, meaning that the currentbetween collector and emitter, I. is 100 times the currentbetween base and emitter (lb). This ratio is called theforward current transfer ratio, common emitter,mercifully shortened to hie. The phrase 'common emit-ter' is used because both the base and the collectorconduct to the same terminal, the emitter.

NPN AND PNPBipolar transistors can be made in two varieties, NPNand PNP. The NPN operates with its collector positive toits emitter and will conduct when the base (the P -layer) isabout 0.55 V positive to the emitter. The PNP transistor

N

P

N

( a ) )b)

Fig. 9. Junction arrangements and symbols. (a) NPN, (b) PNP.

operates with its collector negative to its emitter and willconduct when the base (the N -layer) is about 0.55 Vnegative to the emitter. These voltages assume thatsilicon devices are being used.

GAINING ON THE DEALLook at the circuit of Fig. 10a. It has a 6 V, 60 mA bulb inseries with a switch and a 6k8 resistor across a 6 Vbattery. What happens when it's switched on? Nothingnoticeable, because 6 V across 6k8 produces less than 1mA of current, not enough for a 60 mA bulb to glow asbright as a wet fag in the fog. Now try the circuit in Fig.10b. This time the switch will operate the bulb. Thecurrent through the 6k8 resistor is more than enough tomake the transistor conduct, passing 60 mA through thebulb. We call this effect current gain, and the quantityhie measures this current again.

6kB

6V60mA

SWITCH SWITCH

16V

(a)

61,8

TRANSISTOR

(to)

Fig. 10. Current amplification. (a) This circuit cannot passenough current to light the bulb. (b) The bulb will light when theswitch is closed because the base current of the transistor willcause collector current to flow.

Don't get carried away though. The transistor hasn'tcreated this current, just controlled it. If the battery hadnot been able to supply 60 mA, no cunning tricks withtransistors could make the bulb light.

Try the circuit of Fig. 11 for yourself. The bulb will


100k

TOUCHCONTACTS

01,02 are 2N697or 2N2219 or BFY50 02

6V60mA

+6V

Fig. 11. A touch switch - touching the contacts with yourfingers will allow the bulb to light.

light when the two terminals are touched with onefinger. The tiny amount of current that flows throughyou causes a current many times greater to flow in Ql.(We use the abbreviation Q for a transistor; an alternativeis Tr). This greater current flows into the base of Q2, andcauses a greater current still, enough to operate thebulb. Suppose we put some sizes to these currents. If thebulb needs 60 mA, then let's suppose, for the sake ofsimplicity, that Q2 has a current gain (hie) of 60. Thenjust 1 mA is needed into the base of Q2 to make the bulbglow. This 1 mA is supplied by Ql. If this transistor alsohas hie= 60, then only one sixtieth of a milliamp (1 6 uA)is needed into the base of Ql. You may have seen thistype of touch -detector demonstrated in 'Tomorrow'sWorld' as a way of identifying the end of one selectedwire in a bunch of several hundred.

This sort of use of transistors is called switching. Thebulb, or whatever the transistor is switching on and off,is called the load. The load is connected in thecollector -emitter circuit, and switched on or off bycharges at the base. To switch the lead on:

(a) the voltage between base and emitter must bemore than 0.55 V for a Silicon Transistor (0.1 5V for a germanium transistor)

(b) the current between base and emitter must beenough to ensure that the correct current willflow through the load.

Just to illustrate these conditions, try to explain why thecircuits in Fig. 12 will NOT work!

1V

(a) (b)

Fig. 12. Two circuits which DON'T work. Can you explaun why?

Variations on the themeFigure 13 shows another circuit which needs somethought. If the load resistor is in the emitter part of thecircuit, then the amount of current needed at the base isthe same as it would be if the load were connected in thecollector part of the circuit. The voltage needed at thebase, however, has to be more than the normal voltageacross the load. The voltage needed at the base, how-ever, has to be more than the normal voltage across the

49

Fig. 13. The emitter -follower.

load. Think of it this way - for the load to operate itneeds the correct voltage across it and the correctcurrent through it. Suppose the load is a 6 V 60 mAbulb. In this circuit, the emitter terminal of the transistormust be at 6 V when the bulb glows. For the transistor toconduct, the base voltage must be 0.6 V or so about theemitter voltage which means around 6.6 V. The shift inthe position of the load in the circuit makes a largedifference to the voltage that is needed at the base. Thisis, in fact, a rather different type of circuit, the commoncollector circuit.

The differences between these circuits illustraterather well differences in the behaviour of transistors.Connect as a common emitter circuit (Fig. 10b) andsmall changes of voltage at the base cause fairly largechanges of base current. The input circuit of the transis-tor (base -emitter) behaves like a low resistance. Connectthe transistor in a common collector circuit (Fig. 1 3) andmuch larger changes of voltage at the base are needed tocause the same changes of base current. Now the inputcircuit behaves like a large resistance. We refer to this asthe input resistance of the transistor; it's importantbecause we have to be able to supply the correct voltageto this input resistance to turn on the current.

Transistors have an output resistance too. A transistorconnected in a common emitter circuit has a high outputresistance. This means that changing the collectorvoltage has very little effect on the collector current. It'sa useful feature, because a high value of output resist-ance means that the current flowing between collectorand emitter is controlled only by the base current, not bythe collector voltage. Connecting the transistor up into acommon collector circuit has the opposite effect; theoutput resistance is now low. Any change in the emittervoltage now causes a large change in the emitter current(assuming the base voltage is fixed). These input and

output resistance values are shown in Table 2, for thecommon emitter, common collector and also for thecommon base connection. The common base connec-tion is seldom used now except in high frequencycircuits or as a part of other circuits.

Laying it on the lineTake a look at the graph of Fig. 1 4 . It's a graph of silicontransistor collector current plotted against the basevoltage (base -to -emitter voltage,that is). What it shows isthat very small changes of base voltage will cause largerchanges of collector current. We make use of this whenwe design voltage amplifiers.

A voltage amplifier has an AC voltage signal at itsinput and produces a copy of the wave, with greater

,amplitude at its output. The ratio voltage gain equals

signal amplitude at outputsignal amplitude at input

How do we use a transistor as a voltage amplifier? Tostart with, we have to convert the collector currents intovoltages. The simplest method is to use a resistor as aload. Take the example of Fig. 15. The load resistor here

50

1000

Ic,mA

IC

500

Vbe

1 2 Vbe VOLTS

Fig. 14. Mutual characteristic for a medium -current silicontransistor.

is a 1 k resistor, and the voltage signal is at the collectorof the transistor. When the collector current is zero, thevoltage at the collector is 6 V, the supply voltage. Whena collector current flows, there is a voltage across theresistor so that the collector voltage drops. The greatestamount of current that could possibly flow in this circuitis 6 mA, because this amount makes the collectorvoltage zero. We could then get a voltage wave at theoutput with an amplitude peak -to -peak of 6 V, providingwe had enough signal voltage at the input to cause acollector current of 6 mA peak -to -peak. That's where thegraph of Fig. 15 comes in, because we can read off thebase voltages for the two current limits of zero and 6 mA.Now we have the output voltage (6 V p -p) and the inputvoltage (0.1 V p -p); so that the voltage gain of thisamplifier is 60 times. Easy!

Is the output voltage signal a good copy of the inputvoltage signal? It can be, but two things can go wrong.One is that the graph of I, (collector current) against Vbemay not be a straight line. Any curvature of this linecauses a poor copy; we say that there is distortion. Theother thing is that the base voltage must never drop

TABLE 2Relative sizes of input and output resistances

IN

IN {

IN

OUT

OUT

OUT

Name Rin

commonemitter medium high

commonbase low high

commoncollector high low

low: below 100R; medium:several k; high: above 10k.


Into ElectronicsLOADRESISTOR

SIGNAL IN

+ 6V

SIGNALOUT

6lc, mA

5

4

3

2

1

0.1 02 0..3 04 0.5 0.6 0.7Vbe,VOLTS

ZERO AT ABOUT 0.51V I DIFFERENCE6 m A AT ABOUT 0.61V =0.10V

Fig. 15. Voltage amplification using a load resistor.

below the minimum conducting voltage of about 0.55V. Both problems are tackled by using bias. In everydaylanguage, bias is anything that tips things to one side.What we need to tip to one side is the base voltage, tomake sure that it can't drop below that figure of about0.55 V (again assuming a silicon transistor).

A bias circuit does just that. Two common types ofbias circuits are drawn in Fig. 1 6. Fig. 1 6a shows acircuit using a resistor connected between the base andthe collector. We can calculate the value of the resistorso that the collector voltage, with no signal present willbe about half supply voltage. This way if we put an AC

LOADRESISTOR

BIASRESISTOR

+6V

+6V

OUT 3V

0

( a ) ( b)

Fig. 16. Two types of bias circuit. (a) Current bias, using asingle resistor. (b) Voltage bias, using a potential divider.

signal in at the base, an amplified signal will appear atthe collector and its amplitude can be anything up toabout half supply voltage without cutting off either peak.

Figure 16b is of another type of bias circuit. In thisdesign, a potential divider sets the voltage at the base.This makes the transistor pass current through theemitter resistor R4 so that the voltage between base andemitter is set to the correct value, around 0.55 V. If wechoose the value of the load resistor R3 correctly, thecollector voltage can be set to about half supply voltageonce again. The capacitor C2 is important. Unless thiscapacitor is connected, we do not have a true commonemitter circuit for signals (because of R4) and the voltagegain will be very low - more of that later.

Both of these methods of bias are self correcting - ifwe change the base voltage temporarily with a signal,the voltages will afterwards always return to the valuethat we set. In addition, slight changes in the values ofhie or resistance will not cause noticeable changes in thebias voltages when these circuits are used.

Bias is not necessarily just a method of preventing thepeaks of the signal from being chopped (or clipped). Biascan also be used to ensure that the distortion is as low aswe can arrange. The graph of against V be is not usuallya perfectly straight line, but some parts of the curve maybe more nearly straight than others. If we bias thetransistor so that a fairly straight portion of line is used,then the distortion is low. The names 'linear amplifier'


and 'linear operation' come from the idea that the bestoperating conditions are along a piece of the graphwhich is a straight line.

We can calculate the gain of a transistor amplifiervery easily. The ratio

change of collector currentchange of base voltage

called mutual conductance, symbol gm, is about 40mA /V for each 1 mA of collector bias current. For atransistor biased to a collector current of 2 mA, forexample, the gm value would be 80. This value of gm isalso related to the current gain hie and the input resist-ance of the transistor hie by the formula hie-gmh,e. Nowthe correct signal out for an input V,e must be gmV,,,, sothat the voltage signal is gmV,RL (RL is the value of theload resistance). An even simpler method arises becausethe DC bias voltage across RL depends on the biascollector current. The voltage gain of any small signalamplifier using silicon transistors is just 40 x (steadyvoltage across RL). For example, if there is 5 V across theload resistor with no signal input, the voltage gain of thetransistor is 5x 40 = 200 times.

