THE ELECTRONTC,51VMtAZ/NZ WITH TYIP PRACTiCAL ...ELEKTOR ELECTRONICS FEBRUARY 1990 Please mention ELEKTOR ELECTRONICS when contacting advertisers DEN HAAG HOLLAND SELF-INDUCTANCE METER

THE ELECTRONTC,51VMtAZ/NZ WITH TYIP PRACTiCAL APPROACHUK SI 70 IP 2,2 52 (orr;!. VAI) February 1990

glellorglecirooscs

ifterthlek Killer

- 11:idit)

Video Mixer (21

Initi3lization Aid

C:Jrieni Sensors

Becket Crigade Devices

For fast delivery telephoneyour order on 01-205 9558using VISA/Access Card

ILIACILITABSI

Orders welcome fromgovernment depts &educational establishments

TECHNOMATICTechno House 468 Church Lane, London NW9 8TQ,

Tel: 01.205 9558 Fax: 01.205 0190

rchimedesModel BasicA3000 £649A410/1 £1199A420/1 £1699A440/1 £2499

Dolour£784

eeSpecialOffer

^.4ith Philips CME,333 Mzriitor.Specify if Acorn Monitor required.Carriage f5/computer £12/system

R140 UNIX SYSTEMTechnomatic is an Acorn Authorised UNIX

dealer. We have the R140 system on demo anda range of R140 peripherals in stock.

£3500(a)£449(b)£275(b)£25(b)£65(b)

£499(b)

R140 Base StationEthernet CardSCSI HD InterfaceSystem Admin. GuideProg Ref Manual4 Port Expansion Card

Maintenance contracts availacie on full systems.

TECHNOMATIC UPGRADESPECIALS

Our specially priced upgrades offer aneconomical upgrade to full capability of 410/1

below:and 420/1 systems as indicated1Mb RAM upgrade £120(c)1Mb RAM + 20Mb HD £299(a)3Mb RAM + 40Mb (Toshiba) £629(a)20Mb HD + Controller (310) £349(a)2Mb RAM upgrade £234(c)3Mb RAM + 20Mb £395(a)3Mb RAM + 49.5Mb £699(a)HD Controller (310) £199(b)Ask for our price on 410/1 upgraded to 420/1 or440/1 spec and save ££££s.

MONITORS

Philips CM8833 14 " Colour £199(a)Taxan 770+ 14" MultiSync Col £395(a)Taxan 770+ LR Low Radiation £415(a)CM168616'Hi Res (1280x 1024) £1249(a)Taxan Viking 1119" Mono £849(a)Maintenance contracts available on above monitors.

Ask for details on techno scanner digitising tablet.expansion cards and software

TechnoThis is a specialoffer not likelyto be repeated 'Or

Archimedes 410-1upgraded to full

440 1 spec plus Taxan 770--LR Radiation Monitor

for only £1999Finance and Maintenance contract available. Offer

limited to current stocks.

We can provide attractive discounts toEducation Authorities, Schools, Collegesand Health Authorities. Simply phone us orwrite, outlining your requirements, and wewill su..ly a . uotation.

All prices ex VAT.Prices are subject tochange without notice.Please add carriage(a) £8.00 (Courier)(b) £3.50(c) £2.00(d) £1.00

What we offer in addition to efficientsales service and professional backup!We not only otter professional advice when youare purchasing your system but we will alsoprovide friendly assistance afterwards. Wealso offer the following incentives to make yourpurchase worthwhile.

FREE COLOUR MONITORwith every

400/1 Base SystemOffer limited to current stocks

SpecialOffer

0% FinanceWe are now offering a totally interest free credit to enable you to purchase a system of your choiceby allowing you to spread your payments over 11 easy manageable monthly payments as shownbelow at no extra cost on our normal prices.

Depositex inc VAT

A3000 £ 65.22 £ 75.00A3000 (use with TV) £ 80.00 £ 69.57A3000 Colour £ 78.26 £ 90.00A410/1 Colour £120 £138.00A410/1 + Taxan 770+ £152.17 £175.00A420/1 Colour £170.43 £196.00A420!1 + Taxan 770+ £204.35 £235.00A440/1 Colour £250.43 £288.00A44011 -- Taxan 770+ £286.96 £330.00R140 £347.85 £400.00

10 instalmentsex inc VAT "

Final Costex inc VAT

£ 58.38 £ 67.14 £ 648 £ 746.35£ .62.84 £ 72.27 £ 698 £ 802.70£ 70.57 £ 81.16 £ 784 £ 901.60£107.90 £124.09 £1199 £1378.85£139.18 £160.06 £1544 £1775.60£152.86 £175.79 £1699 £1953.85£183.97 £211.56 £2044 £2350.60£224.86 £258.59 £2499 £2873.85£255.07 £294.06 £2689 £3092.35£323.04 £371.50 £3500 £4025.00

£5 to be added to 1st instalment for acceptance fee. Phone us with your requirements and we willget our detailed offer on its way to you. We are a licensed credit brokers and can also offer creditsup to 24/36 months. Please ask for details.

ON -SITE MAINTENANCEArchimedes Computers like all other Acorn equipment are very reliable systems however, to giveyou peace of mind in the unlikely event of a failure, we now offer you an on site maintenancecontract.The contract will be through Granada MicroCare. Acorn's on site maintenance contractors who withover 160 field engineers and 8 strategically located service depots in the country are able toguarantee 8 working hour service call. with no restriction on the number of calls you make duringthe year. Not many companies can offer such capability or guarantees. Contracts upto 5 years areavailable. When you enter into a maintenance contract remember ridiculously low cost contractslike many low cost insurnces normally result in problems if not in grief.Granada MicroCare. the only truly nationally established service company for Acorn computers.offers high quality service at a very reasonable rate as shown below.

And that's not all we offer!When you purchase a system from us we will allow you the following incentives on on -sitemaintenace contracts and credits to purchase from our extensive range of add-ons. peripherals.software and accessories.

Payment method: with 0% finance cash credit card

A3000: on -site contract at £30 Free on site contract plusOR £35 to spend £35 to spend

A3000 Colour on site contract at £48 Free on site contract plusOR £35 to spend £40 to spend

A3000 (TV) on site contract at £413` Free on site contract plus'OR £35 to spend £40 to spend

A410!1 Colour on site contract at £80 Free on site contract plusOR £60 to spend £45 to spend

A410/1 Taxan 770* on site contract at £85 Free on site contract plusOR £75 to spend £100 to spend

A420!1 Colour on site contract at £85 Free on site contract plusOR £100 to spend £140 to spend

A420/1 Taxan 770+ on site contract at £105 Free on site contract plusOR £100 to spend £140 to spend

A440/1 Colour on site contract at £50 Free on site contract plusOR £120 to spend £260 to spend

A440/1 Taxan 770+ on site contract at £60 Free on site contract plusOR £120 to spend £275 to spend

R140 Free on site contract plus Free on site contract plus£120 to spend £350 to spend

'Contract does not cover UHF Modulator. Above prices are ex VAT. Offer on current stocks onlyIf you do not like any of the frills we offer ask for our barebone prices.

TEL: 01 205 9558

.°3

- - ---1r I 1

so CONTEN'

Feb 1990.February

.1.- S Volume 16Number 175

Theme of the month inMarch will be LEADER CORRECTION

In the description of the pho-Components

Also in the March issue:Sinewave inverter

11 Launch of the digital communications era

AUDIO & HI-FI

tograph on the front cover ofthe December 1989 issue ofElektor Electronics, we stat-

Bucket brigade memories 23 Calsod now even more versatile ed that the 4.5 m dia. antenna

Surge plug A review had won a 1989 Queen's

Pause switch for cam- 34 PROJECT: Feedback killler Award for Technological

corders by T. Giffard Achievement. This was

Low-cost V/I display based on wrong information.

module AUTOMOTIVE ELECTRONICS In fact. Precision Metal Ltd.

SAVE decoder: Part 2. Temperature compensa-

tion for LCD modules

11 PROJECT: Car theft deterrent__

by David Butler

the designer and manufac-turers of the antenna.received a 1989 Queen's

COMPONENTS Award for Export Achieve-ment.

24 Introducing the OP -series opampsby J. Ruffell

44 AC -DC current sensors

COMPUTERS & MICROPROCESSORS

rt._,.1

-Ili14 PROJECT: Initialization aid for printers

h' A. Rigby.7

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32 Waveform modulation of the mains voltageh) A.M. Karailiev

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GENERAL INTEREST

s3 PROJECT: The digital model train - Part 11 Initialization aid for printers

Front coverNew singers and musicians.

by T. Wigmore62 PROJECT: Dark -room clock

p. 14

like Kathy and Ian shownby A. Rigby

here, can now set up theirown home recording studio,

INTERMEDIATE PROJECT I, *.\ i

using the 16 -track console 57 Part 8: Reflex MW AM receiver WOO0:11116

developed by Remix to pro- by J. Barefordfessional standards but at a 11non-professional price. RADIO & TELEVISIONThe Remix has proved attrac-tive not only to bands, corn- 27 PROJECT: SAVE decoder - Part 1 ANAposers and song -writers who by P.N.P. Wintergreenwant to mix and record at 38 PROJECT: PC Radiohome, but also to smaller, an ELV design Capacitance meter. p. 18

mid -priced commercial music 47 PROJECT: Video mixer - Part 2and film studios. by A. RigbyFeatures include: 6 auxiliarysends, separate tape inputs. TEST & MEASUREMENT t: r"peak and status LEDs, +4 dB Pk. -41.

- a,to -10 dB selectable, 8 bus I S PROJECT: Capacitance meter . %

with 16 -track monitoring. 8 by D. Folgereffects returns, 100 mm . Jr.

smooth action faders, direct MISCELLANEOUS INFORMATIONoutputs on all channels, sendand return patch points News 12 &13; New books 46: Events 51: Corrections 59;

Throughout, and monitoring Readers' Forum (letters & stiff itchboard) 60: Readers services 67

with equalization. Buyers guide 74: Classified ads 74: Index of advertisers 74S AVE decoder. p. 27

ELEKTOR ELECTRONICS FEBRUARY 1990

Please mention ELEKTOR ELECTRONICS when contacting advertisers

DEN HAAG HOLLAND

SELF-INDUCTANCE METERMeasuring self-inductance reliablynotoriously difficult and inductance 77.e!ersare, therefore, few and far between andalso quite expensive. The instrumentdescribed in Elektor Electronics September1988 has an LCD read-out, and achievesan accuracy of ± 1%. Four switch -selectable inductanceranges ore available: rums. -2 rnH, 20 mH. 200 mH

2 H.e d enclosure

£ 29.00S

or"PLOTTER"Contains all rnechc:-.:::(filed and turned,electro-magnets and 2 5-'.,ppermotors (100 steps/rev.)

Individual portsstepper motor:

£23.00£ 11.50£ 9.00

Stepper motor interfaceboardComple,--- - 871

£, 36.- 1 -

Plotter in kit form: £ 120. -Ordering andpayment

_

E.

CCC.2.int%irCete.:.t.:E7All payments must be a::: -:by full nameandPostageoncipoc,:r T.:orders.EXPORT: divide total value c 7:=" :7 .

1.185Meei--it Elektronika/Viz' der Service

.:2,--.sorach135AHAAG

Te - 70609554 (only-2_ - - -2-,r,aibus;riess

hours,

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o ° ° !", fJ

0 o 0* I

COLOUR TEST PATTERNGENERATORA PAL -compatible colour video source thatsupplies a number of test patterns for aligningtelevision sets.

E . - -LOe stabie, (mu: 7,2: on an CE: -

inc. case and front! £

..) 0 0.1. .

. ,. 4.!

. I 1

MICROPROCESSOR -CONTROLLEDFREQUENCY METER

a -_,15ional grade multi -purpose frequency meter, designedcy Eleidor Electronics, that con be built by many at affordableoost. Described in Elektor Electronics December 1984,January & February 1985. L1665B-based prescaler.

Frequency meter:0.01 Hz to 1.2 GHL

Pulse duration meter:0.1 s to 100 s.

Pulse counter:0 to 10' pulses

Period meter:10 as to 100 ns

Sensitivity:input A: 10 mV (Z-=2 Ma;Input B: TTL or CMOS compatible (Z=25 ka);input C: prescoler input:10 (2.-=50o).

Auto -ranging and completely menu -driven. 6 or 7 digit accuracy.

Kit includes power supply,prescaler and enclosure 169.00

VARIABLE DUAL POWER SUPPLYThe most frequently used equipment in an electronicslaboratory or workshop is an all-purpose power supply. Suchunit should not only provide o variable, stable, output, but alsobe able to withstand the occosionol overload. The supplydescribed in Elektor Electronics April 1986 does all that, twiceover!

Main technical features: Output voltage: 2 x 0 to 20V Output current 2 x0 to 1.25 A Internal resistance: 2 m Ohm Output ripple & noise: 5 mV. Minimum dissipation by virtue of pre -regulation circuit

Supplied with enclosure £ 149.00

BASIC COMPUTERIntel 8052AH-BASIC

89.00 1

FUNCTION GENERATORzn generator is without doubt on essential test

Tent in any electronics workshop or laboratory. It is-.2-spensible wherever sines -waves, triangle waves or square

.esare needed.Thefunctiongenerator described in ElektorElectronics December 1984 hos a number of features onlyfound on for more expensive ready-made units.

Frequency range: 1 Hz to 110 kHz in 5 decades External -voltage controlled: 0.1 V to 10 Von the VCO input

gives a frequency range of 1 to 100. Z '-'Co 1 m Adjustable output offset and amplitud

Kit, complete with supply and enclosure £ 69.00

IC TESTER FOR IBM-PC-XT/AT

..: 2 2.7 7.2- CC. t. . : test sofhvare.

Complete kit including software G847481a £ 60.85Ready Assembled Module GB474F £ 113.00Software, single G8474SW £ 17.85

S -VHS -RGB-CONVERTER SVR 7000SVR 7000 video recc-de eas of the new

. 76 generation con E.4. a TV sets- the TV set

. e SVR 70Y3

-:m the7.0

_ --ono 2-2 asca-- za cket are ableTr,e unit.

The !age supply is obtained from a 12V/21-1_ L. -DC vol-tage ma:h&c:dap:a-Complete kit GB497131( L.Ready Assembled Module GB497F

£ 76.25£ 176.00


Tr}

Editortpublisher: Len SeymourTechnical Editor: J. BullingEditorial Offices:Down HouseBroortffIl RoadLONDON SW18 4J0EnglandTelephone: 01-877 1688 (National)or +44 1877 1688 (International)Telex: 917003 (LPC 0)Fax: 01-874 9153 (National)or +44 1874 9153 (international)Advertising: PRB Limited3 Wolseley TerraceCHELTENHAM GL50 1THTelephone: (0242) 510760Fax: (0242) 226626European Offices:Postbus 756190 AB BEEKThe NetherlandsTelephone: +31 4490 89444Telex: 56617 (etekt nilFax: +31 4490 70161Managing Director: M.M.J. Landman

Overseas editions:FEDERAL GERMANYElektor Verlag GmbHSOsterfeld Strafe 255100 AachenEditor: E.J.A. KrempeisauerFRANCEElektor sariRoute Nationale; Le SeauB.P. 53: 59270 BailleulEditors: D.R.S. MeyerG.C.P. RaedersdorfGREECEEtektor EPEKariskald 1416673 Voula - AthenaEditor: E. XanthoutlsINDIAElektor Electronics PVT LtdChhotani Building520, Proctor Road, Grant Road (E)Bombay 400 007Editor: Surendra lyerNETHERLANDSElektuur BVPeter Treckpoeistraat 2-46191 VK BeekEditor: P.E.L. KersemakersPAKISTANEtectro-shop35 Naseem PlazaLasbella ChawkKarachi 5Editor: Zain AhmedPORTUGALFerreira 8 Bento Lda.R.D. Estefani, 32-1'1000 LisboaEditor: Jeremias SequeiraSPAINIngelek S.A.Plaza RepUblica Ecuador2-28016 MadridSWEDENElectronic Press ABBox 550514105 HuddingeEditor: Bill Cedrum

Distribution.SEYMOUR1270 London RoadLONDON SW16 4DH

Written and composed on Apple andIBM corporate publishing systems byEtektor Electronics (Publishing)

Printed in the Netherlands by NDB.Zoeterwoude

Copyright 1990 Elektuur BV

ABCtIEUM,13 OF 11+1 AZ.,OtT

ot

THE DIGILALNCH OF

AL COMMUNICATIONS ERA

Plans for the digital communications era of thefuture - paving the way for even more high-techapplications, such as the picture phone, ultra -fastfax and high speed data transfer - have been un-cited by British Telecom. The company has

launched a new advanced service to carry voice,data and pictures. called British Telecom ISDN2.

The new service-the first ISDN service inthe world to conform to the latest internationalstandards now being adopted worldwide-couldbecome, by the mid 1990s. the standard ex-change line service for all customers who wanttwo or more connexions. It enables a broad spec-trum of British Telecom customers to take ad-\ antage of information technology (IT) servicespreviously available only to large businesses. Inso doing. it will accelerate the introduction ofthe information society.

ISDN2 will provide high-speed digital ser-vices to branch offices of large companies aswell as to small and medium businesses. Suchser% ices have until recently been available onlyto large business sites. ISDN2 started as a testmarket at the end of last November and will be-come a fully public commercial service fromthe end of April.

The launch of ISDN2 follows a £23 millionorder with STC Telecommunications for equip-ment to provide up to 90 000 lines of network -capacity. This is equivalent to 180 000 ordinaryphone connexions.

The new service allows British Telecom'spublic network to meet customers' communica-tion needs for data. text. fax. graphics. and videoover a single high-speed digital connexion. Itcombines the power of advanced private net-works with the simplicity and universality of theordinary telephone.

ISDN2 provides customers with two high-speed digital exchange connexions on one pairof wires. Customers can use the new service tomake national and international phone calls inthe ordinary way and at the same cost. In addi-tion. they are able to make data, video and otherdigital services calls within the UK, and toFrance, Japan and the USA. More internationallinks are planned.

ISDN2 offers many benefits over existingservices, including:

even better quality basic telephone servicewith faster call set-up, clearer speech. andfewer data transmission errors:greater flexibility and efficiency. allowingcustomers to use their lines for data andspeech at will;lower cost data calls because of higher speedoperation:identification of callers on incoming callsand of called lines on outgoing calls on digi-tal end -to -end connexions:setting up of wide -area quasi private net-works over the public network:support for true integrated -services worksta-

tions combining voice and data to achieveimproved communications:longer term lower cost for ordinary telephoneconnexions because ISDN2 should eventu-ally prove cheaper than two separate lines.It is expected that call charges for inland calls

will follow the principle adopted by British Tele-com for its other ISDN services: the cost ofhigh-speed data and voice calls over digital linkswill be the same as inland telephone calls overordinary lines.

Connexion and rental charges have not yetbeen announced. Initially, they will be at a pre-mium compared to charges for ordinary ana-logue exchange lines, but because call chargesform the larger part of most business customers'phone costs, the benefits of ISDN2 will be avail-able for no more than a small increase in users'bills.

The initial network capacity of up to 90 000digital connexions will be rolled out over 18months. This will enable British Telecom tooffer an ISDN2 service from all its System Xdigital exchanges-currently totalling more than2000-by the end of next year. This will coverall business centres and recognized high streetsin the United Kingdom. Service will also bemade available in the near future on British Tele-corn's AXE 10 exchanges.