Up and down the scalesWe can measure the voltage gain of a voltage amplifierat any frequency of signal we care to use, but we won'tget the same results at each frequency. A simple voltageamplifier such as that of Fig. 17 will have its normalvalue of voltage gain (around 200) at frequenciesranging from about 1 00 Hz to several hundred kHz. Atvery low frequencies, of a few Hz, the measured gain willbe less. The transistor is still doing its stuff, but the

Fig. 17. A complete voltage amplifier stage. The dotted linesindicate important stray capacitances between parts of thecircuit.

capacitors that are used in the circuit have such highreactance values at very low frequencies that the signalis potential -divided by each capacitor that is connectedto a resistor or the transistor. If we use circuits thateliminate capacitors (direct coupled circuits), the voltagegain remains constant even for DC.

We cannot simply remove the capacitors in a circuitsuch as that in Fig. 17 because these capacitors areneeded to prevent the bias voltages being shorted out orincreased by the circuits to which the transistor isconnected.

At the high frequencies, it's capacitors again thatcause the trouble. This time they're invisible, the typewe call stray capacitance. Every gap in a circuit is acapacitance, though of small value. In the circuit of Fig.1 7 the dotted lines indicate gaps which must have somestray capacitance. The effect of these capacitances is toprovide an easier route for high frequency signals thanthe resistors or even the transistors, so that the voltagegain is, once again, reduced. For audio amplifiers,

51

Into Electronics100

VOLTAGEGAIN

50

25

10 00 1k 10k 100kFREOUENCY IN Hz

(a)

40dB

GAIN

20

10 100 1k 10k 100kFREQUENCY IN Hz

( b)

Fig. 18. Frequency response graphs. (a) Using a linear scale forgain. (b) Using a decibel scale for gain.

working with the frequency range 40 Hz -20 kHz, thesefrequency limits are not usually troublesome. Amplifiersintended for much higher frequencies use tuned(resonant) circuits in place of the load resistor. The straycapacitance then becomes part of the resonant circuit, sothat its value does not greatly affect the frequency wecan use. At really high frequencies, the transistor itselfbegins to give up, because the charge does not movefast enough to follow the changes of voltages of highfrequency signals. Transistors of rather specialisedconstruction are needed for frequencies of 100MHz ormore.

A graph of voltate gain plotted against frequency iscalled a frequency response graph. A typicalfrequency response graph for an audio amplifier isshown in Fig. 18a. We have to use a logarithmic scalefor frequency, with the same distance between 1 kHzand 10 kHz as we have between 100 Hz and 1 kHz. Thisis because a linear scale would have to be several metreslong to show the same frequency points.

The frequency response of an amplifier plotted in thisway always looks much worse than we expect from theevidence of our senses. The reason is that the differencebetween a gain of 100 and a gain of 50 is not so verygreat when the amplifier drives a loudspeaker or thebrightness of a cathode ray tube, it certainly doesn'tseem like a 2 1 ratio. A better way of showing

GATE ( P)TERMINAL

SOURCE \TERMINAL

DRAINTERMINAL

CHANNEL (N)

GATE

(a)

SUBSTRATE

DRAIN

SOURCE

METAL

OXIDE

N- SILICON"-

SUBSTRATE

SOURCE

GATE

GATE

(5)

DRAIN

SUBSTRATE

DRAINSUBSTRATE

SOURCE

Fig. 19. FETS. (a) Junction type, N channel. (b) MOS type, Nchannel. The substrate is the silicon layer on which the otherlayers are deposited.

frequency response is the decibel (dB) scale. To converta voltage gain to decibels, we use dB = 20 log (voltagegain) so that a voltage gain of 20 is 26 dB, a gain of 40 is32 dB, and a gain of 80 is 38 dB. Note that doubling thegain figure causes the dB figure to increase by 6 dB,typical of a logarithmic scale.

A frequency response graph plotted in dBs is shownin Fig. 1 8b.

52

FETSThe two -junction, or bipolar, transistor is not the onlyway of using a semiconductor material for switching oramplification. Field-effect transistors (FETS) makeuse of another way of controlling the movement ofelectrons or holes in semiconduction without injectingcarriers from junctions. The main current path in any FETis called the channel, it's a thin narrow strip of silicon,usually N -type. There are no junctions along the lengthof this channel, so that it behaves like a resistor, allowingcurrent to flow in either direction. The current in thechannel is controlled by changing the conductivity of thematerial, not by injecting carriers into a reverse biasedjunction.

Two ways of controlling the current are used. Oneway uses'a reverse biased junction, so that the FET is ajunction FET. The junction is formed around the strip atone end (Fig. 1 9a), but the carriers moving through thechannel do not pass across the junction. Reverse biasingthe junction will deplete it of carriers, so that theconductivity of the silicon around the junction becomesless, and the current in the channel becomes less.

The other type of FET is the MOSFET (Metal -Oxide -Semiconductor FET). A thin layer of silicon oxide (aninsulator) is grown on the strip of channel at one end anda layer of metal deposited on top of the insulator. Thisarrangement forms a capacitor with the silicon of thechannel as one plate and the insulated metal as theother. Connecting the metal to a negative voltage causesnegative charges to be repelled from the channel, so thatthe conductivity of the channel becomes less.In either type of FET, the controlling electrode is calledthe gate; the end of the channel nearest the gate iscalled the source, and the far end of the channel iscalled the drain. A voltage between gate and sourcecontrols the current between source and drain, and theratio

change of channel currentchange of gate voltage

is called mutual conductance, gm. The values of gmobtained from FETs are pretty low, 1.2-3.5 mA /Vusually, as compared to values of 40 mA /V upwards forbipolar transistors. The big advantage of FETs is theirvery high input resistance. The MOS types have such ahigh gate resistance that they can be damaged by theelectrostatic voltages produced by rubbing materialstogether! These types of FET (and the MOS types ofintegrated circuits) must be handled carefully, keepingthe leads shorted together until they can be soldered inplace in their circuit. HE

In Part 4In the next issue we look much deeperinto transistor circuitry and show themin real circuits. We deal with couplingstages together, biasing, negativefeedback and the rules -of -thumb thatare used in design.


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53

BreadboardShow ReportTHE BREADBOARD amateur electronics exhibition washeld from 21st to the 25th of November in London. Itattracted over 10 000 people. HE had a stand therewhich it shared with its sister magazine, ElectromicsToday International. While we were there Phil Cohenfought his way through the crowds with his box brownieto bring us these photos of the more interesting bits andpieces on display.

The Electronics Today Triton. This is a home computer costingless than E300, the design for which was published in our sistermagazine. The company which market the machine - Transam- brought several of them along so that prospective buyerscould get 'hands-on experience'.

Mike Hughes, the designer of ETI's Triton computer project,giving a talk on its attributes. These talks were so popular thatthe private hall rented for the occasion was seldom less thanfull.

54

Prom Blower Board

Science of Cambridge's PROM blower kit to go with the MK14microprocessor evaluation kit. This allows the user to build a'dedicated' microprocessor system - one which is program-med to begin its task as soon as it is switched on.

Tope Interface Module

The MK14 tape interface kit. This allows programs to be storedonto cassette for future recall.

Another Electronics Today project, the Tcholinka chessrecorder. This machine was used by the BBC in their coverageof the Korchnoi / Karpov chess match. The name comes fromthe Russian - Korchnoi named it (he also uses one). Themachine is capable of storing an entire game in its memory andcan record it onto a cassette tape for future reference. It alsoallows the game to be played from cassette with a commentary- thus allowing a professional player to review all the commonopenings, for instance, the day before a game.


The MK14 now has a VDU kit to go with it. This plugs into theaerial socket of a conventional TV set and allows the display oftext and symbols.

In order that people could gauge the performance of the HEMixer, Graphic Equaliser and Stereo Amp, we connected them in'series' and fed in a signal. The combination of all the facilitiesoffered by the three units made this a very versatile setup.

The Format synthesiser, the design for which was published inElector magazine. There were two of these on display - one byde Boer Components (this one) and one by Elector. Those withsharp eyes may be able to read the notice on the keyboard of thismachine saying that a demonstration could be had at theElector stand. We had to make do with a photo of this one, signand all, because we couldn't get near the Elector one for peoplelooking at it.


The Watford Electronics microprocessor eJaluation kit - soonto be marketed (about a month, they tell us). It uses the Z-80microprocessor, which is more powerful than the SC/MPprocessor used by the MK14, and is expandable into a fullbusiness system (eventually). The price should be around £70- only £20 more than the Mk.14.

Maplin Electronics had on display a small laser. This was set upas a sort of fairground novelty - the idea being that you had to-nove the cut-out along the beam to the end without cutting thebeam. A small electronic counter with a photo -electric inputmeasured the time the beam was interrupted for. The laserwas perfectly safe and wouldn't burn a hole in anything - well,that would spoil the purpose of the game!

The plant in the background is playing a tune (yes, playing atune) through the 'Bioactivity Translator' unit in the fore-ground. The unit gives out a series of musical notes whichdepend on the electrical conductivity of the plant's leaves. Thecompany which was exhibiting them - Jeremy Lord Synthes-isers - claim that amongst other things you can tell a plant'sstate of health from the tune.

55

Show Report

The Integrex system uses a small TV camera to feed informa-tion into a microprocessor. This displays (with sixteen levels ofgrey) the scene which is to be printed, so that it can bereviewed by the customer before it is printed.

This is a photo of Margaret, one of the more attractive of the HEstaff (excluding the editorial people of course). The charactersused are not letters as in some systems, but a special com-bination of characters made up from a random pattern of dots.This gives a smoother appearance to the final product.

56

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PinballWizardsLong before video games were even a twinkle in any designer's eye, theworld had pinball machines. A lot of people (the author included) stillthink that pinball is more fun to play than even the most advanced videogames. How do they work -- and what goes wrong? Jim Perry inves-tigates.

THREE NAMES THAT ANY CONNOISSEUR of the finerpoints of life will recognize are Bally, Williams andGottleib. They all make pinball machines, and havebrought many a strong man close to tears of both joy andanger - depending on whether the bonus or tilt lit up!The developments that have taken place in the last fewyears have meant that even pinballs are now succum-bing to the onslaught of the microprocessor.