British Telecom has been working closelywith industry to encourage development of ter-minals that customers can connect to ISDN2.These are expected to become available from anumber of suppliers at about this time. Initially,they are likely to be the normal industry -stan-dard personal computers found in offices today,but equipped with ISDN communication cardsfor data/voice conferencing, tile transfer and dis-tributed processing. They will still be able toperform word processing and other office IT ap-plications. as well as operating as ISDN termi-nals. Later, integrated ke stems and small pri-vate branch exchanges are expected to be intro-duced.

ISDN2 will be able to carry many of the ap-plications now being run on private lOng-dis-tance networks and local area networks. Thesecould include EPOS (electronic data transfer atthe point of sale). mortgage and insurance quota-tions, and retrieval of the financial and commer-cial data many businesses use for their day-to-day activities.

In the next few years even more imaginativeapplications should appear, such as the view -phone. high-speed fax. and the transfer ofcoloured maps and diagrams originating fromoptical disc stores. Such applications would beespecially useful to small -to -medium businessesand branch offices: firms such as estate agents.advertising agents. designers. and other graphicsarts companies. publishers and consumer goodssuppliers. IN


NEWSSWITCH OF EUTELSAT SERVICES

EUTELSAT, the European Telecommuni-cations Satellite Organization, hasswitched some services from EUTELSATI -F1 at 16° E to EUTELSAT I -F2 at 7° Eand vice versa. Transferred from Fl arefull-time leases while in return telephony,European Broadcasting Union (EBU)transmissions and some occasional -usetraffic is transferred from F2 to Fl.

The changes are carried out to guaran-tee complete continuity of all services onEUTELSAT satellites. EUTELSAT I -F1 isnow entering inclined orbit after almostseven operational years. All telephony andmost EBU traffic can be carried by a satel-lite in inclined orbit since transmissionand reception are conducted via gatewayearth stations that track satellites in spacewith great precision.

THE ATV COMPENDIUMThe British Amateur Television Club hasrecently published the latest issue of itsmagazine CQ - TV. This includes as usuala varied and interesting selection of arti-cles, ranging from general topics such as"Using oscilloscopes" by Mike blooding,G6IQM, and "Broadcast band DX-TV re-ception" by Gary Smith and Keith Hamerto more technical articles such as "Cameratubes explained" by Peter Delaney,G8KZG. CQ - TV is a must for all thosewho are interested in this stimulatingbranch of amateur radio. You can obtainthis issue and much more by joining theBritish Amateur Television Club. For de-tails write (enclosing a SAE) to Mr D.Lawton, "Grcnehurst", Pinewood Road,High Wycombe HP12 4DD.

NEW SWITCHMODE ICSSiliconix has introduced two new switch-mode power -control ICs, the Si9112 andSi9120, which include the first device ca-pable of operating in 'universal -input'power supplies from either 110 V or 220V AC power lines.

The new devices allow the design of

highly efficient (greater than 80%) powerconverters with fewer components thanother semiconductor solutions, so that de-signers can improve system performance

while reducing board space and compo-nent costs.

CUSTOMER REVOLUTION INTELECOMMUNICATIONS

British Telecom has implemented one ofthe world's most challenging computerprojects to transform its services to cus-tomers.

The project, known as Customer Ser-vice Systems, draws together all the mainelements of the customer services BritishTelecom provides. All 23 million cus-tomers of British Telecom are now able tobenefit from its advantages.

Basically, it is state-of-the-art informa-tion technology in action, ensuring thatcustomers get the best possible response totheir needs.

FIRST COMPUTER MUSIC LEARN-ING CENTRE IN THE UK

Equipped with a wide range of the latestMIDI instruments, video and computers,EMR's Computer Music Learning Centre(CMLC) is based in Southend, Essex, andoffers a unique opportunity to gain first-hand experience of using EMR's extensivemusic software and hardware.

The CMLC will initially provide one -

day courses for beginners and more expe-rienced users during holiday periods andat weekends, as well as training for teach-ers and dealers.

The centre has been in the pipeline fortwo years, with EMR providing more andmore seminars to education and usergroups in the UK and Europe, particularlyon the Archimedes, althoug they have pro-duced systems for the Spectrum, Com-modore, Amstrad, MSX and BBC Microsince the company started in 1983.

The courses are under the direction ofMike Beecher, LRAM, GRSM, whose music

demonstrations at exhibitions will be wellknown to many of our readers. His pastexperience as director of a large Essexmusic school and first London synthesizerschool, creator of Electronics & MusicMaker (now Music Technology) and HomeStudio Recording magazines and frequentlecturer in Europe should make your day aworthwhile event-he has also designedthe software you'll be using with his teamof programmers, so you'll be finding outnew ways of making music with yourcomputer.

Full details from EMR CMLC, Suite 1-3,50 Hamlet Court Road, WESTCLIFF-ON-SEA SSO 7LX, Telephone (0702) 335747.

US TAKE-OVER BOOSTSRACAL'S CAE PRESENCE

Britain's Racal Electronics Group, alreadydominant in computer -aided design (cAD)systems for printed -circuit boards, hasstrengthened its presence in computer -aided engineering (CAE) with a £12 milliontake-over of the American HHB Systemssimulation software company.

HHB Systems, based in Mahwah, NewJersey, was set up in 1977 to develop andsell high-performance logic and fault sim-ulation software for use in the design andtest of complex electronic circuits. Sincethen, the company has gained over 200customers worldwide with sales worth S12million a year and its products have be-come established as industry standards.

The two companies have a working re-lationship dating back to 1984, and HHB'sproducts are already fully integratedwithin Racal-Redac's workstation -basedelectronic design autopmation (EDA) sys-tem, Visula Plus. At present, the combina-tion of Visula Plus and CADAT is the onlyproven and commercially available systemthat can design and simulate applicationspecific integrated circuits (Asics) in thecontext of their target systems.

POWERFUL WEAPON AGAINSTCANCER

A new computer developed by the ActiveMemory Technology (AMT) company ofReading is contributing to what has beendescribed as one of the greatest scientificadventures in human history: to map andidentify all human genes.

The aim of the project is to decipherthe complete set of biological instructionsused by nature to make a human being.The resulting "Book of man" may providethe basis for preventing or treating mosthuman diseases in the next century. Its tar-get completion date is the year 2000.

The human genome consists of chemi-cal beads twisted into the famouse doublehelix of the deoxyribonucleic acid (DNA)molecule. There are four types of bead,


known scientifically as bases. They are thefour chemical letters of the genetic code(A, G, T and C) discovered by James Wat-son and Francis Crick at Cambridge Uni-versity in 1953. Watson is one of the lead-ers of the human genome project in theUnited States.

DUAL -OUTPUT HALL EFFECTSWITCHES

Industry's first family of dual -output Halleffect switches with outputs that are inde-pendently activated by magnetic fields ofopposite polarity is available from theSprague Semiconductor Group.

The UGN3235K and UGS3235K Halleffect sensors are bipolar ICs designed forcommutation of brushless DC motors andother applications that use multi -pole ringmagnets in industrial and automotive envi-ronments.

MORE HEADACHES FOR IBMShortly after the news that IBM's tradefigures were "disappointing" and that thecompany was consequently laying off10 000 employees, it was announced thatthe US Navy is to impose an indefiniteban on doing business with IBM.

The Navy decided on the ban after ithad come to light during a House Govern-ment Operation Committee hearing thatIBM had defrauded the US Navy by sell-ing it used computer equipment as new.

As if that was not enough, a group ofclone makers* have defined their EISA32 bit bus standard, which is compatiblewith the ISA-Industry Standard Archi-tecture-system. IBM had hoped to keepthe clone makers out of the 32 -bit marketby keeping their 32 -bit (with data path)standard-Mi croch an n el Architecture,

MCA-to itself. It seems, however, thatIBM has seriously underestimated thestrength of the clone makers.

*AST Research, Compaq, Epson, Hewlett-Packard, NEC, Olivetti, Tandy, Wise,Zenith.

SATELLITE ACCESS SYSTEMTO BEAT TV PIRATES

A new British Telecom satellite manage-ment system will allow broadcasters tokeep track of their customers and avoidpiracy of TV entertainment services. It canalso prevent pictures being received inspecified areas.

The BT Vision system uses transmis-sion and encryption technology to ensurethat only authorized subscribers can re-ceive satellite TV programming, and of-fers programme providers the technologyto control transmission of potentially of-fensive satellite broadcast material.

The system effectively prevents piracyby securely encrypting the programme sig-nal with the aid of a complex algorithmbefore it is uplinked to the satellite. Astream of authorization messages is sentout along with the television pictures, au-thorizing subscribers' set -top decoders toaccept the programming signal.

A very useful feature of the system isits ability to black out pictures in specificlocations. broadcastswill not be received in countries where thedistribution rights have not been agreed orwhere the broadcasting control body con-siders their content unacceptable (so muchfor those who thought that the age of cen-sorship was over!).

NEW SUPPORT SHIP FORTRANSATLANTIC CABLE

Cable & Wireless Marine's new cableship, the Sir Eric Sharp, is said to be themost technically advanced vessel of itskind in the world. It has to be, because it isdesigned to service and maintain theworld's first private trans -ocean fibre opticlink between Britain and the United States.This is the PTAT-1 system, part of Cable& Wireless Marine's global digital high-

way linking the major financial centres ofthe world. The vessel will also be used tomaintain existing analogue coaxial cables.

To enable the vessel to pick up and laycable in the most efficient manner, it is thefirst of its type to be equipped perma-nently with a submersible remotely oper-ated vehicle (Rov).

EARTH STATION FOROLYMPUS LINK

One of the two £750,000 mobile earth sta-tions designed and built by BritishAerospace under contract to the EuropeanSpace Agency has been handed over toone of Britain's leading academic institu-tions for communications technology andeducational broadcasting, PolytechnicSouth West in Plymouth.

The station, known as TDS 4, will beused for demonstration purposes. It willenable the polytechnic to communicate viaOlympus, the world's most powerful civilcommunications satellite, which was alsobuilt by British Aerospace.

The TDS 4 earth station is mounted ona 12 -metre long articulated trailer and in-corporates its own auxiliary diesel genera-tor. In operation, it will transmit in the13.00-13.25 GHz and the 14.00-14.30GHz bands, and receive signals in the12.50-12.75 GHz band. The antenna fittedis a 3.5 metre design with hinged sidesthat can be folded down for ease of trans-port.

EEC MEASURES TO LIMITADVERTISING ON TELEVISION

Britain is supporting the proposed EECmeasures to limit the amount of advertis-ing on TV and the number of Americanprogrammes shown. The new measureswere agreed at a recent meeting of EECforeign ministers.

Under the new rules, no advertisementwill be allowed during news, religious orchildren's programmes under half an hourlong. Advertisements will be allowed dur-ing films after the first 45 minutes.

The decision is unlikely to have mucheffect in the UK where standards areamong the strictest in Europe.


jAL] AT au K)k MON -

A. Rigby

Many computer users have difficulty in changing printer settingswithout the help of a manual or handbook. The circuit presented here

allows one of sixteen pre-programmed printer settings to be calledup at the flick of a switch. Very useful for a good many computerprograms that lack a real printer driver, the initialization aid is a

low-cost and simple -to -build circuit.

You are working hard on a BASIC pro-gram which is almost finished, and hardcopy on paper is required to do the finaldebugging. Since the program is fairlylong, you think it wise to set the printer tocondensed characters at 8 lines per inch.The printer manual is consulted and thefollowing command is typed in the directcommand mode of GWBASIC:

LPRINT CHRS(15);CHRS(27);"0";

We think you will agree that pushing asmall button on the printer initializationaid is a lot easier than having to look upeight or so ASCII characters, and typingthem into the computer whilst observingthe correct order, brackets and delimiters.

In the above example, BASIC fortu-nately allows the printer to be set to therequired mode, albeit in a rather complexway prone to many errors. There are,however, many programs that lack eventhe most rudimentary means to set theprinter to a particular character or paperformat. For such programs, the initializa-tion aid is a useful peripheral, provided itssetting is not overridden by a printer in-itialization string prefixed to each print-able file by the program in question.

Basic operationThe flowchart in Fig. 1 illustrates the basicoperation of the circuit, which is insertedbetween the computer's Centronics portand the parallel input of the printer. Thediagram shows the functional blocks inthe circuit as well as the interrelated tim-ing at which these blocks operate.

At power -on, the circuit is reset andforms a straight 36 -way connection be-tween the computer's Centronics port andthe Centronics input on the printer. Nochange is made to the previously estab-lished printer setting, and the computercan send printer files as before.

When 52 is pressed, data buffer ICsblocks the datastream from the computer.The circuit actuates the BUSY line to forcethe computer to stop sending data. Mean-while, the data outputs of the EPROM,IC-, are enabled, so that the byte at thecurrently addressed location in theEPROM is sent to the printer. A strobepulse is generated to signal to the printerthat a byte is held ready for transmission.Since the start of the first strobe pulsecoincides with the moment the EPROMdatalines are enabled, the strobe pulse isdelayed by about one microsecond to en-

sure stable levels on the datalines. Afterthe STROBE output line is actuated, thecircuit enters a wait loop to allow theprinter to process the databyte. Depend-ing on the position of jumper JP', the cir-cuit waits for a negative pulse transitionon the BUSY or ACK handshaking linefrom the printer. When the transition ar-rives, the EPROM address counter is in-cremented by one. The counter starts atthe lowest address, nil, by virtue of thepower -on reset, and the fact that the cir-cuit stops automatically when the counterreaches nil. In the latter case, the "all 16bytes sent?" loop in the flowchart is leftvia the "yes" exit, and the circuit forms astraight connection between the computerand the printer until 52 is pressed again.

All 16 bytes stored in the EPROM areto the printer under the control of strobepulses, which are started with the aid ofthe delayed BUSY or ACK printer signal.The delay allows some time for the ad-dress counter to reach the next higher out-put state. When all bytes have been sent,the same delay is used to reset the circuitto its start configuration.

16 <16 in controlA part of the circuit drawn in Fig. 2 consistof a set of printed -circuit board trackscalled the control bus. This bus takes allCentronics control signals not used by theinitialization aid from the input- to theoutput connector. Two lines, C18 and C35,form an exception, because they may beused to power the circuit. Whether or notthey can be used for this purpose dependson the printer used. Diodes D2, D3 and D4form an OR function that allows the cir-cuit to be powered by different supplies.In case the printer lacks a +5 V output onits Centronics connector, the circuit ispowered by a 9 VDC mains adapter via D2and voltage regulator ICs. If the printersupplies -3 V via line C18, the circuit ispowered via D3. If +5 V is available on lineC35, D4 is used for the same purpose. Thecurrent consumption of the circuit doesnot exceed 50 mA.

As already discussed, the circuit isreset automatically the moment the sup-ply voltage is present. Bistable FF2 is resetby 12,-C2, and in turn clears addresscounter IC5. Bistable Hi is configured asELEKTOR ELECTRONICS FEBRUARY 1990

IIINITIALIZATION AID FOR PRINTERS

Switch ICS 6.-Itp..utmto 3 -state. Blockcomputer data;

enable data outputsof EPROM IC7

Put byte at EPROM ad-dress on datelines;start first strobepulse (R10/CS)

Generate firststrobe pulse

(R12/C4)

Increment addresscounter

(get next bytefrom PROM)

Disable EPROM data -lines and enable

buffer ICBJestoredatafiow between

computer andprinter

Star nextstrobe pulse usingdelayed nagrie

BUSY or ACKtransition

900007 -

Fig. 1. This flowchart illustrates both the structure of the circuit an the pertinent timing.

a monostable multivibrator (MMV) anddoes not require a power -on reset becauseit goes to the stand-by state on its own. Atthis stage, the circuit functions as a 36 -way connection between the computer atthe input (K)) and the printer at the output(K2). The two devices are connected viathree -state inverter/buffer ICs, invertersN4 -Nib and the gates inserted in hand-shake lines BUSY, STROBE and ACK.

When S2 is pressed, its contact noise iseliminated by debounce network R7-Ci.The short pulse generated by CI -Rs pre-vents the initialization sequence being re-started on completion if the switch has notbeen released in the mean time. Actuationof S2 causes FF2 to be set. The resultanthigh level at the Q output of FF2 causes theoutputs of three -state buffer IC8 to beswitched to the high -impedance state, andthe BUSY and ACK inputs of the computerELEKTOR ELECTRONICS FEBRUARY 1990

to be taken high by N1-Nr7 and 1\12 -Nisrespectively. The low level at the Q outputof FF2 enables the databuffers in EPROM1C7, and ends the reset state of counter IC5.The first strobe pulse is generated by FFiwhich receives at its S (set) input a shortpulse from the Q output of FF2 via net-work Rio -05. The length of the strobepulse is determined by R12 -C4. CapacitorC4 slowly discharges after FF1 has beenset, and resets the bistable. Consequently,output Q goes high, so that C4 is rapidlycharged again via Di. This ensures thatFFI is reads' to generate the next strobepulse.

The strobe pulse is delayed by aboutone microsecond in network toallow sufficient time for the EPROM datato reach output connector K2. Gates Nsand Ni. feed the strobe pulse to theprinter.

S1

address rangeinitialization data

0 0 0 0 00 ... OF0 0 0 1 10 ... 1F

0 0 1 0 20 ... 2F0 0 1 1 30 ... 3F0 1 0 0 40 ... 4F0 1 0 1 50 5F

0 1 1 0 60 ...OF0 1 1 1 70 ... 7F1 0 0 0 80 ... 8F1 0 0 1 90 ... 9F1 0 1 0 AO ... AF1 0 1 1 BO ... BF1 1 0 0 CO ... CF1 1 0 1 DO DF

1 1 1 0 EO EF

1 1 1 1 FO FF

0, -switch closed1=switch open

9C0007 -T1

Table 1. DIP switch settings.

The circuit waits until the printer isready to accept a new command by moni-toring either BUSY or ACK. The selectionbetween these handshaking signals ismade by the user with jumper JPI; the twoare equally suitable, provided the printersupplies the relevant signal. Whateversignal is used, the negative pulse edgesignals readiness to accept a new charac-ter. The pulse edge clocks address counterICs and causes a new strobe pulse to begenerated. The start of the strobe pulse is,however, delayed by R) -C3 to prevent an-other pulse being generated when the16th (last) byte has been sent to theprinter. This delay is used by the circuit toblock FFi before it receives a new clockpulse. The end of the printer initializationsequence is marked by output QD of theaddress counter going low when count 16is passed. The change from 16 to 0 causesFF2 to be clocked, so that FF1 is blocked -hence, the clock pulse generated in themean time by RII-C3 has no effect. Thetoggling of FF2 also resets the circuit to itsinitial state.

The circuit is fully compatible with theCentronics interface on the computer aswell as on the printer by virtue of pull-upresistors and open -collector TTL driversrespectively. DIP switch block Si selectsone of 16 printer initialization stringswhich have been pre-programmed in theEPROM. The selected address ranges inthe EPROM are listed in Table 1. If fre-quent changes are expected in the settingof this switch, it may be replaced with amore ergonomical type or a set of swit-ches, e.g., four miniature SPST types or athumbwheel switch.

ConstructionThe construction of the circuit on thedouble -sided, through -plated printed-cir-

16 CONIPUTERS AND MICROPROCESSORS

Kt

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ACK

10

5V0 D2 i 8...15V=IC9

IN 00ocIN400, cs C9o (5 6 ci 6 Cii 6 ci 6,1. ...._ .,7805

1C8 IC6 IC5 77,0,,,IC4 mr00,IC3 .700,IC2 .700,1C11730,'YTTI 1 1TIT1' TjOn

.0.0902007. 2

JP1

Fig. 2. Circuit diagram of the printer initialisation aid, a very useful peripheral device for advanced computer users.


INITIALIZAIION AID FOR PRINTERS it

STROBE ; GILDDATA 0DATADATA 2 GILDDATA 3 GHIIDATA

= 7.:GHD

DATA 5 GILDDATA 5 " GI:13DATA 7 p GILD

G/:15BUSY = GILD

PE _ - ISPITTPPIITE Ps URN (GIID)

AUTSOTEIIICT - = FIEgliglINPUT PRIME)

r:=N.C. LOGIC CND

LOGIC MD . H.G.CHASSIS 7-= 451,

5Y SELEC

900007. 13

Fig. 3. Centronics connector pinning.

cuit board shown in Fig. 4 is straightfor-ward. The pins of the Centronics connec-tors are simply pushed over the relevantcopper fingers at the edges of the board

(mind the position of pin 1 of each connec-tor). Next, solder the fingers to the pins,taking good care to avoid short-circuitsbetween adjacent pins. Bolt the voltageregulator direct on to the board -a heat -sink is not required.

Printer command stringsLpon completion of the board, theEPROM must be loaded with data. Themanual supplied with your printershould give ample information on com-mand strings to achieve the settings yourequire. The circuit allows a maximum of16 command strings to be programmed,each with a fixed length of 16 bytes. Themini EPROM programmer described inRef. 1 is suitable for loading the 256 bytes.If the required initialization sequence isshorter than 16 bytes, the remainder mustprovide null characters (00), or any othercharacter which is ignored by the printer(again, consult your manual).

The sequence of commands to be pro-grammed in the EPROM Type 2764 is firsttested 'on line' with the aid of, say,Gl\" BASIC. Document the final versionsof the command strings. Next, compile the

7

0 0 0 0 0 0 0 0 0 SI

00000000 0000O 0 0 0

** IC7

O i O e "I

0-101-° Oi0 R'12V't 01F0 R11 to

C 0-01 FO 0 n 'L_110 Woo

IC 6

0 000F.0 01R9 10 00110C'1 shoo 01R13

0,04 E0 oilolp 10

0 0n

A

'3 0-01F°C7

000m

80:>0-11-0

N

:17

0

0

000+

.g. 0--144-0

A

0 o11

Fig. 4. Component mounting plan for the double -sided. through -plated circuit board.

10 RESTORE12 OPEN " 1ptl : " FOR OLTITIIr AS #120 FOR N=1 TO 1630 READ X90 PRINT #1 ,CliRS( X) ;50 NEXT N55 CLOSE60 DATA &1154,E..H45,E..H53,&.H5465 DATA E.110005.1i00,&.1-100,E.H0070 DATA &1454 4H45 ,b.H53 , W-15475 DATA &H0O,E.H00,&}-100,E,H00

Jf.

Fig. 5. GWBASIC test program listing.

databytes to be programmed from

databyte = 255-data.

This inverts each byte to negate the inver-sion in the output buffers.

Reference:

1. Mini EPROM programmer. ElektorElectronics January 1990.

Parts list

Resistors: = SIL resistor array 8x10kRz - Rs = 10kR7 = IMO

- Ris = 2k2Rs = 100k

Capacitors:CI = 10nC2 = 470nC3:C6 = 1n0C4 = 4n7Cs = 470pCnCe;Cio-Cia= 100nCs = 330n

Semiconductors:Di = 1N4148D2;113:D4 = 1N4001ICi = 74HCT02IC2= 74HCT132IC3:IC4= 74HCT05ICs = 74HCT93ICs = 74HCT74IC7 = 2764 cr 27C64tA)ICs = 74HCT540ICs = 7805

Miscellaneous: = 36 -way female Centronics connectorwith straight pins.K2 = 36 -way male Centronics connector withstraight pins. = 4 -way DIP switch block.S2 = Dataswitch push-button.3 -way pin header.jumper.PCB Type 900007 (double -sided: through -plated; see Readers Services page).

ELEKTOR ELECTRONICS I:HIRE-ARV 1991-)

CAPA TAN_

D. Folger

A capacitance meter is indispensable for checking capacitors withillegible or incomprehensible values printed on them, and formatching capacitors in, for instance, higher -order filters. The

instrument presented here is based on a simple circuit, has a handysize and five measurement ranges.

To own a capacitance meter with a maxi-mum error of 1 ci and a capacitance rangeof 1 pF to I F is wishful thinking for many (1electronics enthusiasts because such an in- I

strument is not affordable, if it is at allavailable. Fortunately, not many applica-tions require a capacitance meter withsuch impressive specifications. Consider,for instance, the selection of capacitors fora higher -order audio filter: here, therelative difference is much more import-ant than the absolute value, and a simplecapacitance meter may be used with im-punity to select matching capacitors froman available lot.

Measurement principleThe measurement principle used is fairlystandard - see Fig. 1. The heart of thecircuit is formed by a time -base whichtriggers a monostable multivibrator\INIV). The time constant, t, of the time

base is set to a value that exceeds the maxi-mum monostable delay. The \I\ 1V outputchanges from low to high on the negativeedge of the time -base signal. The time ittakes for the MMV to revert to the startstate, L, is proportional to the value of thecapacitor under test, C.. Since the dura-tion of the measurement cycle is deter-mined by t, an integrator may be used toprovide a voltage which is proportional tothe value of

Circuit descriptionIn spite of the relatively simple measure-ment principle described above, the prac-tical circuit presents a number of possibleproblems related to the translation of theunknown capacitance into an accuratelydetermined numerical value.

The time -base is formed by a 1 -MHzquartz crystal and an oscillator/divider,IC'. Output Q13 supplies a frequency of1 MHz/16,384 = 61.035 Hz. This signaltriggers IC b, one of two NI NI Vs containedin the Type TLC556 LinCMOS dual timer.The choice of the oscillator frequency is acompromise between the stability of theread-out and acceptable current con-sumption when relatively large capacitan-ces are measured.

Given a time constant

t = 1/61.035 = 16.4 ms,

2 20

MAIN SPECIFICATIONS

Measurement ranges:

Max_ resolution:Max. error:Display:Scale factor:Power supply:

Current consumption:

2 nF; 20 nF;200 nF; 2µF;20 µF withoverflowindication1 pF<5%31/2 digit LCDnF/i_tF9-V PP3 bat-tery10-20 mA

a maximum monostable time of 10.5 insand a maximum capacitance of 20 j.tF, theresistor R in the RC delay network is

simple to calculate from

t =1.1RCR = 10.5/(1.1x20 µF) = 477 LI

A practical value of 475 LI (1 `", is usedsince this is available in the E96 series. Theother measurement ranges are created bymultiplying the basic value of R by thecapacitance range factor, i.e., 10: R = 4k75for the 2µF range; R = 4715 for the 200 nFrange, R = 475k for the 20 nF range; R =4M75 for the 2 nF range.

The output signal of ICs has a dutyfactor t -/t and requires averaging to ob-tain a direct voltage proportional to theduty factor and, therefore, to the value ofCs. Integration capacitor Cu is charged viapotential divider R14-P3-Ris if the MMVoutput is high, and discharged if the MMVoutput is low. This arrangement producesan average voltage,

U2 = 1-71(11)(tw /

Note, however, that because of the ad-justable potential divider the high level,U t3,, is lower than the high output levelprovided by the MMV. The maximumvalue of LI: is not reached immediately,but after a delay of about 600 ms, whichcorresponds roughly to the time constantof the integration network.

The 150 pF capacitor connected in par-allel with Cs is required as a minimumcapacitance in the external RC network ofthe LinCMOS timer. Without Cs, the MMVmay not be triggered reliably if small ca-pacitors (in the pF range) are tested. Theresultant off -set is compensated with theaid of a second MMV, [Cm, of which theexternal configuration is almost identicalto that of IC3b.

The low voltage, UL, at the output ofthe second integrator is used as a refer-ence for UH. Capacitors C7 and Cs andresistors Rs -R12 determine the time delaysset with the MMV, and must, therefore, beclose -tolerance types.

Spurious triggering and incorrect ca-pacitance indications may occur ifmeasured values exceed the maximum ofa particular range. Each measurementcycle is, therefore, stopped after 12 ms. Asshown in Fig. 3, the remaining 4 ms areused to discharge the capacitor under testvia FET T2. The timing of the discharge


CAPACITANCE METER 19

AFY,

, voto:

Time BaseCr1

7Kccti-,,

C i i

r.

1C":7.5 IF.C.74. 2;:.,.:7.-s 2Kms 30:7.-4 :::.:7-5 4:

Tile 900012-12

72

1

.: ,.. .:.

KC .

. _

Fig. 1. Measurement principle and graphs to illustrate the operation of the R -C integrator.

period is arranged by the AND diode con-figuration of D3 and D4 at the Q12- andQ13 -outputs of IC!. These diodes causethe MMV output to remain permanentlyhigh when a too large capacitor is con-nected. In this condition, the voltage at thetwo NINIV outputs causes the LC displayto indicate '1'. The measurement error in-

troduced by Tr is compensated by itscounterpart, Ti, in the other MMV circuit.

The display circuit is based on the well-known Type 1CL7106 A -D (analogue-to-digital) converter with integrated31/2 -digit LCD (liquid crystal display)driver. The 1CL7106 is used in a standardapplication circuit with the decimal point

switching arranged by S213, D15+1319 andN: -N;. The scale factor (nF or uF) indica-tors, DI and D2, are driven by a diodecircuit, D2o-D!4.

The power supply of the circuit is of aless usual configuration: a Type 79L05voltage regulator is inserted in the nega-tive supply line to create a supply with+5 V and -4 V outputs. The non -regulatednegative voltage is applied as bias to theADC in the ICL7106, which requires thatthe voltage at the v- input (pin 26) is al-ways 1 V below that at IN HI.

The circuit around T3 controls the LOWBAT symbol on the display. With Ri4=220k,this circuit is actuated when the batteryvoltage drops below about 8 V.

Construction andadjustmentThe complete circuit is built on a printed -circuit board that fits into an ABS, stand-ard -size hand-held enclosure withintegral battery compartment. The circuitto the right of the dashed line in the circuitdiagram, and diodes D15 -D24, may beomitted if the capacitance meter is used inconjunction with a digital multimeter thatassumes the display function.

The population of the PCB should notpresent problems. The high -value 1% re-sistors, Rs and RI2 (4.73 Mil), may be dif-ficult to obtain in small quantities. Theymay, however, be selected from a batch of4.7 MS2 5% types with the aid of a D\IM.

Fit two four- or five -way terminalblocks on to the front panel of the enclo-sure for connecting the capacitors to betested. Be sure to use high -quality termi-nals to avoid problems with wear and tearof the contacts. A two-wav loudspeakerterminal block for wire insertion is a finealternative to multi -way blocks since itprovides fairly wide contacts which allowcapacitors with different terminal spac-ings to be inserted.

Large capacitors of which the wires cannot be inserted into the terminal blocks onthe instrument must be connected withtest leads. Do not use these leads for smallcapacitors, since the wire capacitance willmake the indicated value worthless. Pi is


20 TEST AND MEASUREMENT

5V

cx

7

503

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903312

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Fig. 2. Circuit diagram of the capacitance meter.

either a preset with an adjustment spindleor a potentiometer with a 4 mm spindle. Itallows test lead capacitance to be compen-sated.

The accuracy of the instrument isdetermined mainly by the quality andtolerance of resistors 129-R12.

Fig. 3. The measurement cycle is stoppedafter 12 ms to discharge too large capaci-tors.

Connect a 1%. or 2% polystyrene (sty-roflex) or silver -mica capacitor with aknown value in the lowest measurementrange (e.g., 1 nF). Set S2 to the 2 nF range,set Pi to the centre of its travel and P2 andP3 to maximum resistance to ground. Con-nect the reference capacitor, and adjust P4until the correct value is displayed. Dis-connect the reference capacitor and adjustthe zero -indication of the meter with P2.Repeat the adjustments of P2 (capacitorconnected) and P4 (capacitor discon-nected) until the indicated value and thezero indication are both correct.

DMM as displayThe value of the test capacitor must beadjusted with P3 instead of P4 if a DMMset to the 2 V range is connected to pointsL and H. The 'meter -zero' control, Pi, isreplaced by a 5.11 kfl 1 resistor in thiscase because the adjustments for meterzero and capacitor value interact. Set P3 tomaximum resistance to ground, and ad-

just P2 until the DMM reads 0 V. Connectthe reference capacitor and adjust P3 untilthe correct value is indicated. Repeat theadjustments of P2 and P3 as detailed abovefor P2 and P4.

Close-up of the battery compartment.

EI.EKTOR ELECTRONICS FEBR \R1" Ivvu

111CAPACITANCE METER

a

r

1 .7a

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

R22

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.2.

LO 0-I I-0 rjI

R25 W1:1 P1241 V6

N0_0 CC) a.0-° °-cue-o o-oi Fo a 0a H

0 0C13

-0- U_ r rL teti-P4 111-Et II a a

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r0,1

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0 0 0 0+0

0- N

-WI 610_a_cg-L0/*

ON -ilk- METER

nF

itek. 0

( )

e

j iF

+IIM11=11141111MIIMMIP

e

PF00nF

*-1111111=1111NrrillFig. 4. Component mounting plan of the printed -circuit board tor the capacitance meter (left) and suggested front -panel lay -out (right).

Parts list

Resistors:= 10M

F12;Rzo;R2i;1322;Rz5=100kR3 = 27k134 = 4750 1%1:15;R_ = 4k75 1%1:15;R:a;Rze = 47k5 1%Ft;R: t = 475kRa;RI2 = 4M7 5% (see text)Ris= 15k 1%R:4;R:5 = 10k 1%Ris;F117 = 111#10

Rta= 1k5R g = 220kR23 = 1M8 5%R24 = 200k 1%R27 = 22k1 1%Pi = 10k linear potentiometer with 4 mm

spindle or 10k preset for horizontal mount-ing with spindle.P2;P3 = 10k multiturn presetP4 = 2k mutlitum preset

Capacitors:Ci;C2;011;C12 = 2211;16 V; radialC3 = 100nC4;C5 = 33pC6 = 1n0Ci;Cs = 150p polystyrene (styroflex)Cs;Clo = lOnC13;CI5 = 100n MKTCis = 100pCt6 = 470n MKTC17 = 220n MKT

Semiconductors:Di;D2 = LED; 3 rnm; redD3 - D24 = 1N4148TI;T2 = BS170

T3 = BC5478ICI = 79L051C2 = 74HCT4060IC3 = TLC556 (LinCMOS)IC4 = CD4030ICs = ICL7106

Miscellaneous:= miniature slide SPST slide switch.

S2 = 2 -pole 6 -way PCB -mount rotary switch.X: = 1 MHz quartz crystal.LCD: = general-purpose 31/2 -digit LC dis-play.9-V PP3 battery with clip and leads.Hand-held ABS enclosure, e.g.. MonacorPI.G750BN (160x80'75 mm).Loudspeaker terminal block.PCB Type 900012 (see Readers Servicespage).

ELEKTOR ELECTRONICS FEBRU-kRY 1990

CA THEFT INTby David Butler

Cars seem to attract thieves, probably because of the relative easeof entering them. Anyone who has locked their keys inside will

testify that a bent coathook can usually secure an early reunion.The deterrent proposed is aimed at discouraging casual joyriderswho want not want to risk setting off an alarm - even if, as here,one did not exist. In fact, the deterrent is simply an alarm typeindication that, if accompanied by suitable warning stickers,

should provide basic protection. It must be stressed, of course,that this deterrent does not act as an alarm and it would be wise toinvest in a security system, such as an immobilizer or a Krookloc.

The idea of a dummy alarm is not new, butthe present design adds more credibilityby the use of a single dual function LED.When the ignition is on, the LED radiates asteady green light to show that the 'sys-tem' is 'disarmed'. When the ignition isswitched off, the LED emits a flashing redlight to simulate 'alarm enable'.

The success of this idea rests with dis-playing the LED prominently (say, next tothe ignition switch) and not telling any-body that it is just a flashing light! The pro-ject is inexpensive, simple to construct andinstall, and performs a worthwhile func-tion.

Circuit descriptionThe circuit is designed around a dual func-tion LED. This device looks like a normal

mm diameter clear LED, but has threeterminals. The casing actually houses agreen LED and a red LED driven by aflasher unit. The cathodes of the LEDs arecommoned to the centre pin. When a volt-age in the range 4.75-7.0 V is applied tothe 'R' terminal, the red LED will flash at arate of about 1.3 Hz. The green LED is con-nected to the other terminal (indicated by a

Features:

Deters casual joyriders fromstealing your vehicle.

Inexpensive, simple design.

Compact dimensions.

Easy to install (3 wires).

flat on the casing) and requires a seriescurrent limiting resistor to operate fromany supply.

The relay used is of extremely small di-mensions, and has a single -pole change-over contact set. The coil is energized bythe +12 V ignition circuit in the car, withback e.m.f. protection provided by D2.

The LED terminals are connected to therelay contact set so that normally the redflashing LED is actuated (ignition offstate). The regulator circuit formed by RI,Di and CI provides a fixed voltage for bothLEDs. This was chosen to avoid problems

Fig. 1. Circuit diagram (ignition switched off).

COMPONEIITS LISTR,-.s.tstors:

= ,:L25 ±5.r. :r titter: carbon- L5 defter: carbon

o c Ito r

Semiconductors:DI = EV1:400 mW: ze re oddeD2 =1N4all

contnuous green,fiashing red LED(Maplin Electronics order code

Miscellaneous:RLA = rMorc _ _

:= -2 V 22C3R

ectronics orce- _ E.- -

= E.. PCBS- = -:_sing Ors conia::i- :

F::? :!-- 150

Fig. 2. Printed -circuit board of the deterrent. Thecomponent side is shown true to scale.

ELEKTOR ELECTRONICS FERRI -ARV 19911

AUTOMOTIVE ELECTRONICS

Fig. 3. Pinout of the relay (seen from the base.)

with the cars voltage supply.The circuit consumes about 50 mA

when the ignition is on and around 15 mAwith the ignition switched off.

Construction notesThe prototype was constructed on a scrapof vero board and then transferred on to aPCB (see Fig. 2). Although the method ofconstruction is not important, care shouldbe exercised in connecting the polarizedcomponents: Di, D2 and the LED.

Installation notesThe prototype unit was installed in the au-thor's Metro, which conveniently has aspare blanking plate next to the rear screenheater switch. This is quite close to the ig-nition switch and would be instantly no-ticed by any curious potential thief.

No doubt, other vehicles will have sim-ilar places to mount the LED, which re-quires a 6.35 mm dia. hole.

Once the LED is mounted, three wiresrun from the PCB may be connected to itwith the aid of Scotchlock type break-interminals. Usually, the set of wires leadingto the radio can be used: +12 V continuousmeans that +12 V is available even whenthe ignition is switched off; +12 V ignmeans that the +12 V line is switched viathe ignition.

For extra electrical protection an in -linefuse holder with a 150 mA fuse may beused: this rating depends on that of the carfuse fitted to the circuit being used.

Apart from these brief notes, no otherfitting instructions can be given as eachvehicle will vary.

Generally, the unit should be fittedaway from sources of direct heat, with theLED displayed in a prominent positionwhere it does not distract the driver.