Part of the reason for my 'nostalgia' is that I receivedmy grounding in electronics via repairing pinballmachines (working in my uncle's arcade duringholidays!), but the main reason is the gradual reductionin mechanical thumps, clangs and dings which were thehallmark of old-fashioned models!

If you look inside an ultra -modern machine you willsee very little, apart from a few solenoids and a black boxcontaining a dedicated computer - not a very enligh-tening experience. So to understand the principlesbehind the machines let's take a step back in time andsee how the older models developed into their moderncousins.

TIME MACHINE.. .

Originally pintables were little more than extensions ofBagatelle games - with movable flippers to keep theball in play; the scoring systems were mostly mental!Then some budding genius had the idea of usingelectrically operated flippers and bumpers; so that theflippers could be positioned anywhere on the playingarea, and the bumpers would give more 'action' byphysically repelling the ball harder than unassistedrubber could manage.

By positioning switches behind the bumper rubbers,solenoids were activated when the ball hit the rubber -and the solenoid operated a lever to push the rubbertowards the ball. From this system it was but a small stepto operating electric counters for score -keeping. Thenext major innovation was the mid -field 'mushroom'which was formed from two bumpers and a solenoid.When the ball hits the mushroom it forces the bottombumper down; this operates a switch to power thesolenoid. When energised the solenoid pulls the upperbumper down sharply - thus forcing the ball away from


the bumpers. Naturally the solenoid circuit is thenbroken as the lower bumper springs back into place, andthe upper bumper also returns to its normal position.

The electrical circuits were connected and discon-nected by relays, operated by the bumper switches and'roll-over" contacts scattered over the playing area.Advantages of using relays included less contact wearon the switches operated by bumpers (as relays con -

PLAYFIELDSUPPORT

RAILS

LEFTFLIPPER

STARTBUTTON

PLAY FIELDBOTTOM 44,40

NOTCH

RIGHTFLIPPER

CABINET

CABINETBACKBOX

NOTCH

An artist impression of the new Heathkit DIY pintable, thewhole machine is MPU controlled - with only four bumpersolenoids.

57

Pinball Wizards

Above is a rough sketch of the midfield 'mushroom' mechan-ism, found on all pinball machines. Bumper switch mechanismcan be seen below, the switch operates a solenoid to force theball away.

ACTUATORBLADE

STOPBRACKET

BENDHERE

-..-1/16 RUBBERBUMPER

i'DIRECTIONOF

BALL

A modern machine from a Spanish company Recel, this one i'called 'Mr Evil' and is electronic rather than electro-mechanical.

TILT! The device used to pre-vent you totally abusing anypin table is elegant but simplein operation. When the hangingbob touches the bob ring a relayis operated, and you lose thegame or ball in play.

Bob

Bob Ring

Thumb Screw,

sume relatively little current compared to solenoids), andpossibilities of more sophisticated switching with the aidof complex interlinking of the multiple contacts availableon each relay. In fact the relay systems in pinballmachines were the equivalent of dedicated computers(which explains why MPUs have been used to replacethem).

. . BREAKS DOWN . .A headache with Pinball machines is when they work -but not quite correctly. Mechanics have been known tospend days on a single machine, trying to trace anelusive intermittent fault. It can be the equivalent of aproverbial needle in a haystack, with upwards of 3000contacts in an average machine - each one critical tothe functioning of the complex logic needed for scoringand bonus awarding, etc. But the big benefit of relay andsolenoid control was the ease of fault finding. In mostcases it is easy to spot a burning coil, buzzing relay orother similar symptom of catastrophic failure.

If a machine appeared to be in order physically thelogic had to be checked through systematically, withchecks on all the possible interactions from other parts ofthe circuit - a very good way of learning about logiccontrol and methodical fault finding!

. . . NOW AND THENSo because of the close similarity between what theelectromechanical circuitry did, and MPUs are designedto do, the older types of Pinball machine are rapidlybeing replaced by modern equivalents. The other mainfactor is that of reliability and ease of repair - M PUsystems are inherently more reliable and a single modulecan be used for several similar but different models.

As a result it is possible to buy second (well, probablyfifth or sixth) hand Pinball machines for as little as £20 -well worth it if you fancy learning about complex (yetsimple?!) electromechanical devices, and even just tohave a good time. HE


Flash TriggerTrigger your photo -flash with light, sound, or switchsimple unit.

IT'S NOT TOO HARD for even the amateur photo-grapher to take quite spectacular photographs, freezing,motion at a critical stage. To operate the shutter oncamera manually, even if this could be done at thecritical moment, simply won't work in freezing most fastmotion as even the fastest shutter speeds will cause theimage to blur.

However, by darkening the subject, opening up theshutter and operating a flash gun we can obtain very,very short effective exposure times because of the highlight level for a brief period. In addition we can use anelectronic circuit to fire the flash gun which in turn canbe triggered by light or sound.

Our project here will operate with any of thesetriggers and in addition enables you to delay the flash fora small but variable time after the triggering impulse;this will be found very useful for some effects.

Although the circuit will enable spectacular shotsto be taken, it is quite simple and makes use of theNE555 Timer IC (often abbreviated to just 555) which isadvertised usually for under 40p.

This IC has a very sensitive input and the ability toprovide the required variable time delay as well assufficient output to trigger the SCR (thyristor) which isfitted across the normal contacts for firing the flash.

CONSTRUCTIONUsing the PCB pattern shown makes constructiion veryeasy, only one thing needs to be watched. The triggeringcurrent needed is only 0.5 microamps (this goes into pin2 of the IC) and with the adjacent pins being the negativesupply and the output, leakage across the board can be aproblem. Normally excess solder flux, dirty thumb -printsetc. on a PCB don't make much difference to theoperation of a circuit but here we have to be careful.Once the PCB is built up, clean it up with methylatedspirits. A dirty board will show itself up by continuoustriggering; dampness around pin 2 will affect the circuitin a similar way.


ON

HIFLASH

TRIGGER

light

contact with this

SELECT SENSITIVITY C>E1 AY TRIGG

. 41.INPUTS

4.10twit SOS

Since flash tubes do not have an unlimited life, it's not agood idea continually to use the flash itself as anindicator that the unit is set up properly; instead the LEDserves this function.

It will be necessary to carry out a couple of 'dummy'runs in order to get the sensitivity control (RV1) setcorrectly.

Connect eitner a crystal microphone or a loudspeakeracross Input 2; due to the sensitivity of the circuit either willgive an output of sufficient level to operate.

Start by setting the sensitivity to maximum (this iswith RV1 at a minimum); in this state the circuit willprobably be triggering continuously. Then back off thecontrol until the LED goes out but comes on again when therequired sound is made. There is no advantage in usingmaximum sensitivity all the time as you will find that theunavoidable noises in resetting the equipment after the firsttests have been made will fire the equipment.

LIGHT TRIGGERAlthough a light dependent resistor (such as the ORP12)can be used, these are fairly slow to react and you willhave a built-in minimum time delay. It is better to use alight sensitive transistor; although this costs more, it willintroduce little, if any, time delay.

Light triggering can take one of two forms. With thedelay set at minimum, the unit can be used as a slaveflash, triggering by the principal flash unit. It is notdifficult to set up a light beam which is broken just beforethe photo is needed. The circuit can be arranged totrigger for both an increase and a decrease in light level.

For an increase in light the phototransistor should befitted across Input 1 with the resistor across Input 2; fora decrease the two should be reversed.

The unit can also be operated with either a switchclosing, or opening. As with the phototransistor, aresistor must be fitted across the other input.

OPERATIONAL TIPSIf you're an experienced photographer the following willbe like teaching granny to suck eggs, in that case ignore-this section.

59

As the shutter will have to be open for several secondsawaiting the trigger flash, clearly background light is notdesirable but you don't need pitch black (which makessetting up almost impossible).

If you get movement in the developed pictures, try adifferent flash unit, we have come across at least onecheap flash that doesn't give the highly intense, veryshort duration flash needed.

You will also find that with a flash you will beoperating frequently much more closely to the subjectthan that allowed for on the tables which you have toconsult for aperture settings - in this case move theflash away from the camera.

Oh - and if you want to try breaking light bulbs(which can give beautiful results) don't underestimatethe mess created; form a complete 'cup' of paper ormaterial to catch the bits before you start!

UNSWITCHEDSW1SW2SW2

SW2 COMMONRV1

-VE TO FLASHLED A

+VE TO FLASHRV2

Fig. 3 Component overlay ofthe Flash Trigger.

60

How it WorksA negative pulse at the input is fed via capacitorC1 to the input pin (2) of the IC. Pin 2 is heldslightly above its triggering voltage of I /3 V bythe voltage divider comprising RI, R2, and RV1.The negative pulse triggers the IC and output(pin 3) goes high for a time period controlled byRV2, R3 and C2. When the output goes lowagain at the end of the time interval capacitorC3 charges through the gate cathode circuit ofthe SCR switching it on and firing the flash.

Capacitor Cl isolates the input from thevoltage divider so that the unit isn't sensitive tothe DC level at the input. RV1 acts as a sen-,sitivity control by allowing the voltage to beadjusted to a suitable level so that the inputsignal will trigger the IC. Resistor R4 limits thedischarge current from C2 at the end of thetiming cycle so protecting the IC. The LED andsits protective resistor R5 act as an indicator toshow that the unit has triggered, so simplifyingthe setting up process and minimising thenumber of times the flash has to be fired. Thismeans that the flashgun needn't be fired until aphoto is to be taken.

-VE

RV2 WIPER

LEDK

RV1

TO COMMON OFALL JACK SOCKETS


Flash Trigger

Fig. 1 Circuit diagram. Note that R4 andR5 make up to a non-standard value.

0

I/P A

C1# 10n

SW2 COMMON 0-II

I/P B

0

R4330k

B

a MICROPHONE

oSW2INPUT 2

0/P TO FLASH

SCR12 AC3

T 10n

1+NOTE:IC1 IS NE555SCR1 IS C106D1LED1 IS TI L209Q1 IS OCP71

OSW1

R3330k

RV22M linDELAY

IC1

TPP39VLED 1

R51 R6

330k 100k

R7470R

RV1 C250k lin 100nSENSITIVITY

Parts ListRESISTORS (all 5%, 1/4W)R1 330kR2 165k (2x330 k in parallel)R3 100kR4 10kR5 560 ohmR6 33 k (optional depending on

input)

CAPACITORSCl 10nF ceramicC2 100nF ceramicC3 10nF ceramic

SWITCHAny SPST switch suitable

POTENTIOMETERSRV2 2 Meg linearRV1 50 linear

SEMICONDUCTORSIC NE 555

C1

0-SW2INPUT 1

pA

33k

CH I-C1

SW3

1---10. 0--SW2INPUT 3(SW2 INPUT 4 IS AUX)

Fig. 2 Alternative input connections.