CALSOD NOW EVE\ MORE VERSA171The computing and optimalization of loudspeaker enclosures isnowadays normally effected by computer. One of the programsthat enables the complex calculations to be carried out on an

IBM PC desktop computer is CALSOD, the first version of whichwas reviewed in this magazine last year. We have recently received

an improved version of this program that offers even morepossibilities to the professional designer to approach realistic

sound reproduction.

Designing loudspeaker enclosures re-quires such extensive measurementsand calculations nowadays that it hasbecome virtually impossible without theuse of a computer and a suitable pro-gram. Last year we reviewed* CALSOD,a unique combination of a simulationand an optimalization program. We thenthought that there was very little left tobe desired. None the less, the designershave succeeded in adding some morefacilities to their latest version 2.00,which bring the results even moreclosely to realistic sound reproduction.

The new version offers the possibil-ity of working with a coprocessor. Thisis an especially welcome addition forXTs, since computations on these of three -or four-way systems are relatively slow(but still a lot faster than with comparableprograms). However, this facility is merelyto do with speed of processing.

To us, the most interesting addition isthe RAB sub -module. This makes it possi-ble to calculate the frequency characteristicof the entire system at a given angle (bothhorizontal and vertical) with respect to thelistening axis, for instance, ±30'. Even inthe optimalization of the filters this off -axisresponse may be taken into account. It isthus possibe, for example, to design a filterwhose characteristic remains within cer-tain limits for an off -axis response of ±10'

:CA) To

;

101 ;TA,

with respect to the listening axis (see illus-tration above).

The loudspeaker placement has alsobeen extended. The original version en-ables the loudspeaker location to be calcu-lated in a three-dimensional space: thenew version makes it possible to take intoaccount that loudspeakers are placed at anangle (for instance, in case of a backwardsloping enclosure front). Also, the effectivediameter of the drive units may be takeninto account, so that the program maymake provision for the radiation pattern ofeach individual drive unit.

Apart from optimalization of the filtercomponents to obtain a given characteris-

tic, it is now also possible to optimalizethe impedance of an individual loud-Teaker or of the entire system. To thatend, compensation networks are addedthat are calculated by the computer in amanner that keeps the impedance in apredetermined frequency range within agiven percentage of a certain value. Thisis a very useful facility for compensatingthe behaviour of individual loudspeakersand for straightening out the impedancecharacteristic of a complete system, toensure that the power amplifier is con-nected to a truly resistive load.

There is also a 'student' version (1.20)of the program available at a sharply re-duced price ($A99.00). This version offers

all the facilities we have discussed. Its datafiles are compatible with Version 2.00(SA349.00) so that transfer to the profes-sional version at a later date is facilitated.

CALSOD is available from the design-ers, Audiosoft, 128 Oriel Road, West Hei-delberg 3081, MELBOURNE, Australia. Itmay also be obtained from Old ColonySound Lab, P 0 Box 243, PETERBOR-OUGH NH 03458, USA, or, in Europe,from Audio Specialists, Weichselstrasse22, 1000 BERLIN, Federal Germany.

*Elektor Electronics, January 1989, p. 62


24

INTRODUCING OP -SERIESOPAMPS

J. Ruffen

Dozens of new, improved operational amplifiers are introduced everymonth by leading IC manufacturers around the world. A number of

high-performance opamps from the 'young' OP -series are describedin relation with construction projects featured in this magazine overthe past year or so. The reasons for preferring these new devicesover, say, a Type 741 are manifold and call for an introduction to

opamp selection criteria.

Designing a wide variety of clever elec-tronic circuits on the basis of ideal oper-ational amplifiers is fairly easy. In not afew cases, however, the efforts remainpaper designs after the disappointing re-sults obtained with practical construc-tions. The problem is clear: the idealopamp does not exist. The ideal modeldoes have its uses, however, because itallows the operation of complex circuitsbased on opamps to be analysed andunderstood.

In practice, the ideal opamp is onlyuseful for the design of low -performancecircuits, and for function analyses. In allother cases, the most important non -idealcharacteristics must he taken into account.The relative importance of all the designparameters involved depends on the typeof circuit in which the opamp is to be used.In an alternating -voltage amplifier, for in-stance, off -set drift is less important thanthe slew rate. Conversely, the perfor-mance of a DC amplifier is limited by off-set drift rather than the slew rate.

Distinguishing between the importantand the not so important opamp charac-teristics on the basis of the function of thecircuit not only raises the quality of thefinal product, but also avoids needlesslycomplex calculations. Clearly, a well -

ELECTRICAL CHARACTERISTICS at Vs = :15V -55.0 -, -1. ---. -125.0 -- ess cterese,rte=

0 P.07A 01..07PAPALIETIA SIAM. CO103,210,5 ti_Id rip 1,./A hip TIP YAP , ,s

.., .-t' - ZS 57 - 5: a:: -.

,...-, - 32 .35 - *3 '3L'....,,,.. '., - 33.. N.._3 - *2 *. - wz '3

..,....:- sw ..;......, '-. _ z a t 2 3 A - :

Aeon, -.., Cr,Arf C....,7,1 Set, .._=2 - 3 is - E 5.3 r 20-w.

kw' 6,41.0..,..-1 's - ,a 72 ' t 1

A...V.' 2., as C -rr':C., Nrst 2 - I 35 - .1EN, 5:

-ew 10371PLA-9. ,3 =,3 5 - .,E r"ZE

C.=_-rn",-IA:Ce Rele,c,- . , :.,..= .... - =In' '76 '22 - `X-

21 -ft..* 5.15rt ../...1.,,,,i' , .S .A =_731.. 7 'A, - 3 23 5 -

LA---0-Ss-a.......Ape 'GA- _=80.1'7-='_r 270 .03 - tic LX -0. -7....,03.,1E -S. " : ._,rn .tz srzt - .12 C.._ - .

NOTES,r CP-:,A VE,.I. : , , - , , z ,

2 5.r.c..e xirez2 G.A3.71,PC Pt on -sr

Parameter ConditionsLull tk/L1.174tE t.M741 LIII741C

Lin 3li_n Typ Vas 6i.":1 Typ Usa Ikn I Typ Mu

r,o,....t.leset V5,,S TA = 25%::P. , 1-v:::R. -_-. E.::: : 3 .:, :

1 0 50 20 SO ,,. iri2

TAI.I, - TA - 73.1. AtRs *. W11 AD I

Rs , 1014: E0I

751712

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T,, - 25';:.. 'Ws z 2; . -

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TA,,,. f. TA s TAAAAA ,,,. zt E x.i Mera...,a Frpc...C1Iset

Carent Dr! 0.5

5.0;,1 3,as C1rw,1 TA - 25C 30 I sa i.-::-:.

7. , TA TOO), . 10.210 , 4

-:._ri R.Stf.,',7= TA - 25*C. Vs - .20V to so I ; ; - ; ;Ts.u.. ,. TA , T.,,,,,,Vs = -,-. 20:

0.5 .::

1.-o.i Vu- Range TA = 2VC I 2 7 ' a

TAA, , TA , TA.,._, . ,2 , ' v

13.-.e. S .--.s." V=P Gan TA = 25.C. R, .., 2s.c1vs = .20V. Vo = .15Vvs - .. 15V Vo - 7 1DV

50 I

20 200vimVVic -N

P1.., -. - , - Tasera

I 25V5- =V. v.: .

15 V -.V 17.

590151 - II

Fig. 1. Main specifications of the OP07 and OPO7A compared to those of the 741A E C.

ELF.KTOR ELECTRONICS FEBRUARY 1990

INTRODUCING OP -SERIES OPAMPS

founded choice can only be made on thebasis of knowledge and experience.

Infinite amplification and input im-pedance and zero output impedance areprobably familiar terms in relation toopamps. The real thing, however, startswith off -set voltages, input bias currents,noise, slew rate, and many other factors.

Opamp technologySeveral integration technologies are usedin the production of opamps. In theirquest for the ideal opamp, manufacturersare faced with a real dilemma: improve-ment of one opamp characteristic resultsin degradation of another. Inevitably, aparticular integration technology islinked with near -optimum performancein one or a few respects only. A clearexample of this vicious circle is that anincrease of the slew rate is inevitablycoupled to increased current consump-tion. Although not all opamp charac-teristics can be optimized at the sametime, today's production techniques doallow improvements in the performanceto be achieved if the aim is clearly defined.This results in a particular opamp perfor-ming better than another in the same ap-plication. The use of the most favourableproduction technology for a particularcharacteristic has resulted in many opampfamilies with many members. Currently,the four major technologies arc bipolar,BiFFT. BiMOS and LinCNIOS (Ref 1).

Off -set and driftBipolar opamps such as the ubiquitous741 consist of npn and pnp transistors.Bipolar amplifiers have the best specifica-tions as regards stability of the input off-set voltage. This characteristic is oftenreferred to as input voltage drift, or thetemperature co -efficient, d T, of theinput off -set voltage.

Field-effect transistors are less easy tomatch than bipolar transistors, and as aresult BiMOS and BiFET opamps gener-ally suffer from the resultant drift effects.Modern bipolar opamps such as the TypeOP07 are specified for a maximum off -setvoltage drift of 1.3 gV/K, although valuesbetter than 0.2 AV/ I: are no exception

Fig. 2. Standard bipolar opamp input.

0

;,'t,t.0,11

Fig. 3. Opamp input with external offsetcompensation preset.

This means that the drift specification ofthe OP07 is roughly 15 times better thanthat of the 741.

Extracts from the datasheets of theOP07 and the 741 are given in Fig. I.Clearly, the OP07 has a smaller drift, andthe absolute value of the off -set voltage isalso lower. Apparently, the drift specifica-tion may be improved by keeping the ab-solute off -set voltage. Li small.

Zener-zap trimmingThis technique is used to reduce LI,., of theOP07 to a value smaller than 200 jAV with-out the use of external components. Thefunction and operation of zener-zaptrimming is best explained by studyingthe cause of the off -set voltage.

The basic structure of a standard bipo-lar opamp input circuit is shown in Fig. 2.The output voltage, is the potentialdifference between the collectors of Ti andTi:

Ua = A ll-,111)Ra - /0T -IRA

If the collector resistors Rc are equal inboth branches, U, = 0 if I. equals I.Also, assuming that Ti and T-2 are perfect-ly matched, i.e., identical, equal collectorcurrents result in equal threshold voltagesLI: - and U,., From this it follows thatthe output voltage, Lld, can only becomezero if the input voltage, Lli, equal to

U`'-e'r') - Llb-e41-2)

is also zero. In practice, Ti and T2 are neveridentical, so that the threshold voltages at1_ r: = 1;,I-2, are always (slightly) different.Hence, LI, must be made equal to the off-set voltage to achieve L1 = 0 V.

In most opamp circuits, the output volt-age is made zero by applying the requiredbias voltage to the input terminals. Someopamps, however, have separate termi-nals that provide access to internal com-ponents. This allows an external trimmerpotentiometer to compensate the off -set

voltage without the need of a zero -adjustcircuit at the sensitive inputs of theopamp. The principle is illustrated in thecircuit diagram in Fig. 3.

The designers of the OP07 have gone alittle further, however, as shown in Fig. 4:the collector resistor consists of a numberof series -connected individual resistors,of which two have a zener diode in paral-lel. In the production process, the off -setvoltage is reduced to the minimum valueimmediately after the chip is ready. Ac-curately controlled current pulses are ap-plied to blow ('zap') one or more zenerdiodes, which then turn into shortingwires.

This automatic 'zener-zap' processallows off -set voltages to be reduced to50 p.\- (typical)- In some cases, provisionis made to compensate even that levelwith the aid of an external trimmer preset,which is usually included in a resistivebridge circuit. This arrangement is usedbecause the temperature co-efficients ofthe components in the bridge have a muchsmaller effect than those in the circuit inFig. 3.

All these goodies may lead you to startreplacing all 741s by OPO7s just like that.Remember, however, that the resultantimprovement in the circuit performance,if at all required, does not come cheap: the0P07 is about 10 times as expensive as the

Input bias currentAt room temperature, bipolar transistorshave a much higher input bias currentthan JFETs (junction FETs), although thiscurrent is virtually temperature -inde-pendent. The input bias current of BiFETopamps roughly doubles for every ten de -

Fig. 4. Basic structure of OP07 input withzener-zap collector resistors.


26 COMPONENTS

grees of temperature rise. In general, theinput bias current (at normal operatingtemperatures) of a commonly used BiFETopamp such as the TL074 is often higherthan that of a good bipolar type. Thisshould not be taken to mean that BiFETopamps do not have advantages over bi-polar types, since they offer much higherslew rate values in many cases. The BiFETType OP16, for instance, has a slew rate of25 V/us, which makes the device about73 times faster than the OP07 and the 741.

BiMOS opamps such as the TypeCA3140 have MOSFET inputs and a bipo-lar output circuit. Their input bias currentis strongly temperature -dependent owingto the presence of integrated input protec-tion diodes.

The use of current mirrors in the OP07has resulted in a drastic reduction of theinput bias current with respect to the 741:designers should allow for about 50 nAfor the OP07 compared with 500 nA forthe 741.

Opamp selectionSummarizing the above, the choice of anopamp is governed by the type of circuitit is used in. Broadly speaking, there arefive application areas for opamps:

general purposelow -powermicro -powerhigh-speedhigh accuracy

Each of these applications requires a par-ticular type of opamp for optimum perfor-mance- Each opamp family, in turn, hasmembers that are tailored to give opti-mum performance in one respect only_

The overview in Table 1 may proveuseful as a first guide to opamp selection.The right-hand column gives the best-known types in a particular series.

Reference:

1. LinCMOS circuits. Elektor ElectronicsJuly/August 1989, p. 20 ff.

Application area

general-purpose

low -power

micro -power

high speed

high accuracy

Primary characteristics Opamp type

low-cost

low supply current (1<1 mA)very low supply current (1<100 pA)high gain -bandwidth

low input off -set: high DC gain:hich CM R R

LM741; OP02(=741 upgrade)TLC271; OP21

TLC271: OP20LF356: CA3140:0P15; OP16OP07; 0P77;OP27 (low -noise)

ELECTR[CAL CH4.1R4CTEP,STICS - -.. _E-2- ..:;:.-,.. i, - Ve-C

PARAMETER SYMBOLOP -177A

23-4C,T.3'..5 IJ . 1 I ..= Us:OP477B

UM SOP iflE OATS

--v.a.C,,sat V0-mca. 'OS ,,,d

L r -C. - - -, 01`24.. .=e1). - .2 - - 22

6W. PS, -,.....,..,w, ", = ,....25.7 ,0:::im 2; - .3 - , E ,sa - _u,0.176.241C., -e -f . . ,Kr 3' ,O.,,,/..,... 2 - - 3 3 0...pus

12,43 RV./ .4.-+ 3. 33 25 .5 - Salt

P'...2u - 2::. - 233 - GO

: ..- - -.==e-.3e f eq V, -,-,g A, : 1 ,ta ., 4 - '1

CO-,...- tt,,,e

: - - :, .,, nail vs -.3, -.: r,3..i 123 'OS , S. !M -

__a-,..S.ra va,..-_:. as, A:0 P. a Zn_ vo ... :IC, 223..S:,33 !MCC - 5.332 ,233[C,

,',= :'0.:1 ,' 3 5= 3,13

s0_T.

a' 2 3

m135 :,SO -1125 :130e2C a,a5 -

. n SR _a - C O3 -3 ,.E.r: -.IV 33'Cor., 5.3 - .a L..,Cce--1.00c13,., r.st:th-ce P.. - - , - .

SC,:are Ccrs..-enc, P.-33 - 30 EC - ..

5,03., 2.,3"-t Is, e 2 c -.

--t--:- ;I - ,

0-3e1 a.a...,-,a, Rat -c4 ... . 2:.:: - - :3

NOTES: 2 Sarc -a -- _ ,..' L.'S 7."'-' .1.-20....2%....**Saasar, rri.-s>= re alleasaet-c-a ant at 3 Cas,- ::: :.. :::, ;.-

.1,, es Taro .,,,, aWC,E017...,71 VW re fig 305533 a aca,ro-. Fa.4.C1,-.27.-.2 ma ,-a0.c,cce.-m,..c-rs.,Vc,o_ergr-et,223:5..,-_:- 5 To re.-- .

-g c.ais V3 7,0,..a..la .2 7.21 2_ an? a: -, : _ -

s -3. ...--..x-a-ca 4._

INPUT BIAS CURRENT33 TEMPERATURE

it;i3

.

.. es

INPUT CFFSET CURRENTvs TEMPERATURE

;iD

!

¢

i

1r

..i. .

t. '

C106E0.1001. RESPONSEFOR VARIOUS GAINCONFIGURATIONS

------------

TOTAL INPUT NOISEVOLTAGE vs FREQUENCY

s) ss"raPIA.CiVE ,

INPUT WU:1E13MM NOME vsBANDWIDTH (111 Hs TO

FREQUENCY INDICATED)

... =.e.,833,51-15

MAXIMUM OUTPUT SIV.IMvs FREQUENCY

--,-...... :-

- .. ,....-:..1 ..t.

_--'Y

- - ----- - -....

-

-.,..-...-.

WV., , EU , le

aacnsi-ls

Fig. 5. Introducing the OP177 ultra -precision opamp (illustrations courtesy PrecisionMonolithics Inc. )


SAVE D ECOPart 1: Operation and technical background

P.N.P. Wintergreen

The scrambling system employed for the BBC -TV Europe and thenow defunct Premiere channel on the Intelsat-VF11 TV satellite is

generally classified as a low-level video encoding scheme. Based oncontrolled interference in the video spectrum it is, however, not so

simple to defeat with traditional filter designs that have alreadycaused much hair -pulling and teeth -gnashing among technically

inclined dish owners. The decoder discussed here leaves traditionalfilters for what they are, yet offers a perfectly decoded picture at a

cost of 225 or so.

The Intelsat-VF11 TV satelliteis positioned in geostationaryorbit at 27.5' west, and can bereceived in large parts ofwestern Europe if a dish of1.2 m or larger is used. TheBBC -TV Europe programmeis transmitted at 10.987 GHzwith vertical polarization.The channel is encoded ac-cording to the SAVE (sound

encryption) system,with occasional changes inthe interference frequencyused.

Tackling SAVEThe SAVE scrambling systemlooks relatively simple at firstglance, because the encodingis effected entirely in the fre-quency domain. This is in contrast tomany other analogue encoding systems,which use signal inversion in combinationwith amplitude -shifting and frameand/or line polarity control (Ref. 1).

In the SAVE system, the video signalfrom the TV studio has its amplitude re-duced by 50% before it is inverted. Theoriginal amplitude is then restored by theaddition of a pure sine -wave with a fre-quency of about 94 kHz. This encodingscheme has prompted many technicallyinclined dish owners to design and builda wide variety of filters, only to find thatthe interfering signal is much harder toget rid of than expected. This is mainlybecause any residual interfering signal,however small, gives an irritating effecton the TV screen. As found out the hardway, the 94 kHz notch is simply not ca-pable of ensuring a perfectly stable pic-ture.

Without a suitable decoder, a SAVE -encoded video signal produces a totallyunintelligible picture. In principle, thesignal becomes at least visible by invert-ELEKTOR ELECTRONICS FEBRUARY 1990

ing it and doubling the amplitude. Al-though these measures result in a picturethat can be recognized on many TV sets,the quality is very poor owing to the inter-fering carrier that is still present at its full

SAVE DECODER

low-cost design; no cut -to -order quartzcrystal(s)

complete suppression of SAVE inter-ference

L -C oscillator with amplitude and phasecontrol loops for optimum stability

simple -to -connect to any indoor unit

standard clamped video output:1 Vpc175

automatic switch -over between non -en-coded and SAVE -encoded channels

automatic search for interference fre-quency within 1.5 kHz band

level. The suppression, inparticular, of the inter-ference poses considerableproblems if conventional fil-ter techniques are used.

The phase and amplituderecovery of the 94 kHz inter-ference signal is affected bythe video signal, and de-signers must take into ac-count that this can have aninfinite number of instanta-neous amplitude -frequencyconfigurations because thespectrum is a function of thecurrent picture content,which changes in real-time.

A narrow 94 kHz filter,however, will give reason-able results in many cases. Acrystal filter as used in com-munications receivers is

ideal for this function. Unfortunately,crystal filters are notoriously expensiveand hard to obtain, especially for a non-standard frequency like 94 kHz. Theseproblems may be resolved, however, by acompensation circuit with a PLL (phase -locked loop), and a VXO (variable crystaloscillator) that functions as a VCO (voltagecontrolled oscillator). An advantage of thisarrangement is that it can be designed tooperate at a multiple of 94 kHz.

The block diagram of Fig. 1 shows thebasic structure of a VXO-PLL (Ref. 2). Al-though not used for the present design,the principle merits a short discussion.The crystal is cut to order for one particu-lar interference frequency around 94 kHz.In practice, however, the interference fre-quency is changed on an irregular basis,so that a number of crystals are requiredwhen BBC -TV Europe is to be decoded.Clearly, this is a relatively expensive sol-ution since the station operators canchange the interference frequency withina certain range, requiring new crystals tobe cut. The present decoder is much more

28 RADIO & TELEVISION

flexible in respect of frequency control be-cause it is based on an L -C oscillator thatuses inexpensive components, and worksat 94 kHz. A number of special tech-niques, combined with 'all -analogue' sig-nal processing have resulted in astop -band filter (notch) whose perfor-mance in respect of selectivity comes closeto that of a crystal filter.

The low-cost decoder presented herefeatures automatic processing of anySAVE -encoded video signal that containsan interfering carrier between about93 kHz and 94.5 kHz, so that BBC -TV Eu-rope (A=93,275 Hz) is reliably recognizedand decoded, irrespective of the currentinterference frequency.

Spectral analysisThe frequency spectra of Fig. 2 illustratethe basic operation of pre- and de -emphasistechniques used in the present decoder.The spectrum of Fig. 2a shows the typicalamplitude distribution, within the fre-quency range from 0 to 500 kHz, of avideo signal. It should be noted that thedrawing is purposely simplified: the spec-trum applies to a completely black pic-ture, while the effect of the rastersynchronization pulses is not shown. Thesimplified spectrum with line sync com-ponents and the interference signal is,however, still useful for a basic analysis.

The frequency range between 90 and95 kHz is shown enlarged in the lowerspectrum of Fig. 2a. Clearly, the inter-ference carriers are quite close to the sixthharmonic of the line frequency. The fre-quency differences are small - only475 Hz in the case of BBC -TV Europe, and

a

c amplifier

- Video

Unclampedvideo

- Video

Synchronousdetector

946Hzlow-pass

elliptic filler

VCX0611Hz

16

V

Summingamplifier

1

--ANTe

cancel

9.4kHzlow-pass

elliptic tiller

+90'

Decodedvideo

Fig. 1. The first approach to a SAVE decoder is nearly always a VCXO design. The inherentdisadvantages are, however, relatively high cost and the 'one channel, one crystal' rule(source: Ref. 2).

575 Hz in the case of Premiere - andmake selective suppression of the inter-ference frequency fairly difficult becausethe phase and amplitude of the sixth har-monic of the line sync must be left com-pletely unaffected. If this condition is notmet, the picture is visibly distorted.Hence, a simple notch is unsuitable forbuilding a reliable SAVE -decoder.

For those less familiar with video tech-niques, it may come as a surprise that theinterference signal must be amplified be-fore it can be suppressed. Pre- en de -em-phasis are used to achieve this. These

b

teB]-^:.

1E -.:-

1

C

544

SIM ha j S43251a

33 33 5 9, St 5 33

-it. 1 (kHz]

rds

73

techniques are applied in FM communica-tions systems to improve the signal-to-noise ratio. At the transmitter side, acertain part of the frequency spectrum ofthe modulation signal is given a greateramplitude, which is reduced again in thereceiver by a matched filter to restore theoriginal signal level. Ideally, the fre-quency response between the input of thetransmitter and the output of the receiveris flat because the pre- and de -emphasisfilters are closely matched by means ofcomplementary curves.

A two -section L -C filter in the present

33,J 733 3., 35--ph t [kHz)

I I]6931E5-120

Fig. 2. Frequency spectrum of a SAVE -encoded video signal with no picture content (2a); theoretical pass -band of the pre -emphasis filter(2b); and the result of the pre -emphasis operation on the original spectrum (2c). It should be noted that the spectra shown are theoreticaland purposely simplified.

ELEKTOR ELECTRONICS. FEHRL ARV 1990

SAVE. DECODER 29

SAVE decoder raises frequency compo-nents around 94 kHz and 188 kHz about20 dB with respect to the rest of the videospectrum. The theoretical pass -bandcurve of this filter is shown in Fig. 2b, andits effect on the spectrum of Fig. 2a is ap-parent from Fig_ 2c. The interference sig-nal has the highest amplitude in the latterspectrum.

The control principle adopted for theSAVE decoder requires the second pass -band at 188 kHz. The analogue controlcircuit that is to supply the regenerated94 kHz signal does not have filters to en-sure a well-defined phase response, andmultiplication of the 94 kHz signal inevit-ably produces a 188 kHz component.Owing to cross -talk, a part of this compo-nent ends up in the video signal, where itproduces interference. This is prevented,however, with the aid of a de -emphasisfilter, of which one section is tuned to188 kHz.

Since the interference frequency is al-ways an odd multiple of the interlace fre-quency (25 Hz), residual levels of the94 kHz signal that remain after decodingare virtually unnoticed owing to the opti-cal averaging function of the human eye_

After removal of the 94 kHz compo-nent, the signal amplitude must be re-stored to the level before pre -emphasis.This is achieved with a de -emphasis filterwhose pass -band curve is accuratelydimensioned to form the inverse of thepre -emphasis curve.

Block diagramThe basic operation of the SAVE decoderis best described along the lines of theblock diagram shown in Fig. 3.

The heart of the circuit is formed by anL -C oscillator, which is controlled by loopcircuits for amplitude and phase. In prin-ciple, the regenerated 94 kHz sine -wave isadded to the scrambled video signal. Theamplitude of the regenerated sine -wave isidentical to that of the interference compo-nent, but the phase is opposite. This isachieved with the aid of a control circuitthat is capable of continuously monitor-ing, and, if necessary, correcting, thephase and the amplitude of the regener-ated 94 kHz signal. The oscillator used isa voltage -controlled Colpitts type basedon an L -C tuned circuit with a relativelyhigh Q -factor. In combination with adouble loop filter that forms part of a PLL,it ensures good phase stability of the re-generated sine -wave when the circuit islocked.

The block diagram shows that the un-clamped video signal supplied by the sat-ellite -TV receiver is amplified before it ispassed through the pre -emphasis filterdiscussed above. The output signal of thetwo -stage L -C filter is compared to that ofthe 94 kHz oscillator. The phase compara-tor is an analogue multiplier available inIC Type XR2208 from Exar. The result ofthe multiplication operation is a dif-ference frequency that is fed to the firstloop filter. This filter is realized with theELEKTOR ELECTRONICS FEBRUARY 1990

VIDEOAMPUFIER

PHASECOMPARATOR

E4 kHzSINE -WAVE

"Co

A. PLITUDECONTROLLER

FRE-

LOOPFILTER

4

VIDEORP UT

LO 4 -PASSFILTER

SUMMATIONNETWORK

MULTIPLIER

T

PHASE

SCRAMBLINGDETECTOR

..ITC:: VIDEOSUFFER

ENCODE:,0VIDEO

OUTPUT

00

80165-13

Fig. 3. The block diagram of the SAVE decoder is fairly complex. The operation of thecircuit is based on a 94 kHz L -C VCO surrounded by control loops for amplitude and phase.

opamp available in the X122208. When thecircuit is not locked, the first loop filter isgiven a bandwidth of a few hundred Hzto enable the PLL to lock on to sufficientlystrong signals in a frequency band ofabout 1.5 kHz around 94 kHz. The PLLdoes not lock on to signals below a certainthreshold level, so that, for instance, thesixth harmonic of the line frequency cannot switch the circuit to the decodingmode. Without special measures, how-ever, the sixth harmonic of the line fre-quency is likely to cause problems oncethe PLL has locked on to, say, the SAVE -encoded BBC -TV Europe signal. In thatcondition, the phase comparator supplies475 Hz, the frequency difference betweenthe sixth harmonic and the interferencesignal. Without a correctly dimensionedloop filter, this difference frequency givesrise to phase modulation of the VCO,which, in turn, leads to incomplete com-pensation of the interference. The result isan annoying interference on the TVscreen.

The solution to this problem has beenfound in automatic bandwidth reductionof the loop filter. After the PLL has locked,

the loop filter is given a pass -band ofabout 15 Hz rather than a few hundredHz. This arrangement requires high VCOstability, however, since frequency devia-tions can only be corrected relativelyslowly. Fortunately, the L -C oscillatorused meets this requirement, so that aphase -locked reference signal is obtained.What remains is control of the amplitude,and a shift of phase by 90'. Both functionsare realized by an operational transcon-ductance amplifier (OTA), a TypeLNI13700 from National Semiconductor.A useful background on OTAs can befound in Ref. 3.

The VCO reference signal is passedthrough an amplitude control circuit andan integrator before it is added to theinput signal after pre -emphasis. The re-sult of the addition is phase -shifted by 90'and fed to a multiplier that determines thefrequency difference between its inputsignals. Evidently, the difference is 0 Hzonly when the VCO frequency equals theinterference frequency. As shown in the'Theoretical background' inset, completecompensation is achieved when the am-plitude of the 94 kHz VCO signal equals

30 RADIO & TELEVISION

the amplitude of the interference signal.The control loop performs automatic cor-rections to the amplitude of the VCO sig-nal until a stable condition is reached.Once aligned, the decoder is, therefore,insensitive to small amplitude variationsof the input signal, and other forms ofinstability.

The integrator that shifts the phase ofthe VCO signal by 90' gives a frequency -independent phase shift, and is locatedafter the amplitude control circuit. An ad-ditional benefit of the integrator is itsability to suppress harmonics generatedby light distortion. These harmonics aresuppressed at 6 dB per octave.

The control voltage for the amplitudecontroller is used to light a LED that indi-cates the decoding of a SAVE signal.When the LED lights, the loop filter isautomatically switched to reduced band-

width, and the output signal of the de -em-phasis network is fed to the video bufferwith associated damping circuit. Thedecoded video signal has an amplitude ofabout 1 Vpp at a load impedance of 75 aand is suitable for direct connection to amonitor or remodulator.

From theory to practiceThe previously discussed functions areeasily found back in the circuit diagram ofFig. 4. The unclamped video signal fromthe satellite -TV receiver must have an am-plitude of at least 0.5 Vpp. Components PI,Ci, R3 and ESi take the non -encoded videosignal to amplifier T5. From there, it ispassed through clamping circuit Di -D2 -T.before it is applied to output bufferTr. Theoutput amplitude is set to 1 Vrr into 751.with the aid of preset Pt.

Theoretical background

The pre -emphasis L -C filter in the col-lector line of Ti supplies a video signalwith a 94 kHz component of about 3 Vppwhen the receiver is tuned to a SAVE -en-coded TV channel. The transistor works asan inverting amplifier. Capacitor Cr feedsthe signal with the 94 kHz component tothe phase comparator in IC3, where it ismultiplied with the VCO signal appliedvia Cis.Phase controlThe 94 kHz Colpitts VCO built around

T- is tuned by means of a direct volt-age applied to dual variable capacitancediode D7. The circuit around D. and 1-4 isan amplitude stabilizer. When the ampli-tude of the oscillator signal exceeds a cer-tain level, conducts via D., and drawscurrent through R44. This causes the draincurrent of the oscillator FET to drop to alevel at which the feedback gain in the

1. Pre- and de -emphasis When pre -emphasis network La -Cs (Z) resonates at 94 kHz. it formsa very high impedance. R13 alone then determines the impedance of

Current source T uses pre-empnasis impedance Z formed by Z With R13 = 27000:C= -Cs -Rs to generate a signal voltage at TP1:

U3 T1 = UUanim U

T11 s 4E1'1} = Rs

UTP1 = Inn) ( Z+ )=-U Z -R7 1 Rs

Since T3 offers high current amplification, Z is hardly loaded.

The regenerated 94 kHz sine -wave is added in anti -phase to the videosignal. Emitter follower T4 forms a virtually ideal voltage source forthe de -emphasis impedance. Z:

Ua 74, = U574, = Z R7 / R5 + Up Sin (WI+ 9)

whereca = 2 rz 94000 [ rad/s]

The decoded video signal across R15 may be written as

Urbs= UDU0= R 15 / R 15+ Z UEIT41

R Z -t -R7 .U0 Rls+Z)

(

R,upsm (o.) t 9)]

The input voltage. U may be written as

Ui= Uirideo+ Up sin (o) (p).

Owing to noise in the input signal. the residual 94 kHz component.Ures. has to be taken into account:

U0=R 15 i( R15-4- Z)1-Uvrtrect R7)/R5+U.-es)res

R15( Z- R7 ) R 15Uo- Wide° R Z R 15 ) Z+R15

With R7 = R15:

U0= -R 15 R 5 Uvideo÷ R 15 Z R 15 ) Liras

U0=-1.23 Uvideo+ R isi(Z+ R 15) Ures

U0 = -1.23 Uwde0-- 0.09 Ures

Correctly aligned, the pre- and de -emphasis networks yield more than20 dB suppression of the 94 kHz component, without distorting thevideo signal.

2. Amplitude control circuit

The frequency of the amplitude control voltage. U,eo, is 0 Hz if theregenerated frequency equals the frequency of the interference volt-age, U,:

Ureg= Jo sinuo t !I( U,- (keg)sin (co rildt

Ureg= jo 0.5 ( -Limo)[1 - cos ( 2 co t)id t

The integrator gives virtually complete suppression of the 188 kHzcomponent, so that:

Ureg= Jo 0.5 ( U- Uteg ) d t

This equation can be solved if U= Urog. which corresponds to com-plete compensation.

3. 90 phase shifter

Pin 5 of OTA1 supplies a sinusoidal current. I. which is integratedby C25:

= ip COS (0) )

UC29=1 /Cf d0

/Jag= 1 /Cr' ip. COS ICO Ildt0

Lic29= mit!) )

Cwhere

co = 2 rr 94000 [ rad s]

The phase is shifted 90' independently of frequency.


IIISAVE DECODER

Fig. 4. Circuit diagram of the SAVE decoder.

oscillator is unity. The result is a clean andstable oscillator signal that is coupled outinductively via Ls. Transistors Till and Tnamplify the oscillator signal and convertit into a symmetrical current that is fed tothe pair of diodes at each of the parallel -connected OTA inputs (IC4).

The OTA inputs are also connected to112 and Ti_ to form a gain -cell. In principle,a gain -cell provides linear conversion ofan input signal into an output signal. Thegain of the cell is determined by a currentsupplied by an external source. In thepresent case, this control current ema-nates from R32, while the basic gain of thecell is defined by the ratio Rs2:R5s.

The 94 kHz signal is taken asymmetri-cally from the collector of TI2 and fed tothe phase comparator via Cis. The dif-ference frequency is available as a bal-anced signal between pins 1 and 2 of theXR2208. The balanced difference signal ispassed to the opamp in the XR2208, bymeans of networks Res -R3 -C21 and R3s-R37-C22.

Depending on whether the I'LL isELEKTOR ELECTRONICS FEBRUARY 1990

locked or not, the bandwidth of the loopfilter is changed by ES2 connecting R27 inparallel with Res, and ES1 selecting a dif-ferent R -C network at the output of theopamp in the XR2208. The control voltageis passed to the varicap via R4I and R-16.This closes the phase -locked loop.Amplitude controlThe amplitude of the regenerated 94 kHzsine -wave is determined with the aid of acurrent sent into pin 1 of OTA:. This cur-rent is supplied by OTA2, which works asa multiplier. The result is that pin 3 ofOTAI supplies a sinusoidal current to in-tegrator The mathematical deductionin the 'Theoretical background' inset dem-onstrates that the phase is shifted 90' in-dependently of frequency. The voltagedeveloped across C24 is fed back into theIC via pin 7, and reappears buffered atpin 8. The regenerated, amplitude -con-trolled and 90' phase -shifted 94 kHz sig-nal is fed to T2 via Cs, and from there tosummation point T2 -R12. This forms thenucleus of the circuit: the interference dis-appears against its regenerated counter-

part, which has the same amplitude butthe opposite phase.

The summation signal is taken throughthe de -emphasis network in the emitterline of T4. The first of the two L -C sectionsremoves residual 94 kHz levels, and thesecond ensures sufficient suppression ofthe 188 kHz component. The decoded sig-nal is fed to the clamping circuit via ESi.

The amplitude control circuit receivesits error signal from the summation point,via Rn.s. Components C33, Rn, Czo andR59 shift the phase of the error signal by90' so that it can be used for driving multi-plier OTA2. This supplies a current that isintegrated by C3I. The resulting voltage onthis capacitor is buffered by the darling -ton transistor in the OTA, and serves tosupply the control current to the pre-viously mentioned gain cell (OTA1) viaR57. This closes the amplitude -controlledloop.

The construction and alignment of the decoderwill he described in Part 2 of this article.

or

DESIGN IDEASThe contents of this column are based solely on information supplied by the author

and do not imply practical experience by Elektor Electronics.

WAVEFORM MODULATION OFTHE MAINS VOLTAGE

by A.M. Karailiev

In order to simplify the transmitter and to improve the noiseimmunity of the remote control receivers described in his earlierarticle*, Mr Karailiev here offers another method of modulating

and demodulating the mains voltage.

A thyristor. rated at not less than 25 A. isconnected directly across the mains sup-ply. It is controlled by a circuit as in Fig. 1in a manner to make it conductive for atime Ar us at the end of every positive halfcycle of the mains voltage.

The programming unit is an integratedcircuit Plessey Type SL120. or SGS TypeLI20. or equivalent.

Assume that the thyristor will short-cir-cuit the mains at the instant this reaches avalue of +24 V - see diagram in Fig. 2.Then.

= U sin wr = sin(Zit 11T )

24 = 310xsin2rcx50At.

Since. for a small angle, sinO = e.

= 310x2rtx50AL and

= (24x106) 310x2x50 = 246 us.

Thus, for a period of 246 }is. the mainsvoltage is practically zero and this mani-fests itself, at minimal power dissipation.in a distortion sienal on the mains voltage.It is interesting to note that since the signalis caused by a lack of voltage, it will nothe attenuated by the power lines, so that itwill be detectable over fairly large dis-tances.

Type of modulationThe effect of the momentary short circuitof the mains voltage will be a change of itssinusoidal waveform, so that we may

`Mains Signalling Elektor Electronics.November 1988. p. 27.