SCR C106 D1 or similar with a 400 voltrating

LED Miniature red

INPUT DEVICEOCP71 transistorCrystal mike insert etc

OUTPUT DEVICE

Electronic flashgun, preferably computertype as these give a much shorter flashduration at close range

MISCELLANEOUSInput socketsOutput to suit flash unit being usedKnobs 2 for potsBattery ConnectorPCB

KA

SCR1C106D1

OCP71NOT USED

CASE TYPE: T97

4 JACK SOCKETS

COMMON BUS, TINNED COPPER WIRE

_I_ _ft_ TOCOMMONON PCB

-IL--a_-

UP 4 I/P 3 I/P 2 I/P 1

Fig. 4 Jack socket connec-tions.


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)0^

the quickest fittingCLIP ONcapacitive dischargeelectronic ignitionin KIT FORM ij Smoother running // Instant all-weather starting Continual peak performance Longer coil/battery/plug life Improved acceleration/top speeds

Optimum fuel consumptionSparkrite X4 is a high performance, high quality capacitive discharge, electronicignition system in kit form. Tried, tested, proven, reliable and complete. It can beassembled in two or three hours and fitted in 1 / 3MinsBecause of the superb design of the Sparkrite circuit it completely eliminatesproblems of the contact breaker. There is no misfire due to contact breakerbounce which is eliminated electronically by a pulse suppression circuit whichprevents the unit firing if the points bounce open at high R.P.M. Contact breakerburn is eliminated by reducing the current to about 1.50th of the norm. It willperform equally well with new, old, or even badly pitted points and is notdependent upon the dwell time of the contact breakers for recharging the system.Sparkrite incorporates a short circuit protected inverter which eliminates theproblems of SCR lock on and, therefore, eliminates the possibility of blowing thetransistors or the SCR. (Most capacitive discharge ignitions are not completelyfoolproof in this respect). The circuit incorporates a voltage regulated output forgreatly improved cold starting. The circuit includes built in static timing light,systems function light, and security changeover switch. All kits fit vehicles withcoil/distributor ignition up to 8 cylinders.

THE KIT COMPRISES EVERYTHING NEEDEDDie pressed epoxy coated case. Ready drilled, aluminium extruded base and heatsink, coil mounting clips, and accessories. Top quality 5 year guaranteedtransformer and components, cables, connectors, P.C.B., nuts, bolts and silicongrease. Full instructions to assemble kit neg. or pos. earth and fully illustratedinstallation instructions.

NOTE - Vehicles with current impulse tachometers (Smiths code on dial RV11will require a tachometer pulse slave unit. Price £3.35 inc. VAT. post Ft packing.

Electronics Design Associates, Dept. HE 382 Bath Street, Walsall, WS1 3DE. Phone: (9) 614791

Name

Address

Phone your order with Access or Barclaycard

Ache.' flastsgs cad imam LIK .11 QUANTITY READ.

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(01)


SolderSolder is solder is solder . . . or is it? K. T. Wilson takes a closer look.

SIMPLE ENOUGH, isn't it? You just buy a reel of coredsolder and that's it. Or is it? In fact there is no singlematerial called solder, and there's a very wide range ofbehaviour that you can expect from solder, dependingon their composition. Add to that dozens of materialsthat can be used for a flux core, and it doesn't look sosimple.

Basically, solder is a mixture of lead and tin. Pure leadmelts at about 327 C, tin at 232 C, but mixtures of tinand lead melt at temperatures which depend on thecomposition. Fig. 1 is a diagram which shows themelting points of various alloys - the important soldersare the alloys which contain up to 60% tin. This type ofdiagram shows an important point - that there aremixtures of tin and lead which have melting points lowerthan either tin or lead. This lowest melting point is for a63% tin mixture, called the eutectic - a name given toany mixture of materials whose melting point is theminimum.

327

183

20 40 60

PERCENTAGE OF TIN (1.)

PURE

EUTECTICMIXTURE

60 100

Fig. 1. Graph of melting point plotted against composition fortin/lead mixtures.

The graph of Fig. 1 shows only melting points - butthere's more to it than that. Pure materials, such as puretin or lead, have sharp melting points - meaning thatthey go from liquid to solid for only a tiny fraction of adegree change in temperature. The eutectic mixture(63% tin) does this also, but all other mixtures of tin andlead which contain more than about 15% tin have a'pasty' stage (Fig. 2), neither liquid nor solid. This pastystage is important in soldering, because slight vibrationduring the setting of solder can cause fractures if there isno pasty range of temperatures. Fig. 2 shows thetemperature range of the pasty stages for the variousmixtures - not that these are always completely solid at183 C, so that this is the final setting temperature of anytin/lead solder.

TIN

327

183

PERCENTAGE OF TIN (1.)

Fig. 2. The pasty part of the temperature / composition graph.

SOLDER ONThe reason for the popularity of the 60 / 40 alloy(60% tin) is fairly clear. It has a low melting point and asmall pasty range; a good combination of qualities forhand -soldering. The low melting point lets us uselow -power irons, and also avoids damage caused by thequick burn -off of flux which would occur at highertemperatures. The small pasty range (flux 180 C to188 C) means that the alloy will set fairly quickly -blow on it once or twice and it's then strong enough toforget about. There are some merits which do not appearin the graph, though. One is that the 60 / 40 mix is oneof the strongest, another is that it is the best electricalconductor of all the tin/ lead alloys (about 11.5% of theconductivity of pure copper).

The 60/40 alloy, along with the 50/50, 45/55 and40/60 alloys are the solders most commonly used forsoldering small electrical equipment. Solders with lowertin contents are used for purposes where higher runningtemperatures are encountered, such as in lamp bases,electric motors, dynamos and fuses.

The lead/ tin alloy isn't the only type of soldermixture, though, particularly for the industrial user. Astraight lead/ tin mixture dissolves copper, and copper isthe material that we use for the business end of thesoldering iron. The result is that bits of irons wear awayvery rapidly as the copper of the 'iron' bit dissolves, andthe tracks of PCB's can also be dissolved in the sameway, causing thinning of the copper layer. This problemis serious for large-scale production soldering work inparticular, and can be overcome by making solder whichalready has a content of about 1.5% of copper. This isabout as much copper as a tin -lead solder would nor-mally dissolve, so that the inclusion of copper into thesolder virtually puts an end to the dissolving of copper bythe alloy from the bit or the PCB. Copper -alloy solder,invented by Multicore, and sold under the trade name of


SAVBIT, is used extensively for large-scale work, and issufficiently well proven to be approved for solderingwork on military equipment.

GOLD SOLDERSNEVER DIENot all soldering makes use of 60 / 40 alloys. Forhigh -temperature soldering, alloys with only 5% tin areavailable (melting at 301 C); at the other end of thecomposition scale there is non-toxic solder which has96.3% tin and the remainder silver. Such lead-freesolder can be used when soldering has to be in contactwith foodstuffs (as in tin -cans for example, or waterpipes). Just for a bit of variety, there is also a low meltingpoint solder, melting at 145 C, which contains 50% tinand 18% cadmium. This is of particular use in solderingonto gold, and the very low soldering temperature is anadvantage for IC internal soldering. Another low meltingpoint alloy is 62% tin, 2% silver, 36% lead, whichsolders particularly well to silver -coated sutaces. It findsparticular use in soldering ceramics to metals. Table 1shows some alloy compositions.

The metal, of course, is only half of the solderprocess. When we solder metals together, the tempera-ture that has to be used is high enough to enable theoxygen in the air to attack the metals and the solder aswell. In addition, we want the solder to spread over thesurface of the metals. Now the spreading of a liquid overa solid is greatly affected by the presence of othermaterials - for example, water will not spread on glass ifthere is a trace of silicone grease on the glass. Liquidsolder is equally fussy, and traces of dirt on metalsurfaces will simply prevent solder from spreading.

A flux is a material which is used to avoid both ofthese problems. A good flux should help to clean up themetal surfaces (though it can't be expected to performmiracles) and should form a protective coating aroundthe solder and the metals being joined so as to avoidoxidation.

For non -electrical soldering, acid fluxes like thetraditional 'killed spirit' can be used. These materials areacid enough to dissolve away impurities; the sort of workthat is soldered in this way is usually 'pickled' in acidanyway, so that the acidity of the flux is unimportant. Forelectrical work, however, strongly acid flux of this typehas to be avoided like the plague. It's not very often thatwe can boil our printed circuit boards in water for severalhours to get rid of acid, and if we don't remove it then thelife of the conductors will be pretty short.

SOLDER GLUEElectrical fluxes are therefore based on resin, thegummy material which is extracted from wood. Moltenresin flows evenly over metals, giving protection for thejoint for some time. Any resin which remains on a joint ishard and non -corrosive; a useful protective coating infact.

Unfortunately, resin by itself does not dissolve a filmof oxide from a metal surface, so that it doesn't have thecleaning effect of an acid flux. Fortunately, we can makeuse here of the fact that soldering is a high temperatureoperation. Some chemicals, such as the range of saltscalled halides, will dissociate when heated, meaningthat they will release acid vapours which will be neu-tralised again when the material cools. Chemicals such

Fig. 3. Cross-sectionof 5 -core solder.

as tin or lead chlorides can be used for the purpose. Theaddition of such materials, called activators, to a resinhas a very noticeable effect on the fluxing ability of theresin. The release of chlorine from a chloride, forexample, cleans metal oxides very effectively, but hasless adverse effects than acid on the life of the jointbecause the chlorine is reabsorbed wherever thematerial cools. Less strongly active materials can beused when there is any risk of contaminating the areaaround the joint. In general, fluxes for electronics usehave a fairly low halide content. A few types of halid-freefluxes have also been developed, and are used for suchapplications as circuits which are to be encapsulated. Forcircuits which must have very long corrosion -free life inhot climates, pure resin -flux is available.

In the early days of soldering, the flux was alwaysapplied separately from the solder. Since Multicorepioneered the idea of flux -cored solders, the separate -flux system has died out almost completely. Thoughseveral manufacturers now make flux -cored solders,Multicore are still unique in offering five cores (Fig. 3),ensuring quick and even dispersion of the flux. In caseyou were in any doubt, by the way, the resin cored solderis made in the same way as Blackpool rock - a thick rodof solder is cast with five holes running through it, theseholes are filled with flux, and the whole thing is drawnout into the fine solder which we use. It's a fascinatingprocess to watch too!

KEEP IT CLEANAnd after all that, let's hope we are soldering cor-rectly. Boards such as Veroboard which have coppertracks should be scrubbed clean - don't let the flux haveto do all the job of cleaning the board. Similarly,tarnished leadout wires of components should becleaned by pulling them through loops of emery -paper.For really good joints, it pays to use leads and tracks thatare tinned in advance (like the Blob -board tracks); goldplated leadout wires on transistors will also solder veryeasily.

Make sure the iron is hot enough. Some irons alwaysseem to run a bit cool and if a very small bit is used, theheat sinking action of a circuit board can be enough to


Solder

ALLOY GRADE MeltingTemp.

Solidus Liquidus

° C ° C

USES

2/2/96 Sn/Sb/Pb pouf.,TWO

305 315 High temperatureHigh creep strength

5/93.5/1.5 Sn/Pb/Ag HMP 296 301 High melting point

95/5 Sn/Sb 95A 236 243 High melting pointLead free

Pure Tin Sn PT 232 232 Lead free

15/85 Sn/Pb W 225 290 Lamps

96/4 Sn/Ag 96S 221 221 Stainless steelBright, strong,non-toxic

20/80 Sn/Pb V 183 275 Lamps

30/70 Sn/Pb J 183 255 Lamps, motors

31.2/67/1.8 Sn/Pb/Sb L 185 243 Radiators, generalpurposeNon -electrical

40/60 Sn/Pb G 183 234 General purpose

45/55 Sn/Pb R 183 224 General purpose

50/48 5/1.5 Sn/Pb/Cu SAVBIT 1 183 215 Saves copper erosion

50/49.7/0.3 Sn/Pb/Sb Sn 50 183 212 General purpose

50/50 Sn/Pb F 183 212 General purpose

60/39.7/0.3 Sn/Pb/Sb Sn 60 183 188 Electrical

60/40 Sn/Pb KP 183 188 Electrical

63/36.7/0.3 Sn/Pb/Sb Sn 63 183 183 Electrical

62/35.7/2/0.3 Sn/Pb/Sb/Ag Sn 62 179 179 Silver-platedsurfaces

62/36/2 Sn/Pb/Ag LMP 179 179 Silver-platedsurfaces

18/180.1/1.9 Sn/Pb/A. ALU SOL45 178 270 Aluminium

50/32/18 Sn/Pb/Cd TLC 145 145 Low melting point,soldering on gold

70/30 Sn/Zn"r-- - 196 307 Spray wire for metal

film capacitors80/20 Sn/Zri* - 196 26830/70 Sn/Cc'

' These alloys are only available as solidprecision made solid wire can be made

TC 30

wire. For those purposesin any alloy to special

176 240

where solid wire is stillorder.

Low thermalEMF solder

used on automatic appliances

LIVE

N

TO IRON

N

Fig. 4. Using an autotransformer, such as a Variac, to controliron temperature.


keep the bit too cool to melt the solder properly. A veryhot iron, on the other hand, will cause oxidation andburning if it's left on too long. The power output ofsoldering irons for electronics use is so low that a simplethyristor controller can cope, and excellent heat regula-tion can be obtained if an auto -transformer is also used(Fig. 4). Incidentally, a lot of awkward problems can beovercome by using ready -formed solder shapes, such asrings and spheres, or by the use of solder and flux mixedin the form of a cream or paint. HE

May the flux be with you!

The author gratefully wishes to acknowledge thehelp of Multicore Solders Ltd in providing informa-tion for this article.

65

electronics todayWhat to look for in the February issue: On sale Jan 5th

TODAYS 100 WATT AMPLIFIER AT

YESTERDAYS PRICESETI, Britain's most ingeniousmagazine has come up with a100W mixer amplifier, withdistortion below 0.1% at all signallevels, S/N ratio greater than80dB, inputs for four sources,including one or two disc inputsas you wish. Somehow or otherthe design, by Richard Bekker,cost less than £50 to build

complete with metalwork.A complete kit of parts will be

made available and fullconstructional details will begiven next month. The unit isfinished to match the five channellight show presented in theDecember issue of ETI.

Crowds are expected to throngshops early next month -newsagents are preparing.

BUILD YOUR OWNVCT

AND FIND OUTWHAT VCT ME

The revolutionary device that will replace theop -amp.We got fed up waiting for it to be released.We did something about it.We show you how to construct your veryown VCT next month!Astound your friends!Confuse your budgie!Amuse your boss!No home dare be without its VCT!ETI brings home the bacon next month!

VOICE SYNTHESIS CRISIS -

MACNINEPanic in the streets! Women andchildren unsafe! Machines canspeak! Prime Minister to go onsteam radio tonight! From our un-cover agent - Tim Orr - comesfull details of the invention thatcould cause a bigger stir than the

S SPEAK OUT!double breasted jacket! Severalmethods are in use, and a new unitis soon to be available which pro-mises to confound us all.

Speech synthesis is here to stay,and Special Agent Orr is right

SLIDING INTO SYNCH?OK you guys youse asked for this and now yousegonna get it, see? Youse bin ringing and hassle us boysdown at ETI to do youse a slide synchroniser so longnow dat the broad on de phone is going bananas see?So we gotta give it to youse see? Nuffin personal see?OK?

Articles mentionedhere are in an ad-vanced state of pre-paration butcircumstances mayaffect the final con-tents.

66

SCILLY SCOPEMake more use of your tele tolks! Here is aunit to make the room pulsate with colour

in time to your hi-fi! Hooks into musicsignals to give an oscilloscope type display

on a television screen, in full gloriouscolour! What will they think of next?

Pocket calculating machines?

there in the forefront reportingback for ETI readers exclusivelynext month. If you value yoursanity you cannot afford to missthis! Thinking people everywherewill be talking about this - don'tbe left out at the dinner table!

NEXT MONTH: COMPUTING TODAY GOES TO 48PAGES! CAN MANKIND SURVIVE? WILL YOU BYTEOFF MORE THAN WE CAN CHEW? FIND OUT INCOMPUTING TODAY NEXT MONTH!

Composer goes SCAMPAn amazing revelation came to the attention of theBritish electronics public today. ETI have plans foran MPU composer! Bach and Handel have beenheard to revolve in their graves at 2000 RPM at thisstunning news! This audacious machine employs aSC/MP processor and an amazingly low com-ponent count. All will be finally revealed in the nextissue of ETI, and anyone remotely interested inmusic, synthesisers or electronics is urged not tomiss it! A machine that thinks up and plays its owntunes has to be seen to be believed.


C.)

Kit ReviewWEIN BRIDGEOSCILLATOR

A WEIN BRIDGE oscillator is a circuit which produces asine wave output. This kit makes a test -bench unit whichgives a variable frequency sine wave for a variety ofpurposes.

A sine wave source is very useful in many applica-tions. It is the only waveform which contains only asingle frequency. By 'sweeping' it - varying thefrequency a little at a time - and finding the response ofa particular circuit (how much it reduces the level of theinput waveform), the overall frequency response can befound by drawing a graph of response against inputfrequency.

This procedure requires a sine wave source of aknown, but variable, frequency.

This unit will produce a sinewave at frequenciesbetween 15 Hz and 20 kHz, accurate to about 10%.This should prove sufficient for most test applications.

FIRST IMPRESSIONS

The kit cost £7.99 and for this price we expected a small,cheap case. Nothing could be further from the truth. The

The completed unit - very smart, except for the frequencyscale (which goes from 15 to 150). White on aluminium is amistake.


case is heavy. It's made (we think) from steel plate - orat least, that's what it feels like!

The front panel is printed in red and white and looksvery impressive, with the top painted with grey hammerfinish enamel. It wouldn't look out of place on board abattleship.

The components came in two plastic bags, oneholding all the board and panel mounting componentsas well as PCB pins and wire. The other bag containedmore PCB pins and more wire! Very strange. Later wediscovered that the pins in the first bag (which wasstapled shut) were the wrong size for the holes - thesecond bag had obviously been added after the first hadbeen sealed - fair enough. Why the extra wire, though?We never did find the answer to that one.

There was also a huge length of solder included -much more than was required.

BUILDING IT

Let me say right at the start that if you can't follow acircuit diagram, you can't build this kit. It's excellentvalue in terms of the hardware and components pro-vided but it's not easy to build.

The main problem is the book of instructions - it'sbadly written and produced and, in places, positively

0

67

cryptic. Anyone who could build a project from thecircuit diagram could find their way round it easilyenough but if you want step-by-step instructions then itwill give you problems.

COMPONENT CHECK

The first thing we did was to check to componentssupplied against the parts list in the manual. Sur-prisingly enough, the instructions gave the resistor andcapacitor colour codes where necessary. This is verymuch at variance with the level of knowledge assumedin the rest of the book.

A couple of components were not as marked in the list- a close value to that required was supplied instead.

The list itself was full of ambiguities:C6 -C10

didn't mean C6 to C10. it meant C6 and C10.

TR1-TR2 BC177-178-179

meant that TR1 and TR2 are both either a BC177, aBC178 or a BC179!

Having came to terms with this sort of thing (and hav-ing reterred to the circuit diagram once or twice), wediscovered that all of the required components werethere and so we could proceed with the construction.

PUTTING IT TOGETHER

The board has the component positions printed onto it.While on the one hand this is a good idea, it's also a good

68

What you get. The case makes up most of the weight.

idea to print the component positions somewhere in thepaperwork also. This means that when you've solderedeverything down (covering the component positionmarkings as you go, of course) and the manual tells youto solder a PCB pin to a position "alongside C3", youdon't have to refer to the circuit diagram and the PCB foilpattern to find out where C3 is!

We had no trouble following the componentpositioning, however, and managed to get everything inthe right place.

The resistors were a bit fiddley - they were of the'pre -formed' type, with the leads bent ready for insertioninto a PCB. The only trouble was, they weren't bent inthe right places to fit the board! As the instructions put it,-Resistors are pre -formed. Straighten and re -bendbefore use". Easier said than done.

The manual is also worth quoting when it comes toD1 and D2. These are wired in parallel, facing inopposite directions or, as the book puts it, "D1-2.1 N401 1 Diode. These are fixed in an alternate man-ner"!

THE FRONT PANELThe multi -way range switch (a 2 -pole, 6 -way rotary) hasno less than four wires and eight capacitors mounteddirectly onto it. This is not easy and must make itfault -prone during extended operation. It could havebeen avoided by making the PCB a little bigger. Therewas more than enough wire, solder and PCB pins toconnect the switch to the capacitors mounted on thePCB.

All three of the front panel components had mountingtags on them. These are designed to poke through a


Kit Reviewsmall hole beside the spindle of the device to stop it fromrotating. As there were no such holes in the front panel,the tags had to be clipped off. This was not mentioned inthe instructions.