Readers are advised that modulating themains voltage in the United Kingdom issubject to the provisions of British Stan-dard BS6839. Further information on thesubject may be obtained from BIMSA

f BEAM Interactive and Mains SystemsAssociation), Leicester House. 8 LeicesterStreet, LONDON WC2H 7BN, Telephone01-437 0678.

speak of waveform modulation. However,as the depth of modulation is defined bythe angle 0 during which the short circuitoccurs. or by the time duration of the shortcircuit. other definitions of the type ofmodulation may be applied.

DemodulationThe information on the mains voltage isextracted from it by means of a circuit asshown in Fig. 6. The instantaneous volt-age. It, at the secondary of the input trans-

former. IF. I,

= Um S111127t11 I II T ± 1)

and this is represented graphically inFig. 3.

If the full -wave rectified voltage(Lj,,= 17 V) is applied to the invertinginput of an operational amplifier acting asa zero crossing detector. and a direct volt-age of 0.25 V is applied to the non -invert-ing input, the output of the detector willconsist of 93.66 us wide pulses at a pulserate equal to twice the mains frequency.

When the positive voltage applied tothe non -inverting input is higher then thevoltage caused by the modulation. thepulse spacing (pulse repetition period) de-creases. When. however, the voltagecaused by the modulation is higher thanthat applied to the non -inverting input, thepulse spacing increases - see Fig. 4.

In the absence of modulation. the zerocrossing detector should be adjusted tomake the width of the output pulses as

02

1/44001

1 O1

1144001

(Cl 4011

03

IC1

TV

CI

R2

ID. 3C2

104R3

co -4 a

torp

T

10

BC108

R4

PACGRAMMINGUNIT

RS

*D4Ren44

*--C9220V

__T-11

Fig. 1. Basic control circuit for the modulator: the value of R5 depends on the type of thyristor.

ELERTOR ELECTRONICS FLIII-ZL RN- 199(1

Fig. 2. Fig. 3.

sl-}4 zas4

Itmmuuttlllll li1ll 111111 111111 111111 1111 11111 111111111

1

Fig. 5. Pulse diagrams at selected points in the circuit of Fig. 6. The numbersat the left correspond with the circled numbers in Fig. 6.

small as possible. For example. if the de-tector produces pulses 100 ps wide in theabsence of modulation, and a modulatedsignal with a depth of modulation of250 ps is applied. the width of the outputpulses will increase to 300 gs. If the detec-tor is made more sophisticated. it may pro-duce pulses only when there is modulation

WAVEFORM MOM_ LATION OF THE MAINS VOLTAGE

Fig. 4. In diagrams a and b there is no modulation: in diagrams c and d.modulation is present.

present and no pulses at all in the absenceof modulation.

Receiver

In the circuit of the receiver shown inFie. 6. Ti serves as the zero crossing de-tector. It should preferably be a germa-

F,

P3

C41[11,752,

3

3

2PcExT

zcExT ICU

2C20

LP

IRCEXT

10EXTICiG

ICLOO

10

IA

5V 12V

IC2 1.1

7805s

T200

mi=

FS IC3a

IBMi2

CLIP,

A701.

Cl'CIT713,

IC57474

S

I' V

rl

220V

ICI =74123IC3=74132

11 l32. C, 02

IC47493

FS

MEI

Fig. 6. Circuit diagram of the proposed receiver.

nium type to ensure minimum -v. idthpulses in the absence of modulation.

Network R3 -R4 -C.; forms a charge/dis-charge circuit that converts the pulse -width modulation into pulse -amplitudemodulation. The pulse height can be ad-justed by R3. so that monostable ICI maybe triggered only by pulses above a certainheight.

The monostable is retriggerable andproduces a long output pulse at its pin 12when there is modulation present. Here.the length of the pulses is predetermined at500 ps. Shorter pulses will he suppressedby a noise protection circuit consisting ofRft-R7-C(i and the two Schmitt triggerscontained in IC3.

The output pulses of ICI are countedby 1C4. When the pulse train ceases. re-triggerable monostable IC2 emits a pulsethat is applied to the clock input of D -typebistable ICs. This causes the contents ofone stage of the counter to be transferredto IC:. which acts as a latch. Since ICs is afour -stage counter, it is thus possible tocontrol four different devices or machines.

Diagrams of the pulses at selected partsof the circuit are shown in Fig.. 5.

The higher noise immunity of the re-ceiver as compared with that in my earlierarticle is obtained because:I. its input is open to the signal andclosed to noise, since the thyristor short-circuits all noise:2. of the noise protection circuit:3. of the special form of the signal (ab-sence of voltage).


34

FEEDBACK KILLERT. Giffard

This circuit helps to prevent feedback in public address (PA)systems. Feedback occurs at relatively high volume settings when a

microphone is held too close to a loudspeaker. The effect ischaracterized by whistling or howling sounds that are annoying to

the audience, the sound engineer and, of course, the speaker on thestage.

Feedback is a constant sourceof worry to mixing desk oper-ators in the backstage line.since the now shrill thenhowling sounds always seemto occur unexpectedly, andtend to irritate both the per-formers on the stage and theaudience, who respond withboos, catcalls and plugging ofears. No good this equipment!

At a certain frequency, orfrequencies, the phase shift inthe closed loop formed by themicrophone, the amplifier,the loudspeaker and the lis-tening room is 0 degrees, 360degrees or a multiple thereof.If, in this situation, the ampli-fier gain is greater than theroom attenuation, the feed-back signal rises to a level atwhich it tops all other sound.This is the well-known high -pitch whistle:the I'A system oscillates.

Figure 1 illustrates what happens. Thesound produced by the loudspeakers isreflected by the room or by objects or per-sons in the room, and is subsequentlypicked up by the microphone. The phaseshift and attenuation depend on theacoustic parameters of the listening room(note that the audience forms a movingpart of the room and must be taken intoaccount as regards the loudspeaker posi-tions).

Evidently, the best way to preventfeedback is to place the microphones inpositions where they are unlikely to pickup reflections from the loudspeakers

Fig. 1. Sound reflections.

Sound technicians know a lot aboutmicrophone placement and often spendhours on small adjustments of the monitorloudspeakers. However, many artists areinclined to take the microphone from itsstand the moment they are on stage. Thisrequires great attention on part of thesound technician, since there remains alarge risk of feedback occurring.

The circuit described here raises thesound level at which the feedback startsby a few decibels. This should not be taken

4

to mean that it makes feedbackimpossible: only the point atwhich the effect starts is shiftedto a much higher level.

The risk of feedback occur-ring is reduced by creating asmall frequency difference of1 Hz to 30 Hz between the am-plifier input- and output -signal.This operation causes the fre-quency of a (potential) feedbacksignal to be raised a little so thatthe condition for oscillation isno longer satisfied.

Block diagramA number of modulation tech-niques may be applied to obtaina fixed frequency shift betweenan amplifier input- and output -signal. In practice, virtually allthese techniques make use of

amplifiers, filters and mixers.The block diagram of the feedback kil-

ler is shown in Fig. 2. The principle ofoperation is fairly simple. The input sig-nal is passed through two all -pass filterswhose output signals have a phase dif-ference of 90°. These signals are multi-plied with two clock signals that are also90' out of phase. The resulting signals areadded. As shown by equation (A) inFig. 2, the multiplication and additionoperations produce a new signal of a fre-

(;) a ^ ___..01=0.s sr,n(c3:-- C,

a e- nrelt - E - ucrt

o.,9TE S = Engnal. :name,

E z5a,,, - 2. - 6;t - 0.5 c. -to -bst

in.:.:4-2 -1:

Fig. 2. Block diagram of the feedback killer, and its mathematical background.


FEEDBACK KILLER

as

C

MIM MIN

Fxx

ES]

Li

C22

RI

IC'

40E0

SY

ES t

ay

Cap6

97 40.53S

sr. 22a

OM.

/taxa

A1 -_A/ =ICI = 21024A5_,55 = IC2 T1074AS_Al2 = IC3 =71071A13__A15 =1CA r -7L071ES1--ES4 =ICS =10E6ESS_ESS =106 =1:E5FF1. FF2 =ICE .1013FF3. FF 4 =1010=1013

CAI :7:,IC1 .,.....

T102 9114 ,e3 ÌI ,,, (-C---.C22ics OM

E-----." ma ,7Ca.00 0

Cr)

(3

T

CSa

LT14 i (CI. cly

I TUBC 2

ir--""j LI:CIS .7. ----7C:

---7C3I

.--7 .____T'V C24 -CASs 22,,Xst

SYSICCC2 -13

Fig. 3. Circuit diagram of the feedback killer.

quency equal to the sum of the frequencyof the input signal and the frequency ofthe carrier with which it is mixed. Theupshot is that the input signal is shiftedover a small range in the frequency spec-trum. Although the equation shows thatthe sum signal is a single frequency com-ponent only, it should be noted that this isonly valid if the input signal is shiftedexactly 90'. Unfortunately, this is onlypossible in theory. Any practical circuitproduces spurious signals that cause am-plitude -modulation (AM) of the mixeroutput signal. The all -pass filter used inthe feedback killer ensures a phase shift ofvery nearly 90° over a frequency range of50 Hz to 7 kHz:

The output signal of the mixer is takenthrough an active low-pass filter. Thisoperation is required because the inputsignal is multiplied by 90° -shifted rectan-gular waves, rather than a sine -wave anda cosine -wave which would have givenrise to a considerably more complex cir-cuit. From Fourier analysis, a rectangularwave is composed of a fundamental fre-quency and an infinite number of odd -numbered sine -wave harmonics as

4irc(cos(cot)-1/3cos(36in /5cos(5o)t)

The amplitude of the harmonics decreaseswith frequency. The use of two 90' -shiftedrectangular waves results in a series of

sine -wave products and a series of cosine -wave products. The harmonics areremoved in the previously mentionedlow-pass filter, so that only the wantedmixer product remains.

There are two ways of shifting theinput signal over a small range (1 Hz -30 Hz )in the frequency spectrum: 1) use alow -frequency modulation signal or 2)mix the signal a second time with a signalof which the frequency is almost the sameas that of the first rectangular wave. Next,remove the undesirable sidebands withthe aid of a filter.

The second principle is adopted in thefeedback killer. Two carriers of a relative-ly high, but slightly different, frequency


36 AUDIO AND HI -F1

are used.The sum signal, A, obtained from the

first mixer is filtered before it is mixedwith a carrier, sin(ck+8)t, where 8 is therequired shift in radians. The resultingsignal, B, may be expressed as shown inFig. 2. In the equation, -cos(o).:+26),+8)t isan unwanted sideband, and cos(o.),8)t themodulated input signal shifted by anamount 8 in the frequency spectrum. If thecarrier frequency is relatively high (ap-prox. 125 kHz), suppressing the un-wanted sideband is relatively easy withthe aid of a band filter, which doubles asa filter for the sidebands that are gener-ated by the use of rectangular waves in thesecond clock generator.

Circuit diagramFigure 3 shows the circuit diagram of thefeedback killer. The essential blocks in thecircuit are an all -pass filter around 1C:: -IC:, an output filter around A1s and Ate,and two clock generators around IC7-ICio.The audio signal is buffered by Ai beforeit is applied to the all -pass filter whichintroduces a phase shift of 90±0.5' for sig-nals between 50 Hz and 7 kHz. The filtercomponents have a tolerance of l';;: to en-sure that the deviation from 90' remainsas small as possible (remember that thenumber of unwanted sidebands rises withthe deviation from 901. The frequencyrange of the all -pass filter restricts thepractical use of the circuit to speech. Thefeedback killer, must, therefore, not beused with singers or musicians, since theabsolute frequency shift upsets harmonicrelations and so causes the performers toplay or sing out of tune.

The all -pass filter is followed by a buff-er, A1, -A.1, which introduces a phase shiftof 180' required for the first multiplica-tion with the rectangular signal suppliedby the first clock generator. The multipli-cation proper is effected by electronicswitches ES:-ES4 as illustrated in Fig. 4.The non -inverted and the inverted sine -wave is passed during the positive andnegative half -cycle of the clock signal re-spectively.

Opamp A: adds the two mixed signals,and supplies, in principle, the compo-nents of which the rectangular wave con-sists. The frequencies of the componentsare, however, raised by the frequency ofthe input signal. Opamp An suppressesall unwanted higher harmonics generatedby the use of rectangular -wave clock (car-rier) signals. As a result, the sum signalcontains only the mixing product of theinput signal and the fundamental fre-quency of the clock signal. The 'digitalmixer' approach uses fewer componentsthan an equivalent all -analogue (sine-wave/cosine-wave) circuit, and has theadditional benefits of lower cost and theabsence of adjustments.

The filtered signal is available in in-verted form after opamp A13, and ismixed a second time with a clock signal.Like the first mixer, the second one useselectronic switches. The frequency dif-

rrppr

pry.444A4rvpr44.0.A4

Fig. 4. Principle of frequency multiplica-tion with the aid of electronic switches.

ference between the two clock signalsdetermines the effective frequency shift ofthe input signal. The output signal of thesecond mixer is taken through a second -order high-pass filter, Ain, and a second -order low-pass filter, A15, to remove allunwanted harmonics. The AF output is avirtual copy of the input signal: the onlydifference is that it covers a slightly differ-ent range in the frequency spectrum.

The clock generators around IC4 andIC- are identical with a Type CD4060 os-cillator/divider and an 8 MHz quartzcrystal as the frequency determining ele-ment. The Q3 -output of each CD4060 sup-plies the oscillator frequency divided by16, i.e., 500 kHz. The two bistables (FFI-FF2 and FE-FF-; secure the required phaseshift of 90' between the two clock signals.In this operation, the clock frequency isdivided by four, so that the carriers havea frequency of 125 kHz. The trimmer ca-pacitors in the crystal oscillators allow thefrequency shift applied to the AF inputsignal to be adjusted to requirement.

ComponentsThe capacitors in the all -pass filter arepreferably 1`7, -tolerance polystyrene ('sty-rollex') or silver -mica types, although theboard allows MKT (plastic encapsulatedmulti -layer polytherephtelate) types to befitted also. These capacitors from Siemensgenerally have a tolerance of 5c'e. Notethat when MKT capacitors are used, thereis little point in using 1% resistors. The useof 54 -tolerance components in the all -pass filter increases the amplitude modu-lation of the output signal.

Two types of PTFE foil trimmer,10x5 mm raster and 5x7.5 mm raster, maybe used in positions C21 and CB. Do notuse ceramic types: they are not partlytransparent as PTFE foil types and do nottherefore allow the set capacitance to bededuced from the position of the rotorrelative to the stator blades.Be sure to use quartz crystals and

CD4060's from one and the same manu-facturer and, if possible, from a singlebatch (look at the production date indica-tion, the type and batch number).

The circuit is a combination of anal-ogue and digital electronics and has,therefore, relatively many decoupling ca-pacitors to keep cross -talk and inter-ference to a minimum. Fortunately, thereis virtually no tolerance or quality re-quirement for the decoupling capacitors,so that inexpensive types may be usedwith impunity.

Construction andadjustmentThe circuit is preferably constructed onthe single -sided PCB shown in Fig. 5. Theconstruction is entirely straightforwardand best started with fitting the wire links.Note that a fair number of resistors ismounted upright.

The adjustment of the circuit is simple.As already discussed, the frequency shiftof the audio signal is determined by thefrequency difference between the twocrystal oscillators. This frequency dif-ference is set with the respective trim-mers, C21 and C.

The function of the circuit is relativelyeasy to check. Set both trimmers to aboutone-third of their travel. Connect a two -channel oscilloscope to the input and theoutput of the feedback killer. Apply asine -wave to the input and synchronizethe top channel of the scope. The circuitworksif adjustment of one of the two trim-mers causes the (output-) signal on thelower channel to start moving horizon-tally across the scope screen.

The frequency shift required for opti-mum suppression of feedback dependslargely on the equipment used, the powerratings and anticipated sound levels, and,of course, the acoustic characteristics ofthe listening room. Finally, it will be clearthat the effect of the circuit is less noticedat relatively small frequency shifts.

The circuit draws less than 50 mA froma regulated ±5 V to -±8 V power supply.


FEEDBACK KILLER

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Parts list

Resistors:= 10k

R2 = 8k283;843;844;845 = 1k0R4:R5:R3;R9:R tz:R13:R 16:R i7:Rat:RatRzs;R2s:Rza:R32:R33;R37;Ras;R33;R46;R47 = 10k 1%Rs = 91t31 1%87 = 619k 1%Rio = 17k4 1%Rn = 909k 1%R14 =18k7 1%RI5= 750k 1%Fbe = 22k1 1%Rts=866k1%Rz2 = 15k4 1%R23 = 1MBR26 = 16k2 1%Ro=2M2

Raf_% = 19k6 1%8! = 931k0R. = 24k3 1%R35 = 1M5R4o:R41:1142= 100k 1%1148:1149;R52:Rs3;Rs4 = 47k

R53;Rst = 12kRss:Rse = 470k

Capacitors:CI = 680nC2 =,1n0 1%C3 = 4n7 1%C4 =22n 1%C5 = 100n 1%

= 2n2 1%C7= 10n 1%Cs = 47n 1%Ca = 3,30n 1%Cio;Cti = 820pC12:C13 =1nC;4 = 1n8

Cis = 680pC16;C17= 18nCis = 220pCI9 = 100pC20:C22= 47p

= 100p trimmerC24 -C45= 100nC45:C47 22pC4s = 8p2

Semiconductors:IC; - iC4 = 11074ICs;ICs= 4066IC7:1C9 = 4060

= 4013

Miscellaneous:= quartz crystal 8 MHz.

PCB Type 900002 (not available ready-made through the Readers Services).


38

2AD[10This FM radio, designed by ELV GmbH, consists of an insertion card

for IBM PC -XTs, ATs and compatibles and is available as a kit or aready -built and aligned unit. The radio has an on -board AF power

amplifier for driving a loudspeaker or a headphone set, and ispowered by the computer. A menu -driven program is supplied to

control the radio settings.

Take a look around many modern officesand you will notice PC users or terminaloperators wearing headphones while sit-ting in front of their screens. In most cases,a portable radio or cassette recorder isplaced on the desk or near the PC tolighten the work with a little music with-out annoying colleagues.

This PC radio with integrated poweramplifier is constructed on an insertioncard that can be plugged into any free busextension slot on the motherboard of anIBM PC -XT, PC -AT or compatible com-puter running under MSDOS control.

Control program andcursor keysThe control software for the radio is sup-plied on a 51/4 -inch MSDOS formatted dis-kette with 360 KByte capacity. After thecomputer is switched on, this disk is in-serted into one of the disk drives. Use theDOS to select the relevant disk drive andtype PCRADIO to load and run the controlprogram, which is automatically con-figured for the graphics adapter card usedin the computer. The cursor keys are usedto select the menu areas shown on theintroductory photograph. The adjust-ments that can be made include bass,treble, volume, manual or automatic tun-ing and a signal -level- dependent muting.The volume area is selected as a defaultwhen the program is started. The left andright keys on the cursor keypad allow thevolume to be reduced or increased respec-tively, while a coloured horizontal bar onthe screen indicates the relative setting. Afurther volume indication is provided bya large two -digit number to the right ofthe bar. The volume setting can takevalues between 0 and 63.

The up and down cursor keys select themenu areas. Pressing 1, for instance,moves the PC radio setting from the vol-ume to the treble area, which uses a col-oured bar and a two -digit indication likethe volume setting. Pressing 1. again se-lects the bass setting. Here, again, the %-and keys may be used to change thesetting.