WIRING IT UP

The rest of the construction went off without a hitch.Sleeving was provided for 'flying' connections. A fewcomponents mounted directly onto the front panelcomponents. While this is not elegant, it shouldn't causeany problems in building or operation.

The PCB was mounted on plastic pillars which push -fitted into holes in the chassis and the PCB, providing astable and strong construction.

A piece of paxolin (the stuff PCBs are made of) andanother couple of PCB mounting pillars held thebatteries (two PP3s) in place fairly securely. The lastconnection was made and a 'scope plugged into thecoax socket on the front panel. The unit was thenswitched on.

The completed board. Can you follow the component positionsthrough the components? We can't!

The board before construction. The component positionmarkings are fairly easy to follow.

This is what the multi -way switch looks like after everythinghas been soldered onto it.

OPERATION

It worked first time, the only problem being that theoutput level control operated in the wrong direction -clockwise rotation reduced the signal level. Again theambiguous instructions had let the product down. Easilyenough fixed, though.

The unit produced a range of frequencies from about30 Hz to over 150 kHz - no doubt the 'nearly correctvalue' components contributed to the lack of lowfrequencies.

Another problem was the DC shift on the output. Theaverage value of the signal voltage was not zero volts,even though the output was decoupled by a capacitor!

Replacing the decoupling capacitor revealed thesource of the trouble - it was an electrolytic type. Theseare 'polarised' and have slightly different characteristicsto current flowing in one direction, compared to currentflowing in the opposite direction.

This means that there is effectively a diode in serieswith the output. Anyway, replacing the capacitor with apolyester type cured it.

SUMMARY

This is not a kit for the complete beginner. To anyonewho is not a complete beginner, however, I wouldrecommend this kit highly. When completed it is veryrugged, reliable and would prove very useful to anyoneinterested in (particularly) audio work. It's the best valuefor money I've seen in a kit for some time. If what you'reafter in a kit is for someone to do your component buyingfor you in bulk, then this is for you. If you're looking forsomething easy to build, buy something else. HE

This kit is available from:Arrow Electronics,Leader House,Coptfold Road,Brentwood,Essex,CM14 4BNFor £7.99p all inclusive. Order as LTO 104.


Short CircuitsA PREAMP WITH DB STEPPED GAIN CONTROL

A handy little piece of test equip-ment is a preamplifier with steppedgain control selected by a rotaryswitch. The circuit here uses asingle IC, a 741, 14 resistors and asingle -pole, 12 -way rotary switch.

The voltage gain of an op -amp(and that is what the 741 is) isdetermined by the ratio of RFB/R,n;thus by having Ft, switched, thevoltage gain can be varied.

The input impedance of thepreamplifier is set by R, to 10k.Having the gain set in decibel (dB)intervals is most useful in audioapplications because our hearing,like dBs, is logarithmic. The gain indB is defined as being equal to20 X log io (Voltage Gain) whichequals 20 X log t0 (R/R,n). There-fore a voltage gain of 1 is20 x log to 1 =0dB but a voltagegain of 2 is 20 X logic) 2=6dB.

Although this may at first seemlike a complex approach, the de-cibel is an easy to use method fordescribing gain and attenuationsince all you have to do is add andsubtract them. For instance, say a

DIAL MARKEROUTER RING - VOLTAGE GAIN AS A MULTIPLIERINNER RING - VOLTAGE GAIN IN dB

xX1.4

X2

X2.8

X4

X5.6

signal passes through four deviceswith gains of 9dB, 15dB, -3dBand -3dB, the overall signal gainis 9+15-3-3 which is 18dB(this is a voltage gain of times 8).Note that negative dB means atten-tuati on (reduction in strength).Now consider the same situationwithout using dB; a signal passesthrough four devices with gains of2.8, 5.6, 0.7 and 0.7. The overallsignal path is 2.8 X 5.6 x 0.7x 0.7which comes to the same result buta lot more difficult to calculate thanadding and subtracting.

PHASE SHIFT OSCILLATOR

C1 I C247n c 47n

Q1BC 182

A single transistor can be used tomake a simple phase shift oscilla-tor. The output is a sinewave with a'lump' in it, which means that thedistortion content is rather high,about 10%. This is not always aproblem, quite often whengenerating audio tones a high har-monic content will make a moreinteresting sound. The sine wavepurity can be increased by puttinga variable resistor (25 ohms) in theemitter lead of Q1 (x). The resistoris adjusted so that the circuit is onlyjust oscillating, then the sinewave

OUTPUT

250 Hz

is relatively pure. However, if thepower supply level varies, theoscilation may cease altogether.The operating frequency may bevaried by putting frequency may bevaried by putting a 10k variableresistor in series with R3, or bychanging C1, 2, 3. Making C1, 2,3 equal to 100nF will halve theoperating frequency. Also, theoperating frequency can be voltagecontrolled by a FET in series withR3, or optically controlled by anLDR in series with R3.

Vin

SINGLE POLE 12 WAY SWITCH,MAKE BEFORE BREAK

Rin

10k

-3dB

0dB

+12V

-12V

3dB

6dB

9dB

12dB15dB 56k

82k

6k8

18dB

21dB

24dB

27413

30dB

VOLTAGE GAIN = Rfb/RinNOTE: AMPLIFIER INVERTS SIGNAL

100k

150k

220k

/7771

A LOW FREQUENCYOSCILLATOR

Cl10 NON-POLARISED

R1

IC1 is a MOSFET OP amp. Thus itsinput bias current is very low in-deed, typically 10 pA as comparedwith 100 nA for a 741 (10 000times larger!). This allows very lowcurrent designs to be produced.The circuit above shows an inte-grator (IC1) and a Schmitt trigger(IC2). Imagine that the output ofthe Schmitt is high (+10 V). Thevoltage at the junction of R4, R5 isapproximately +1 V. This pushes acurrent of 1 uA through 81 whichthen charges C1. Thus C1 (theoutput of IC1) ramps down at a rateof I /C which is in this case 1 V/Sec. When this voltage reaches

R3100k +10V

-10y

R4100k

1uA = 10-6 A

lnA = 10-9 A1pA = 10-12 A

-5 V, the Schmitt trigger flips overinto its low state (-10 v). Now thecurrent through R1 flows the otherway, and so the output of IC1ramps up at 1 V/sec. This con-tinues until it reaches +5 V (theupper hysterysis level of theSchmitt). The Schmitt trigger thenjumps to its high output and so theprocess repeats itself. The circuitproduces a square wave (± 10 V)and a triangle (± 5 V) output.Using the components shown theperiod is 20 seconds. To get 200seconds make R1 = 10M. To get2 000 seconds make R1 = 10M,R5 = 1R.


TAMTRONIK LTD. (Dept. HE)' 217 Toll End Road, Tipton

West Midlands DY4 OHW. Tel. 021-557 9144

TELEPHONEOR LETTER

ONE -STOP SHOPPINGPCB's, Components, Hardware, Cases, Part Kits and full Kits. A comprehensive service to thel-I.E. Constructor, Over1 10 kits now available for projects from Electronics Today International, Hobby Electronics & Everyday Electronics.Prices include VAT P&P 30p per order

alaS issue. Proles! Rel. PCBCamponen1

Pack

Hardware

PackCase Total

Dec. '78 Pushbutton Dice H005 79 3.19 32 45 4,75Dec, '78 Metronome H006 61 1 24 1.26 1 65 4.76Dec. r 78 Photon Phone H007 1 40 3 52 2.66 7.58Dec. '78 HE Mixer H008 2.95 5 36 4 76 7 35 20.69Jan '79 Four Channel Equaliser H009 TBA TBA TBA TBA TBAJan. '79 HE Slave Flash H010 TBA TBA TBA TBA TBAJan. '79 Van Wiper H011 TBA TBA TBA TBA TBAJan, '79 Touch Switch H012 TBA TBA TBA TBA TBA

KITS ARE ALSO AVAILABLEFOR NOV. PROJECTS

THIS MONTH'S BARGAINS100x1/2W 1K carbon resistors 30p1001W 5% 7k5 resistors 12p1021W 5% 1k5 resistors 10p1001W 5% 1508 resistors 12p10x6W 5% 3908 wirewound 50p10047a 10v Ax. Electrolytic Capacitors 25p1010k 1 in Rotary potentiometer 15p1 plug-in mains PSU 3V/6V/9V/12V DC300MA. suitable for calculators and TV Games

£2.99

A SPECIAL INTRODUCTORY OFFERA voucher worth 10% of the advertised prices will be issued topurchasers of full or part kits of H E Projects This offer is valid until 31stDecember, 1978 The vouchers may be redeemed against futurepurchases of ANY items offered for sale by Tamtronik LtdSend s a e for free catalogue and any kit list, naming kit and kitreference

TRADE ANDEDUCATIONALENQUIRIESWELcONE

Visit our shop at32 Market Place

Great Bridge, TiptonWest Midlands

PARTS FOR PROJECTSIN HOBBY ELECTRONICS

BC107,8 11p MC3360 130pBC109 12p NE555 27pBC184 10p TIL209 13pBC212 12p ZN414 100pCD4017 85p 741 24pTransformer 240V Pr18V lA Sec 290p (+ 50p p&p)

THIS MONTH'SSPECIAL OFFER2 SN76018 1 SN761311 TIL209 2 Heat Sinks1 1 A 100V Br Rect £5.40.

We carry LARGE stocks ofSEMICONDUCTORS/RES/CAPS/ACCESSORIES. Send 6 x9 Stamped Ad-dressed Envelope for your free list or see our fulladvert in E.Ti. /PW7PE/WWAdd 25p p&p All above prices inc. VAT.

TECHNOMATIC LTD.17 BURNLEY ROAD, LONDON, NW10

12 mins from Doll's Hill Tube)Hrs Mon Fri 9 30 5 'in

Sat 10 30 4 10

PRINTEDCIRCUITS

HARDWAREComprehensive range Constructors' Hardwareand accessories.

Selected range of popular components.Full range of HE printed circuit boards.normally ex -stock, same day despatch atcompetitive prices.P.C.Boards to individual designs.Resist -coated epoxy glass lamina4 for thed.i.y. man with full processing instructions (nounusual chemicals required).

Alfac range of etch resist transfers, and otherdrawing materials for p c. boards.

'Send 1Sp for catalogue.

RAMAR CONSTRUCTORSERVICES

MASONS ROADSTRATFORD-ON-AVONWARWICKS. Tel. 4879

1.24

too'For UK orders

add 50p for postand packing

All pricesinclude VAT

BUTTON CELLSDia. Hgt. Price/Cell

225mAh 25 0 7 0.65600mAh 74 5 Iii 11 £1.25

VENTED CELLS(for fast charge)

045 Ah 17 5 28 0 0.9505 Ah HP7 or Size AA 1.10

1 2 Ah 22 5 49 1.401 8 Ah HP11 or Sire C 1 954 0 Ah HP2 or Size D 2.755.0V Pack 501 2 Ah nicads 7.50

Tags available at extra 10p per cell for 0.5Ah, 1.2 Ah and1.8 Ah nicads only.Charger - Suitable for any of the above vented nicads -charges up to twelve similar cells in a series at a 50mA or200mA rate £9.95

OE SO 1.1.010tP0P1.10t1

OILSizes approxorder

Only

38 Stone1eghRoad,

000ess%

01-5606642


BARGAIN PACKS C=111111

TTL7400, 01, 02, 03, 04. 08. 10, 20,30,51 ANY MIX10/E1.00, 100/£9.00.7433. 48, 104. 105, 109, 122, ANY MIX 10/£1.90,100/618.007445, 46, 92. 95, 151, ANY MIX 4/21.007443, 83. 96, 156, 160, 162. 163,74165, 180, 193, 194, ANY MIX 3/£1.30, 10E5.20.BC184, 2N711, 12/£1.00. OCP70 5/£1.00SIMILAR TO 2N2192 20/£1.00. MAN 10 1.27"41£3.00BAX13,1N4151 100/61-50. TBA 120A 2/E1.00'IN5400 10/E0.80. 74510 20/61.00PANAPLEX 9 DIGIT 7 SEG DISPLAY 2/£2.50THYRISTOR 3A 25V 3/20.50'LISP 21/2 DIA 40 OHMS 2/£1.00'RESISTORS 10/E0.09, 100/60.80, ANY MIXE12-10 OHMS TO 1 MEG'CAPACITORSCERAMIC 27P to 8,700P E12 Series 10/£0.30.POLYESTER 0.01 to 1 10/£0.50, ANY MIX 1 MF10/21.00ELECTROLYTIC 63V IMF TO 10MF 10/60.70. 16VIOMF TO 100MF 10/60.70TELEPHONE ORDERS: COV (02031611597 USING

ACCESS CARD NO.V.A.T. add 121/2% TO ITEMS MARKED TO ALL

OTHERS ADD 8%

IBEKSYSTEMS

32 DUNSVILLE DRIVECOVENTRYCV22HS

BUILD THE

TREASURETRACER

MK IIIMETALLOCATOR

AS SEENON BBC -1

& BBC -2TV

Genuine a silicon transistor circuit.does not need a transistor radio to'operate.Incorporates unique varicap tuningfor extra stability.

et Search head fitted with Faradayscreen to eliminate capacitiveeffects.

*Loudspeaker or earphone opera-tion (both supplied).

Britain's best selling metal locatorkit. 4,000 already sold.

Kit can be built in two flours usingonly soldering iron, screwdriver,pliers and side -cutters.

411, Excellent sensitivity and staonity. Kit absolutely complete including

drilled, tinned, fibreglass p.c. boardwith components siting printed on.,

Complete after sales service. Weighs only 22oz.; hanale knocks

down to 17" for transport.Send stamped, self-addressed

envelo a e for literature.

Complete kit-

EI 5.95with pre -builtsearch coil

Plus E1.20 P&PPlus E1.37 VAT (8%)

-Lilt, tested £ 20.95and',Guaranteed Plus £1.20 P&P

Plus E1.77 VAT (8%)

MINIKITS ELECTRONICS,6d Cleveland Road, South Woodford,

LONDON E18 2AN(Mail order only)

71

SECOND GENERATION

INDUCTIONBALANCEMETAL

DETECTORDESIGNED SPECIALLYFOR THE HOMECONSTRUCTOR

EASYTO

BUILD

EASYTO

USE

A second generation Induction Balancesystem with improved Variable -Tone de-tection.

Designed by professionals for easyassembly by amateurs but with very goodperformance.

The search coils are fully assembled andadjusted for you.

Automatically rejects ground effect

Uses include:

* Treasure hunting - it's amazing what youcan find in the garden or on the beach.

* Finding lost metallic items.

* Locating waterpipes and cables underfloorboards on in walls.

* Checking old timber for nails before cut-ting, etc.. etc., etc., etc.

KIT - COMPLETE WITH PRE -ASSEMBLED SEARCH COILS

£16.50Plus E1-00p&p Plus £1-32 VAT

ASSEMBLED & TESTED

£22.50Plus £.1-00p&p Plus £1-80 VAT

Send sae for free components stoctdIst

Communitation MeasurementLtcr15 MALLINSON OVAL, HARROGATE,YORKS,

72

;Ado) iixeu ramoA4e6 04 umop emsdn

SA illUe6DSMeU 044UM111. away cuop

nog( wins elowTo: SubscriptionsHobby ElectronicsP.O. Box 35Bridge Street,Hemel Hempstead,Herts

Name

I would like a postal subscription to HE star-ting with theissue. I enclose payment (£6.50 for UK andEire, £7.50 elsehwere, £11.50 air mail).

Address

ETCH RESIST TRANSFERKIT SIZE 1:1Complete kit 13 sheets 6in x 41/2in£2.50 with all symbols for directapplication to P.C. board. Individualsheets 25p each. (1) Mixed Symbols (2)Lines 0.05 (3) Pads (4) Fish Plates andConnectors (5) 4 Lead and 3 Lead andPads (6) DILS (7) BENDS 90 and 130(8) 8-10-12 T.0.5. Cans (9) EdgeConnectors 0.15 (10) Edge Connectors0 1 (11) Lines 0.02 (12) Bends 0.02 (13)Quad in Line.

FRONT AND REAR PANELTRANSFER SIGNSAll standard symbols and wording. Over250 symbols, signs and words. Alsoavailable in reverse for perspex, etc.Choice of colours, red, blue, black, orwhite. Size of sheet 12in x 9in. Price £1.

'GRAPHIC TRANSFERSWITH SPACERACCESSORIESAvailable also in reverse lettering, coloursred, blue, black or white. Each sheet12in. x 9in contains capitals, lower caseand numerals Vein kit or 1/4in kit. £1complete. State size.All orders dispatched promptly.

All post paidEx U.K. add 50p for air mail

Shop and Trade enquiries welcomeSpecial Transfers made to order

E. R. NICHOLLSP.C.B. TRANSFERS

Dept. HE/246 LOWFIELD ROAD

STOCKPORT, CHES.061-480 2179

ELECTRONIC MUSIC MADE EASY!Break into this fascinating world and buildyour own Sound Effects units, even a com-plete Synthesiser, from ready -built circuitmodules. Simple, versatile, low-cost projects,ideal for both experimenting and seriousbuilding. Send 2 x 9p stamps for very infor-mative booklet: B. J. Tyler, 21 BeaufordOrchard, Norton, Taunton.

AUTOPROGRAMMER remote controlequipment. Construction unfinished. Mass ofvaluable electronic components. Sale due tospace shortage. Offer over £25.Phone 352 0015 after 6.30 p.m. orweekend.

AD INDEXACE 74AMBIT 2ASTRA-PAK 13BNRS 19CODESPEED 13COMMUNICATIONS MEAS 72DORAM 19E.D.A. 62ELECTRONI-KIT 74HENRY'S 13,19&74IBEK SYSTEMS 71MAPLIN 76METAC 62MINIKITS 71NICHOLLS 72RAMAR 71STEVENSON 6SWAN LEY 74TAMTRONIK 71TECHNOMATIC 71V&F SMALLCRAFT 71VIDEOTIME 56

1


Good EvansAmaze your friends! It's notnecessary to know anything at allabout electronics to hold your headup high in the most august circles -simply remember our Gary Evans'jargon generator.THE BAUD RATE on the CUTS serial I/O connecting myCPU to my background storage medium needs adjust-ting because at present I am getting a large quantity ofread errors.

My Hi-Fi Amp is suffering from HF instability and theLF component of the S /N figure has increased due, I

think, to a corresponding increase in the ripple on theLT rails of the system.

Lost in the Jargon Jungle? - well I can help. I maynot be able to teach you about all the ins and outs of thetechnical shorthand that permeates the world of elec-tronics in a few words but at least I'll be able to help youreply in kind when exposed to the talents (sic) of a jargonjuggler. The point is you don't have to understand whatyou're saying, in fact it helps if you don't, you just haveto sound convincing in your performance.

Study the ready-made jargon generator below -some of the phrases are more applicable to electronicsthan others but all should baffle your friends andinfluence people.

The jargon generator will produce up to 40 000discrete, well balanced "State of the Art'' sentencesquickly and simply with very little propagation delay.

To put the generator to work, arrange the modules inW, X, Y, Z order. Next generate a four digit randomnumber, this may be the most difficult phase but if apseudo -random number generator is not available, youcould, at a pinch, use your grey cells to generate saidnumber. The first digit of the number will select a phrasefrom module W, the second from X etc. The result is ajargon filled sentence. Add more sentences for a parag-raph and if you carry on you could soon have a book inprint.

After you have mastered the basic technique you cantry the advanced version of the generator by altering theorder of the modules - be warned though, in theseadvanced configurations some additional comas may berequired.

Jargon Generator Module W1 . in particular2. on the other hand,

.3%151 WW1 Eat rt

-to widtve UP,Agt

3. however,4. similarly,5. in a real time environment,6. in this connection,7. as of now,8. for example,9. thus,

10. the "State of the Art implies,

Jargon Generator Module X1. a large portion of interface coordination com-

munication2. the concept of electron mobility3. the characterization of specific criteria4. the worst case load situation5. the fully integrated test program6. the incorporation of DIN requirements7. any associated supported component8. a constant data path9. in independent functional principle

10. a primary interrelationship between systemand /or sub system technologies

Jargon Generator Module Y1. must utilise and be functionally interwoven with2. maximises the probability of project success and3. adds explicit performance limits to4. necessitates that urgent consideration be applied

to5. requires considerable systems analysis to arrive

at6. is further complicated when taking into account7. presents extremely interesting challenges to8. recognises the importance of9. effects a significant improvement in the perfor-

mance factor of10 adds over-riding performance constraints to

Jargon Generator Module Z1. the sophisticated hardware2. the anticipated fourth generation equipment3. the sub system compatibility testing4. the structural design concepts5. the preliminary qualification limit6. the evolution of specifications over a given

period of time7. the philosophy of commonality and standardisa-

tion8. the greater fight -worthiness concept9. any discrete configuration mode

10. the total system rationale.

tir4

\AtAft\ I vrwst RE.v.ntrnsER

WI -to sthao opt 14

THE vekttlk of"41143,0. -to -40E

ta<forel Of 1T14.