The next area reached by pressingfrom the bass area is the tuning control.Press the -) and <- keys to tune up anddown respectively. The frequency stepsize in the VHF FM band is 10 kHz. Thetuning rate is increased automatically to

I Ulu tut tit

1 1 :1* 1411

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_Art1.1111The PCRADIO menu screen (control pro-gram version 1.2).

about 2 MHz per second if the or -4keys are depressed longer than necessaryfor a single keystroke. The fast tuning fea-ture allows the full VHF FM broadcastband to be covered in about 10 seconds.

Manual tuning is accomplished by typ-ing the required frequency on the numberkeys. Automatic station scan is selected onpressing the I or L key. In this mode, the<- and =, keys select between scanning upor down from the current frequency. Thescanner stops when a station is received.If the band is empty (hardly likely thesedays) or if the stations are too weak, theradio automatically reduces its detectionthreshold and starts a new scan. The highsensitivity mode may also be entered bypressing the scan cursor keys twice withinone second at the start of the station scan.

The relative field strength of the re-ceived station is indicated in the lowerleft-hand corner of the screen. The 'mute'function allows the level to be definedbelow which the AF amplifier of the radiois automatically muted. This functioneliminates noise between stations as thescan mode is in operation. It is selected viathe I and I keys, while the <- and -, keysallow the mute level - displayed in adifferent colour - to be adjusted to indi-vidual requirement. Reception of veryweak signals is made possible by movingthe mute bar to the extreme left of thescreen.

The menu area in the lower right cor-ner of the screen is reserved for the stationname, provided this is among the oneslisted to the left of the screen. Any stationcan be pre-programmed. When it is listed,it may be selected rapidly by pressing theassociated function key Fl through F10.

Stations are pre-programmed as fol-lows. First, tune to the desired station,then hold the SHIFT key depressed beforepressing the function key to define thestation number in the list. Next, type thestation name on the keyboard and pressENTER.

The PCRADIO control program is lefton pressing ESC (escape). Next, the user isasked whether or not the previously madechanges to the settings and the station listare to be saved or cancelled. After goingthrough this menu, the user is returned tothe DOS prompt. The radio remains on,however, using the current settings, al-though the control program is removedfrom the computer's memory. The com-puter is ready to load and run other soft-ware at this stage.

For advanced PC usersThe following information is aimed at ad-vanced PC users, allowing them to use anumber of special features of the PC radio.

The PC radio kit is supplied with adiskette that, apart from the previouslydescribed user software, cointains a do-cumentation file, READ.ME, and an installa-tion program, INSTALLEXE. Hard copy ofthe (ASCII-) READ.ME file is obtained in theusual way by typing CONTROL -P followedby TYPE READ.ME and a carriage return.Turn off the printer again by typing CON-TROL -P.

The PCRADIO program may be madememory -resident by calling it up with aswitch: PCRADIO <CR>. The program maybe called up while another program isrunning by pressing the two SHIFT keyssimultaneously. This key combinationmay be changed to requirement as de-tailed further on. The radio settings in usemay be changed as described earlier, butthey can not be saved to disk in the resi-dent mode of the control program. ThePCRADIO program is left by pressing theESC key. It may be cleared from the resi-dent program memory by typing PCRADIO/R from the DOS prompt.

The installation program allows anumber of hardware and software par-ameters to be pre -defined. Its menu startswith the I/O address, which has been setto default 300n on the PCB as well as in thecontrol program. Changes may be madeas required. Next, the system checks if thePC radio card can be addressed in theselected I/O area.ELEKTOR ELECTRONICS FEBRUARY 1990

PC RADIO

The next menu step allows the user todefine the key combination for calling upthe memory -resident program. The de-fault is the left and right sHIFT keys on thekeyboard_ Depending on whether othermemory -resident programs use the samecombination, this may be changed via asubmenu.

Block diagramWith reference to the block diagram inFig. I, a special feature of the VHF tunerblock that receives the signal from the ex-ternal aerial is that it contains an IF (inter-mediate frequency) stage as well as anaudio preamplifier. This allows the out-put of the tuner to be connected direct toa tone control section, which has directvoltage inputs for the bass, treble and vol-ume settings. The output signal providedby the tone control stage is applied to anon -board AF power amplifier capable ofdriving a 4 S2 loudspeaker.

The control voltages for the tuner andthe preamplifier/tone control section areprovided by digital -to -analogue conver-ters (DACs) which translate the binaryvalues obtained as digital combinationsfrom the PC bus via a databuffer intocorresponding voltage levels_ A 12 -bitDAC supplies the tuning voltage for thevariable capacitance diodes (varicaps) inthe FM tuner. The field strength voltage issent to the computer via an A -D converter.The DACs for volume-, bass and treblecontrol are 6 -bit types which offer ampleresolution for the purpose.

Circuit descriptionThe circuit diagram of the PC radio isgiven in Fig. 2. The operation of the fourmain circuit sections that make up the PCradio is discussed under separate head-ings.

VHF FM tunerThe aerial signal is applied to the mixerinputs of the TDA7021, pins 12 and 13, viabandfilter Li -C27-C25-C:.-. The bandfiltersuppresses signals outside the VHF FMbroadcast band. In the configuration used,the Type TDA7021T single -chip FM re-ceiver achieves a sensitivity of about 4 gV.The local oscillator frequency is deter-mined by external components L2 -Cm -C32and varicap K.Di. The LO is tuned by ap-plying a direct voltage between 0 V and10 V to R71. The IF amplifier on board theTDA7021T operates at 76 kHz, and usesonly three external capacitors, C33, C34 andC35. The field strength (f -s) output of thechip, pin 9, supplies a current of about20 gA when a strong station is received,and about 150 gA when no station is re-ceived. This current is passed through R74on which it causes a proportional voltage.Capacitor C41 eliminates short variationsof the field strength voltage.

The amplified IF signal is internallydemodulated, and the resultant AF signalis applied to an on -board Al' amplifier.The AF output voltage of the chip has anELEKTOR ELECTRONICS FEBRUARY 1990

FM tuner

tuning

A

field strength

preamplifierwith

tone control

V

A

treblebass

A

A

VO[L11110

address decoder and data buffer

9:G501.11

Fig. 1. Block diagram of the PC radio card.

amplitude of about SO mV at pin 14.

Tone controlThe Type TDA1524A preamplifier/tonecontrol chip receives its input signal viaC42. The second input of the chip, pin 4, isdecoupled by C43 because the stereo modeis not used in this application. Direct volt-ages applied to pins 1, 9 and 10 enable thetone and volume parameters to be set withthe aid of the internal electronic poten-tiometers. Capacitors C44 and C45 and re-sistor R77 are used for the bass control,while treble control is effected with C46.Resistor R.3 is optional, and selects be-tween linear and contour operation of thepreamplifier. If chip output pin 17 sup-plies a current between 1.5 mA and10 mA, the preamplifier works linearly. Ifthe current is smaller than about 0.5 mA,the loudness/contour correction is actu-ated.

The bass response of the TDA1524Aalso depends on external components.may be omitted, and C45 or C44 may bereplaced by a wire link to limit the lowfrequency range.

The chip receives its supply voltage viapin 3. The chip supply voltage is cleanedand buffered by C4` and C47 respectively.

AF power amplifierThe AF output signal at pin 11 of theTDA1524A is coupled out with C49 andtaken to input pin 1 of an integrated AFpower amplifier, ICI., a Type TDA2030.This chip provides a voltage amplificationof about 8 as well as the necessary currentamplification in its power output stage.

Since the chip output is at about halfthe supply voltage, a coupling capacitor,C53, is required to connect the loud-speaker. Components R33 and Cm sup-press any tendency of the AF amplifier tooscillate.

Digital sectionThe description of the digital section ofthe PC radio card is best started at theaddress decoder. This consists of an 8 -bitcomparator, IC4, for the base address, andtwo 1-01-4 decoders in IC3 for the ad-dresses within the selected block. Addresslines A2 through A9 are applied to thecomparator to achieve block decoding_The base address is set with jumpers Bnthrough Brs. Omitting jumpers Br andBi-2, for instance, selects base address300H, the default setting of the card (con-sult Table I for the address assignment).

When the CPU in the PC addresses I/Olocation 300H (A8 and A9 are high), outputP=Q of IC; goes low. Assuming that theI/O access is a write operation (i.e., pin 4of IC2 is low), pin 15 of IC3 goes low also.Depending on the bit combinations on ad-dress lines AO and Al, one of four outputs1'0 through 1'3 changes to low. If, for in-stance, address 302ii is selected, pin 11 ofIC3b goes low and provides a latch pulsefor IC7. The 8 -bit word on the databus islatched into D -type register 10-, whichholds the bass setting.

The four 6 -bit D -A converters are dis-crete types set up around four 8 -bit latch-ing registers, IC. through IC9, andassociated R -2R ladder networks. Thevoltage at the output of each ladder isprogrammable between 170 mV and 3.2 Vin 64 steps.

Circuit IC9, an 8 -bit latch, is used fordriving the 12 -bit D -A converter, !Chi, aswell as for field strength measurement.

The 12 -bit DAC receives an 8 -bit wordfrom IC9, and 2 -bit words from IC- andIC,. The DAC reads these words from ad-dress BASE+1. The reference voltage for theDAC is provided by voltage divider Rb2-Ro3 at pin 19. Opamp ICi is supplies a volt-age between 0 V and -1 V in 4,096 (10")steps. This voltage is amplified and in-verted by a further opamp,Kiib. Resistors

40 RADIO AND TELEVISION

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Fig. 2. Circuit diagram of the RF and AF sections of the PC radio.

R71 and R71 feed the tuning voltage to thevaricap in the FM tuner. The gain of thesecond opamp is adjustable over a rela-tively wide range to allow the tuningrange required to cover the FM band to beset with preset 1242.

The field strength voltage provided bythe TDA7021T is applied to pin 10 ofopamp ICiic. A field strength indication of0 on the screen corresponds to about1.35 V, and one of 63 to about 245 mV. Theopamp raises the field strength voltage toa level at which pin 12 of comparator ICI Idis held at a voltage of between 0 V and 4 V.The DAC built around IC' and the R -2Rnetwork allow the comparator thresholdat pin 13 of IC114 to be changed. The out-put state of the comparator can be re-quested at I/O address BASE -I via buffer1C12.i. Since the output voltage of theopamp can take values between -5 V and+12 V, R04, R70, D4 and Do are required tolimit the voltage to 0 V and 5 V.

The remaining three drivers in IC12 areused to check the addressing of DACs IC.,IC:- and ICs.

The circuit is reset at power -on by a100 -ms long low pulse at pins 1 and 2 ofICs.. This causes bistable IC5,1C5a to bereset via gates IC23 and IC5b. Transistor Ti

short-circuits the voltage at pin 1 of theTDA1524A to ground to reset the internalelectronic volume potentiometer. After100 ms, pin 4 of ICs reverts to logic high,

and it can be set, i.e., pin 6 can be madelow, by a read operation to address BASE -t-2via Depending on the position ofjumper JP!, the radio may also be switched

Off -set address read operation

0 (basel 05 - DO = 6 -bit DAC for volume control

DO = not used

D7 = control bit 1

05 - DO = 6 -bit DAC for bass controlDT; D6 = D9; D8 for 12 -bit tuning DAC

D7 = control bit 2

D5 - DO = 6 -bit DAC for treble controlD7; D6 = D11;1310 of 12 -bit tuning DAC

D7 = control bit 3

write operation

DO = comparatoroutput for field -strengthmeasurementD1 = control bit 1D2 = control bit 2D3 = control bit 3

latch data into 12 -bit DAC

turn on AF

05 - DO = 6 -bit DAC for field -strength turn off AF

- DO = D7 - DO for 12 -bit tuning DAC

Table 1. Input output address assignment on the PC radio card.


PC RADIO 41

to mute when the PC is reset. Anotherpossibility to control the mute functionexists in a read operation to addressBASE_? ( IC33 pin 7).

All supply voltages for the circuit are

taken from the PC expansion bus. Note,however, that the 5 V supply for the anal-ogue circuits is provided by voltage regu-lator ICi 3 -

ConstructionThe complete circuit is built on a double -sided through -plated printed circuitboard supplied with the kit. The size of the

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Fig. 3. Circuit diagram of the digital section and the PC -interface of the Ff.1 radio card.

ELEKTOR ELECTRONICS FERRI-ARV 19911


t2, ,If

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Fig. 4. Cutting and drilling details of thealuminium support bracket.

board is 230x106 nun.Start with fitting the low -profile com-

ponents as per the indications on the com-ponent overlay (Fig. 5) and the parts list.Proceed with the taller components. Sincethe board is through -plated, soldering atthe component side is not required. Leavethe wire terminals of Cs() and C53 longerthan usual to enable these capacitors to bebent towards the board as shown in thephotograph. The TDA2030 and its heat -sink are also mounted horizontally andsecured on to the board with an M3 bolt.

The TDA7021T FM receiver chip is anSMA (surface -mount assembly) typewhich requires great care and precision inhandling as well as soldering. First placethe IC on to the board. Next, position itaccurately with the aid of a pair of twee-zers so that the terminals align with thesolder areas. Use a low -power solder ironwith a fine tip and a power rating of notmore than 16 W to solder one of the cornerpins of TDA7021T to the associated track.Next, use a sharp device to align the otherIC pins with the tracks below. Heat thepins briefly in rapid succession while ap-plying very little solder.

Interconnect the points marked A andthose marked B on the PCB with an insu-lated wire of 0.4 mm' cross-sectional areaor larger.

The position of the tin-plate screenaround the FM tuner section on the PCB isshown in the top view of the completedprinted -circuit board in Fig. 5. Bend thescreen in the form of a rectangle and joinits ends by soldering. Place the screen somade vertically on to the component sideof the PCB, and align it with the PCBedges. This is necessary for the PCB sup-port bracket which is secured to the screenlater. Solder the screen to the PCB, ap-plying a generous amount of solder tinfrom the inside, but taking care not todamage components by overheating. Besure to cover the entire length of thescreen, i.e., do not solder it to the PCB injust a few locations. Next, bend and fit asimilarly shaped, but lower, screen on tothe component side of the PCB, oppositethe screen just mounted. Drill a hole in thetaller screen for the aerial input socket(9.4 mm dia.).

Use an M3x6 mm bolt, a washer and anut to secure a small aluminium bracketto the lower side of the PCB. Next, preparea standard PCB support bracket as shown

Parts list

Resistors:Ras. = 1120

R75 = 470Rat = 1000R62 = 2200Reg = 6800R63 = 8200Rte= 1k0Ras = 1k8R69 = 2k2Fiss:Res;Raa:R94 = 4k7R2 - /39;R49:R61 ;1364;R7CER76 - R83 = 10k

R17 - R21:R30 - R34;R53 - R47;1355 - R6,3 =12kRas = 15kRii - R16 ;R22;1:124 -Ft*";R35;R37 -R42;R4e:Rso- R55 = 24kR67 = 33kR71;R73 = 47kR66 = 68kRi;1110:1123;Ra6;R72 = 100kR68 = 4M7

= 50k preset V

Capacitors:C32 =18pCa = 33pC28;C67 = 56pC27 = 68pCao;Cal =180pC29 = 220pCm = 270pC34 = 620pC35 = 1n5C39 = 3n3Cm = 4n7C2s:C36;Cei = 10nC46 = 15nCa4;C65.-.C65:C6s:Cea = 22n ceramic06;C 7:C10;C ti :C25 ;C51 = 47nC44;C4s = 56nCas;Cai:C62 = 100n

Cis;Css = 220nCI - C4:Ce;C41;G49 = 1p0; 16 VC42;C43 = 2p2: 16 VC12 - C24;C40 = 10p.; 16 VCs2;Css = 47g; 16 V

= 10011: 16 VCso:Csa = 2200p; 16 VCs = 40p trimmer

Semiconductors:IClo = AD7545ICis = TDA1524ICia = TDA20301C14 = TDA7021T (SMD)ICs = 74LS00IC2 = 74LS32IC12 = 74LS125IC3 = 74LS139ICi = 74LS245les -1Ca = 74LS3741C4 = 74LS688ICit =1-10841C13 = 7805Ti = BC548KOs = BB809Do;137= 1N4001Di - D5 = 1N4148

Miscellaneous:Bus = loudspeaker socket.Qty 1: SKI 3 heat -sinkQty 1: coax socket.Qty 1: 3 -way pin header.Qty 1: jumper.Qty 1: aluminium bracket.Qty 1: tin-plate for screeningQty 3: screw M3x6.Qty 1: screw M3x8.Qty 4: nut M3.Qty 1: metal cover.Solder pins.90 mm insulated wire, min. OA mm2.140 mm silver-plated wire.3 m coaxial cable.

in Fig. 4. Secure this bracket and the aerialinput socket to the screen on the PCB.Solder the socket to screen, and secure thesupport bracket to it with two M3x6 mmbolts and nuts. Connect a short piece ofinsulated wire between the signal pin ofthe socket and point sT1 on the PCB. Notethe position of CS on the PCB and drill ahole in the metal cover to be mounted onto the screen at the component side of the

A complete kit of parts for the PCradio is available from the designers'exclusive worldwide distributors (re-grettably not in the USA and Canada):

ELV FranceB.P. -10F-57480 Sierck-les-BainsFRANCETelephone: +33 82837213Fax: +33 82838180

Also see ELV France's advertisementelsewhere in this issue.

board. This hole serves to access Cs with atrimming tool. Solder the covers to thetwo screens.

Install the PC radio card into the com-puter, and screw the support bracket tothe back panel. Do not forget to check thatthis panel is connected to ground of thePC - this is essential for the FM tuner onthe PC radio card. Finally, connect an FMaerial via 75 S2 coaxial cable.

AdjustmentStart the PC radio program from floppy -or hard disk. Use the menu to set the fre-quency to that of a station in the lowerrange of the FM band, e.g., to 88.5 MHz.Align Cs with an insulated trimming tooluntil this station is received (use a secondradio as a reference). Next, set a receivefrequency near the top of the band, e.g,104 MHz, and adjust R92 until a pre-viously selected station is received. If theadjustment of Cs is correct, the frequencyshown on the menu screen corresponds tothe transmit frequency of the station.

E1.EKTOR ELECTRONICS FEBRUARY 1990

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AC -De CURRENT SONCurrent may be measured without introducing losses in its pathwith the aid of the well-known series current transformers with

clip -on probes or reed sensorsin which a contact is closed by thepresence of a magnetic field. These devices suffer from a serious

drawback, however: they can not measure direct currents. Thesensors described here depend on the Hall effect and are suitable

for measuring both alternating and direct currents.

The Hall effectThe Hall effect is observed in thin strips ofmetal and in some semiconductors. Whena strip carrying current is placed in a mag-netic field that is perpendicular to thestrip's plane. a voltage appears betweenopposite edges of the strip that. although itis weak, will force a current through anexternal circuit.

The force due to the magnetic flux den-sity. B. on a charge carrier of charge e anddrift velocity. v. is equal to evB. This forcedisplaces the carriers and sets up a non-uniform charge distribution that gives riseto an electric field, E. At equilibrium. theforce on the charge carriers, eE. due to thisfield just balances that due to the flux den-sity:

eE+ evil =0.

Fig. 1. Movement of electrons in: (a) an electricfield: (b) a homogeneous electro-magnetic fieldin a vacuum: and (c) in a degenerate semicon-

ductor material.

The drift velocity is related to the cur-rem density, j, by

j= nev

where n is the number of charge carrier,per unit volume. Thus. the electric fieldrelated to the vector product of the mag-netic flux density and the current densityby:

E = -R11(.lB)

where RH is the Hall coefficient that isequal to line. The electric field results ina potential difference. UH. the Hall volt-age. across the material.