'\50"

0


:11ED ELECTRONICDIGITAL AINW1CLOCK IMMEDIATE

DELIVERY £8.95INCL V A TPOST PAID

HOBBYELECTRONICSRECOMMENDTHISMODEL

BARCLAYACCESSJust phonein number

Hanimex HC -1100Large Bright LED displayWhite case with red display

on black backgroundAdjustable dim /bright controSilent operation, all electronicSpace age technology L.S.I. circuitryAlarm and 9 minute snooze repeaterP.M. indicatorHour and minute displaySize 100mm x 130mm x 60mm highFully guaranteed

THREE FOR £26.00POST FREE

NENRgiSRADIO

HENRY'S RADIO404 Edgware Rd. London W2PHONE (01)723 1008 ENGLAND

ACE MAILTRONIX LTD

Dept HE TOW Stave

Warta W Yorkshre 1061 5,5

Name

address

TOPS THE PACK!COMPONENTS

KITS

MODULESSERVICEQUALITYPRICESMAGAZINEPROJECTS

- Now over 1,000 types instock'

- See the new range of low-cost 'E LEK ITS'.

- New ready -built functions.1st Class same day despatch.All guaranteed products.Many reductions!

Trouble -free!

I enclose 30p, please send catalogue.

Refundable with future orders over £5.00.

SINCLAIR PRODUCTS rnicrovision tv El 72PDM35 (27 25 mains adaptor E3.24 case E3 25DM235 E48 30 rechargeable battery units £8adaptor /charger £3.70. case (8.50 Cambridgeprog calculator E13 13 Prog library £3 45 Memoadaptor E3 20 Enterprise prog calculator E21 95DM350. DM450 p o a

COMPONENTS send sae for full list 1 lb FeCIE1 05 dein pen 73p 60 sq ins pcb 55p laminatecutter 75p small doll 20p zn414 E1 05 pcb andextra pans for radio £3 85 case El 164148 1 4p.154002 3.6p 723 29p 741 15p NE555 23p.bc182b, bc183b, bc184b, bc212b. bc2l3b.bc214c 4 5p plastic equivs bc107, bc109 4.8pV.W 5% El 2 resistors 108 to 10M 1p. 0.8p for 50-Fof one value 16v electrolyncs 5 / 1/2 / 5 / 10/22mf5p.,100mf 6p. 1000rnf 10p polyesters 250v 015.068. lrnf 11/2p ceramics 50v E6 22pf 47n 2p

polystyrenes 63v 612 10 pf to 10n 3p zeners400mW E24 207 to 33v 7p

TV GAMES send sae for data. AY -3-8500 + kit68 95 AY.3-8600 + kit (12 50. tank battles chipE6.90. kit E7 05 stunt cycle chip E6 90. kit E5 60rifle kit £4.95

TRANSFORMERS 6-0-6v 100ma 74p. 11/2a£2.35. 6 3v 11/2a (1 89 9.0-9v 75ma 74p. la E2,2a (2.60.12-0-12v 100ma 90p. la E2 49

IC AUDIO AMPS with pcb JC I 2 6W L1.60 JC2010W L2 95 JC40 20W (2 95.

B ATTERY ELIMINATORS 3 -way type 6/ 71/2 /9v300 ma E2 95 100ma radio type with press -studs90 13 35 9 4- 9v E4 50 stabilized type 3/ 6 r 71/2190400rna E6.40 12v car convertors 3/41/2/6/ 71/2/9v 80Orna E2 50

B ATTERY ELIMINATOR KITS send sae for data10Orna radio types with press -studs 41/2v El .40. 6vE1.40, 90 E1.40. 41/2 -1- 4'hy E1 80, 6+6v El 80,9+9v £1 .80. stabilized 8 -way NIX. 3/41/2 /6/71/2 /9/12/15/180 100ma (.2 80. tabnp E6 40stebilded power kits 2-18010E/me E.3 60. 2-30v 1AE6.95.2.30v 2a El 0 95 12v car convertor6/71/2/90 la E1.35

T -DEC AND CSC BREADBOARDS s-dec C3 17,t-dec E4 u-deca E4.40. u-decb E6 73 16 diladaptor E2 14 .13300 E6.21. exp350 E3 40 exp650 E3 89 exp4b E2 48

B I-PAK AUDIO MODULES s450 E23 51 AL60E4 86 pa100 E16 71 sprn80 E4 47 brnt80E5 95 stereo 30 (20.12

SWANLEYELECTRONICS

(Dept HE), 32 Gelded' Rd., Swenley, KentPost 30p extra prices include VAT

74

ELECTROF11141"EDUCATIONAL KITS OFEXCEPTIONAL QUALITY"

(AUDIO magazine)

THIS IS A POWERFULRADIO RECEIVER!

The same kit is also 150 other differentactual working projects e.g.:

Computer & Logic CircuitsElectronic Organ. Tinter, LightControl, Agility Tester, LieDetector. Siren, Horn. Buzzer.Bird, Metronome.Cds cell light & sound controlPhotogim, Light OscillatorLight Switch. Light and SoundMorse Code.Field Strength MeterHygrometer. Sphygometer, etc.etc.

Radio Receiver, Transmitter,Amplifier, Audio Generator, SignalTracer & Injector, Continuity Tester,Telegraph. Ehotoradio Receiver,Radio Receiver /Microphone Miser,Illuminometer, Voltmeter, Ammeter.Sound Level Meter, Ohmmeter.Diode & Transistor Tester.Transparency Indicator. etc.. etc.

The above is just a selection of the circuits available - youcan also design your own circuits with these superb newDenshi-Gakken "EX" construction kits.

No previous experience of electronics is required but youlearn as you construct and have a great deal of fun too Thekits are completely safe for anyone to use.

Kits are complete with very extensive constructionmanuals PLUS Hamlyn's "All -Colour" 160 page book'Electronics'' (free of charge whilst stocks last).

ALL KITS ARE FULLY GUARANTEED. Add-on sets (toincrease the scope of each kit) are available, plus sparesand accessories as required.

150 PROJECT KIT £39.75120 PROJECT KIT £33.75100 PROJECT KIT £29.25

60 PROJECT KIT £25.7530 PROJECT KIT £18.9515 PROJECT KIT £16.75

Prices include educational manuals, free book. VAT. p&p (in the U K I andfree introduction to the British Amateur Electronics Club.

Callers at 20 Bride Lane will be very welcome. Trade and Educationalenquiries invited.

Cheque P 0 Barclaycard /Access No (or 14p for illustrated literature) toDEPT HE

ELECTRONI-KIT LTD.20 BRIDE LANE, LUDGATE CIRCUS,LONDON, EC4Y 8DX (01-353 6430)


Hobby ElectronicsRESISTOR COLOUR CODE

IIA B C

A and B- - ohms-0 ohms

RED 2 - kORANGE 3

YELLOWGREEN 5

BLUE 6

VIOLETGREY 8WHITE

D (tolerance)NONE 20%SILVER 10%GOLD 5%

BROWN

CAPACITOR COLOUR CODE

ABCD

C D (tolerance)BLACKWHITE

20%

10%

E (voltage)

A and B: As forresistors.

EXAMPLES:

BRITISH STANDARD COMPONENT MARKINGS

rn-AiABC D

A= 1, B = 0, C = - - 0 k, so valueis 100 k; D = accuracy = 2%

A A = 4, B = 7, C = - - n, so value is47 n; D = accuracy = 10%; F =voltage = 250 V

E

M (pronounced mega) means multiply by 1 000 000,k (pronounced kilo) means multiply by 1 000,m (pronounced milli) means divide by 1 000,u (pronounced micro) means divide by 1 000 000,n (pronounced nano) means divide by 1 000 000 000 andp (pronounced pico) means divide by 1 000 000 000 000.

So when we write 10 mV, we mean (10 / 1000) V, or 0.01 V. Note: it is usual to leave the "F" and "ohm"out altogether when writing, but as we rarely talk about resistances less than one ohm or capacitances greaterthan one farad, this does not cause too much confiision.

Examples: 10 uV = (10 / 1 000 000) V = 0.000 001 V; 330 R = 330 ohms (R is used for ohms whenno multiplier is needed); 10 k = 10 000 ohms; 3.9 mV = 0.003 9 volts; 3.9 u = 0.000 003 9 farads.

Now, at some stage, someone decided to jazz up the system by putting the suffix in place of the decimal point,so that: 4k3 is 4 300 ohms and 4u9 is 0.000 004 9 farads.Also, in case one of the digits got lost, it was decided that there should always be at least three numbers orletters in the value, so: 5k is written 5k0, et cetera.

Examples: 3M0 = 3 M ohms = 3 000 000 ohms; 4n5 = 4.5 nF = 0.000 000 004 5 farads.

INTEGRATED CIRCUIT NUMBERS

TTL: f)7manufacturer device function (2 or 3 digits)

Example: DM 74121

CMOS: CD 4

manufacturer device function(2 digits)

Example: CA 4015 B

electrical fragility (A = do not handle - staticcharges will destroy device; B or C - protectedagainst static)

OTHER TYPES: Examples: LM 309, CA 3011, NE 555

manufacturer device function (3 or more digits)

byvargun#alaunch their newa loguA massive new cataloguefrom Maplin that's evenbigger and better thanbefore. If you ever buyelectronic components,this is the one catalogueyou must not be without.Over 240 pages -some infull colour -it's acomprehensive guide toelectronic componentswith hundreds ofphotographs andillustrations and pageafter page of invaluabledata.

We stock just about everyuseful component youcan think of. In fact some5,000 different lines, manyof them hard to get fromanywhere else. Over 1000new lines in our newcatalogue. And with theservice only Maplinprovides, you won't regretsending for a copy of ourfantastic catalogue.Orders paid beforepublication date willreceive a set of 10 specialoffer coupons.Big Discounts on popularlines.

r

uunEli CTRON SUPPLIESPBox a Rayleigh, sex SS6 8LR.Telephone: Southend (0702) 715155.Shop: 284 London Road, Westcliffe-on Sea. EssE_(Closed on Monday).Telephone: Southend (0702)315157

Post this coupon now for your copy of our1979-80 catalogue price 75p.Please send me a copy of your 280 page catalogueas soon as it is published (8th Jan.1979). I enclose75p but understand that if I am not completelysatisfied I may return the catalogue to you within14 days and have my 75p refunded immediately.If you live outside U.K. send £1 or ten InternationalCoupons.NAMEADDRESS

HE3

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