In some materials, the direction of thefield is reversed, which means that thesematerials have a positive Hall coefficient.This indicates that in these materials thecurrent is carried by positively chargedcarriers, that is. by holes.

ElectrOns in an electric field move instraight lines as shown in Fig. la. whereasin an homegeneous electro-magnetic fieldthey follow cycloidal paths (Fig. I b). Indegenerate semiconductor materials, theycan not follow such paths owing to fre-quent collisions with atoms of the semi-conductor material. Because of these colli-sions. the electrons lose velocity and thisresults in a waning of the magnetic field.since the strength of that field is directlyproportional to the velocity of the chargecarriers. The electrons are consequentlyattracted more strongly by the electricfield as shown in Fig. lc.

When a wafer of semiconductor mate-rial is connected as shown in Fig. 2 andplaced in a magnetic field, an e.m.f.. theHall voltage. UH, is generated across thelonger sides of the wafer. Because of this.the wafer is called a Hall generator. Thelevel of the e.m.f. is directly proportionalto the strength of the magnetic field. If thedirection of the field is reversed. the direc-tion of the voltage also changes by 18(1-.Within certain limits. the e.m.f. is

UH= Rain 1 d

where d is the thickness of the wafer. Notethat the Hall voltage is inversely propor-tional to the thickness of the material.

Fig. 2. The Hall voltage. 4. is generated acrossthe longitudinal sides of a Hall generator.

Hall generators have come more intovogue with the arrival of semiconductormaterials such as indium antimonide(InSb) and indium arsenide (InAs), whichhave a high electron mobility. resulting ina large Hall coefficient. and a low specificresistance. Table I iliveS a comparison ofthe electron mobility (average drift veloc-ity per unit electric field). v. in copper anda number of semiconductor materials.

anent payingcorAxIct

Fatiettectcere,atOe

17-tn yoke

wated field

Fig. 3. With the Hall generator in the air gap ofthe iron yoke of a current sensor, the Hall volt-age is directly proportional to the current flow-ing in a conductor that is at right angles to the

cross-sectional plane of the yoke.

ELEKTOR EI.ECTRONICS FEBRUARY 1990

AC -DC CL RRENT SENSORS 45

Material Type

coppersilicon

germanium

InAs

InSb

metal

group 4 semiconductor

group 5 semiconductor

By semiconductor

By semiconductor

Drift velocity(cm2 s I)

27-431350

3900

23 00065 000

Table 1. Electron mobility in copper andconductor materials.

a number of semi -

Fig. 4. Basic circuit of a direct -reading current sensor.

Fig. 5. In a compensating current sensor. the Hall elementgenerates a current in a secondary winding.

Parameter

Current rating

Nominal output

Nominal output

impedance

Supply current

Supply voltage

Ambient temperature

Measuring accuracy

Linearity

Drift with temperature

Max frequency

Response time

Output signal:

Offset

Drift

Hysteresis

Drift with temperature

Drift with supply voltage

Direct -reading Compensating

150-3000 A10V

10 kt-2

15 mA

±15 V

0-80 C

.250 ppm = C

10 kHz

±-30

5_20 mV

20-50 mV

5_25 mV

mV = C

5_0.1-10 mV V

100-250 A

100 mA

50

8 mA

±15 V

-25 to +70° C

_-50 ppm = C

75-300 kHz

±1 us

<1 mA

0.05-0.1 mA

<0.2 mA

c12 C

S0.01-0.03 mA V

Table 2. Comparison of parameters of direct -reading and compensating sensors.

Note that in spite of the highvalue of v in indium antimonidethis material is not used much be-cause of the fairly large change inr with temperature: over the tem-perature range 20-120° C. for ex-ample. the value of v changes bya factor 5.

Practical sensors

In practice. the wafer thickness isof the order of I-5 pm. which notonly gives a larger Hall voltage.but also enables the wafer to be

broaxlar y due tosedheatngof i-ternal

ilelf (A)

Ta ['CItooldary die tosetRieothg ofinternal dean:rice

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used in very small air gaps necessitated byweak magnetic fields. This may be seen inthe schematic representation of a sensor inFig. 3, in which the Hall voltage is directlyproportional to the current flowingthrough a conductor that is perpendicularto the cross-sectional plane of the yoke ofthe sensor.

There are two types of Hall -effect cur-rent sensor: direct -reading and compensat-ing-see Fig. 4 and Fig. 5.

The main advantage of the direct -read-ing type lies in the minimal losses itcauses. which is, of course, particularlyimportant when large currents are mea-sured. The amplified output voltage. u,, isdirectly proportional to current

The compensating type is particularlyuseful at higher frequencies-see Table 2.The current to be measured is coupledmagnetically to a secondary winding onthe yoke. The Hall element serves as a de-tector for the magnetic '0' which is in-duced with the aid of an auxiliaQ, (com-pensating) current fed into the secondarywinding. This compensating current is avery precise measure of the primary cur-rent. This type of sensor is eminently suit-able for measuring square -wave currents.

Equivalent parameters of the compen-sating sensor may be determined from afour -quadrant diagram as in Fig. 6. If, forexample. the r.m.s. value of the current tobe measured is 100 A. its peak value is150 A, and the maximum ambient temper-ature is 70° C. Drawing lines at right an-gles to the three corresponding axes givesa load resistance of 40-50 O.

Fig. 6. The operating parameters may be determined from a four -quadrant diagram.


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YD

PART 2: MODULATION BOARD

A. Rigby & G. Dam

Last month's introductory article described the general setup of thevideo mixer, and discussed the operation and construction of the

video switching board. This month we turn our attention to themodulation board which supplies two sets of modulation waveformsthat together put many attractive picture mixing, wipe, fade-in and

fade-out effects at your disposal.

As already discussed in Part 1 of this ar-ticle, the video mixer consists of threeblocks: a video switching board, a modu-lation board and a keyboard. These blocksform the basic configuration of the mixeras shown in Fig. 1 in last month's instal-ment. The modulation board and the key-board receive a number ofsynchronization signals and the supplyvoltage from the video switching board.The keyboard circuit supplies all thenecessary control signals to the videoswitching board and the modulationboard.

Effects waveformsAll picture mixing effects provided by thevideo mixer, and all combinations thereofselected by the user, are based on threeelementary waveforms: the triangle, theramp (linearly rising sawtooth) and theparabola. All three are available in thehorizontal (line -synchronous) as well asthe vertical (raster -synchronous) pictureplane.

All picture mixing and combination ef-fects rely on the switching between twovideo sources. A left -to -right wipe (cur-tain) effect, for instance, requires a circuitthat switches between two video sourcesat accurately defined instants in the pic-ture line. The HSW signal is used for thispurpose. Similarly, the VSW signal is usedif the wipe effect is required vertically(top -to -bottom). Both HSW and VSW arerectangular signals whose duty factor i,controlled to obtain the wipe effect. Therequired duty factor variation is securedwith the aid of a comparator that com-pares a ramp voltage to a reference volt-age set with the wipe control (a slidepotentiometer on the front panel of thevideo mixer). The other picture mixingand effects are obtained by combining dif-ferent patterns, horizontally as well a!,

vertically.

Block diagramThe waveform generators for the mixingeffects are shown to the left in the block

000°HSYNC

0°LINEARSIGNAL

00-P

HORMOD

0

MOD IN

0oVERTMOD

0

IN ItEt

0 Li

0o

0o00

HORIZONTALKEYING

MC5

SELECT1 OF 4

VERTICALKEYING

HORIZONTALFADER

SELECT

I.IODULATICINPUT

SELECT

VERTICALFACER

[111:t7;:j--

CCMPARATC P.

-4-SELECTI OF 2

AGO SUFFER

INVERTER

SELECT1 OF 2

SELECT1 OF 4

a -

COMPARATOR

VS11

VSla

Fig. 6. Block diagram of the modulation board.



diagram in Fig. 6. Since the same wave-forms are in principle required horizon-tally and vertically (only the frequency isdifferent), almost identical generators areused. In both cases, the waveforms aresynchronized with the video signal. Theparabola- and triangle waveforms arederived from the ramp voltage. All threewaveforms are applied to comparators viaelectronic switches. Depending on theirfunction, the comparators supply either ahorizontal (HSW) or a vertical (VSW)switching signal.

A separate modulation input allowsadditional control over the selected effect.The modulation signal applied to thisinput may be synchronous or non -synchronous in relation to the picture. De-pending on the available video materialand the applied waveform, remarkableand sometimes quite unpredictable effectsmay be obtained in addition to the onesnormally provided by the video mixer.

The horizontal and vertical keying in-puts, finally, enable 'home-made' pat-terns to be added to the picture.

A buffer and an inverter feed the se-lected vertical waveform to the horizontalcomparator, which compares it to thewaveforms generated in synchronismwith the horizontal line pulses. The inver-ter may be switched on and off by a con-trol on the keyboard. Depending on thelevels applied to the comparator, an HSWsignal is supplied that switches betweentwo video sources at a particular instantin the picture line. This combination of

fairly complex mixing effects to be real-ized.

The vertical waveforms are availablein non -inverted as well as inverted form.The selection is made by the user with theaid of a keyboard control, and allows twovideo signals to be transposed on thescreen. Taking the previously mentionedvertical wipe effect as an example, the in-vert control allows the 'top' and the 'bot-tom' pictures to change places in themixed image.

Circuit descriptionThe circuit diagram of the modulationboard is given in Fig. 7_ The three -stagehorizontal waveform generator is shownin the top left hand corner. The almostidentical vertical waveform generator isfound in the lower left-hand corner. Thefollowing description of the operation ofthese circuits refers to the horizontalwaveform generator, IC20-1C27-1C2s-IC2ya,To and IC40.

The HSYNC signal is applied to thewaveform generator to ensure that this issynchronized with each picture line. Dur-ing the HSYNC pulse, capacitor C, : is dis-charged via \4-, so that integrator IC2. canstart from zero at the end of the pulse. Theresult of the integration is a line -synchronous ramp voltage at the outputof 1C20. To produce a triangle voltage, theramp is first converted into a rectangularwave by comparator IC27, which corn -

pares the instantaneous amplitude of theramp to the reference voltage at the wiperof preset P. The rectangular signal so ob-tained is applied to integrator IC23, whichcharges or discharges C as the output ofIC27 goes high or low. The result of thissecond integration is a triangle voltage atthe output of the opamp.

The horizontal synchronization pulse,

to be completely discharged via N5i atthe start of each picture line. Preset INdetermines the switching level of the com-parator and with it the symmetry of thetriangle voltage, while preset P7 sets theamplitude.

Transistor TO converts the triangle volt-age applied to its base via preset Ps into aparabolic voltage, which is subsequentlyamplified by IC2.5b. The amplitude and theoff -set voltage of the parabolic waveformare adjusted with presets Ps and P' respec-tively.

The vertical waveforms are obtained ina manner similar to the horizontal ones.However, the frequency is 50 Hz insteadof 15,625 Hz, and VSYNC is used to en-sure vertical synchronization to the mixedpicture.

Demultiplexers IC29. and IC24stogether determine the waveform selec-tion for the mixing effects. This selectionis accomplished in conjunction with elec-tronic switches N,:-\ andand De-pending on the logic levels on controllines \1C1-MC2 (IC:-, ) and MC3-NIC-1(IC29b), either the ramp-, triangle-, parabo-la- or KEY- signal is used. The selectedsignal is applied to opamp IC -q.; for com-paring to the voltage at the inverting inputwhich takes either a steady voltage or oneof the vertical waveforms.

The signal at the non -inverting input ofIC343 is selected by electronic switches Neior No3. To ensure that the effects selectionremains in synchronism with the raster,

these switches are controlled in com-plementary fashion by bistable IC33b. Theother bistable in the 7-ifiCT7.1 package,IC333, functions as a monostable multivi-brator. It is clocked with VSYNC and sup-plies short pulses at its Q output. Thesepulses clock IC3.3b and time the instant itlatches the logic level of control line MC5connected to its D (data) input_ Thelatched level is subsequently transferredto the Q and Q outputs.

The direct voltage supplied by thehorizontal FADING control, PT', may be ap-plied to the input of IC -;4a by switching on

The toggling of the FISIV signal sup-plied by inverter N., determines the bor-der between the mixed pictures - in otherwords, the location (or instant) in the pic-ture line at which the switching betweenthe two video sources takes place. PresetsP14 and Pit serve to give potentiometer Pisits maximum usable range, i.e., the widthof the picture. Capacitor C allows analternating voltage to be superimposed onto the direct voltage at the wiper of Pi..This alternating voltage is taken from theexternal effects input and first buffered byIC37. If the external input is not used, thenegative terminal of Cs4 is taken to groundby switch This is done to preventnoise being picked up, which would causeinterference in the mixed picture.

The modulation signal supplied byIC37 is also applied to comparator IC14b,which adds it to the direct voltage sup-plied by vertical FADING control P2o. Theselected vertical waveform is applied tothe positive input of the comparator. Theoutput signal is VSW or VSW which indi-cates the vertical (raster -synchronous)switching instant.

Series -connected opamps ICF,a-1C33b-IC3, form a precision inverter or buffer forthe vertical effects waveform selected bythe user via N., - N;-. Presets Pi:, Pia andP22 are adjusted to cancel off -set voltagesELEKTOR ELECTRONICS FEBRUARY 1990

VIDEO MIXER PART 2: MODULATION BOARD 49

4.3

2 //_A.

---7=EEZT=ZZq0 la

---

.1

Q; 0 \.s

22225cooca

1. 4(.1

41

1

1>`'0 1

Fig. 7. Circuit diagram of the second unit in the video mixer, the modulation board.

ELEKTOR ELECI RON1CS FEBRUARY 1990


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Fig. 8. Component mounting plan of the PCB for the modulation board.

in the three stages. The fairly extensivecompensation is required to ensure thatthe output of IC36 supplies the exact in-verse of the signal applied to the externaleffects input. Control line MC6 is used toselect between the inverted and non -in-verted version of the vertical effects volt-age.

ConstructionThe single -sided printed -circuit board forthe circuit is shown in Fig. S. Construction

should not present problems to the experi-enced electronics constructor at whomthis project is aimed.

Start the construction of the board withfitting all the wire links. Next, mount thesolder pins, connectors and IC sockets, ifused. Check your work so far and proceedwith the passive parts and the singlediode. Be sure to observe the polarity ofthe electrolytic capacitors and the diode.Next, mount the voltage regulators andbolt them straight on to the board - heat -sinks are not required. Lastly, plug the 1Cs

in their sockets or, if sockets are not used,solder them direct on to the board. Theslide potentiometers are mounted on tothe keyboard PCB to be discussed in Part 3of this article. Set all presets on the modu-lation board to the centre of their travel.

To he continued next month

Part 1 this article appeared in the January 1990issue 0: EieCITOniC5.

ELERTI)R ELECTRONICS FEBRUARY 1990

VIDEO MIXER PART 2: MODULATION BOARD

Parts list

Resistors:R7a = 220kRao:Ra4;Ra2;R:R97 = 4700Flai;R52;Ras;RacR1o5:R1o7;R1.c.:Riii =10k1383 = 6800R50;R85;R9cnIct;n120 = 1k0Rae = 47kRai = 82kRea = 470kRaa;R 1o3:Rio4;Rii2 - 131 Is:R117 - R119;R123 = 220k

Rao;Riot = 100R91 = 390kRs5;R122 = 5k6Ras = 120kRioo = 150kR102 = 470kRica = 22kR110 =33kR116 = 180kR121 = 560kP5;P7= 10k preset HP6;P1O;P17 = 50k preset HPa;1312:1311;Pia:Pa;P23 = 100k preset H

Pe;P13 = 500k preset HPi4;P1s = 5k preset HPis;Pcso = 10k linear slide potentiometer(not on PCB)Pia;P21 = 2k5 preset HP24 = 50k preset H

Capacitors:C61= 390pCe2:Caa:C:C67:Ca9:C73;C74;C77;C7a;

C7a;Ca2;Ca3;Cas - C85:C91;C52;C97 - C11:6;C107;C111 - C119 =100n

Cse;Cas = 27nCaa;Cao = 15pC70 = 1 nO

C71 12pC72 = 68nC75;C76 = 4p7;16 V radialC81 = 82n

Csa:Cg3;Cs4:Ca5 = 10p; 16 V radialCas;Cao = 100pCes;Cioa = 330nelm) = 27pOwe = 1n9

Semiconductors:De = 1 N41481C24 = 74HCT04

1C25 = 78051C26:1Cao;IC26:1C37 = TL081

IC27;1C31 = CA3130lC2s;IC22;1C3.5 = TL082IC29 = 74HC239IC33 = 74HCT74IC34 = LM319NlCss - IC4.3 = 4066lC44 = 7905T6 = BC516T7 = BC557B

Miscellaneous:KMC1 = 20 -way PCB header.PCB Type 87304-2 (see Readers Servicespage).

IEE MEETINGS5 Feb - Evaluating student performance:

key issues and strategies.6 Feb - Integrating control system design

with the analysis of flexible structures.7 Feb - The role of venture capital in the

UK electronics industry.8 Feb - Broadcast TV recording.8 Feb - Technical visit and discussion

meeting at Colchester TechnologyPark.

12 Feb - The radio data system.13 Feb - Safety critical software in vehi-

cle and traffic control.13 Feb - Data storage technology.14 Feb - Stereo sound for television: im-

plementation.15 Feb - Interactive computing: a revolu-

tionary medium for teaching and de-sign.

20 Feb - Definition of traceability.21 Feb - The interaction of radiowaves

with the sea surface.28 Feb - Safety regulations and codes for

power and lighting in public areas.28 Feb - Nucleonic instrumentation.

Information on these, and many other.events may be obtained from the IEE,Savoy Place, LONDON WC2R OBL,Telephone 01-240 1871.

The UKCMG conference entitled "Newdimensions in application development"

EVENTSorganized by the UKCMG in associationwith Blenheim Online will be held on 7-8February at the Metropole Hotel. Brighton

The conference is designed to assist inmaking sense of the critical developmentsin the IT industry today. Application de-velopers need to understand the issues in-herent in the move towards high produc-tivity development environments, the de-sire for open systems and applicationportability and the continuing trend to-wards more flexible and adaptabledatabase management systems. Further de-tails from Blenheim Online, BlenheimHouse, Ash Hill Drive, PINNER HAS2AE, Telephone 01-868 4466.

Frost & Sullivan have organized a numberof seminars for this month on Informa-tion Technology; Telecommunications &Data Communications; and ElectronicEngineering. Details from Frost & Sulli-van, Sullivan House, 4 Grosvenor Gar-dens, LONDON SW1W ODIL Tele-phone 01-730 3438.

CALL FOR PAPERSPapers are invited for the Second Interna-tional Conference on Rural Communi-cations. which will be held at the Institu-

tion of Electrical Engineers, London. on29-31 October this year, and for the

Conference on Television Mea-surements. which will he held in Mon-treux. Switzerland, on 20-22 June 1991.Details from IEE, Savoy Place, LON-DON WC2R OBL, Telephone 01-2401871.

A number of courses are conducted byLearning Tree International betweennow and May in London. Stockholm.Paris. Los Angeles, Washington. Torontoand Ottawa. There are courses for "Man-agers in technical environments", on-Networks, Datacom and Telecom". andon "Software Development and Sys-tems". For further information phone0800 282 353 or 01-748 6667 in London(fax 01-748 5005) or 08-18 99 00 inStockholm (fax 08-18 99 07).

Saudicom 90, Riyadh's third electronics,communications and computer show takesplace from 28 January to 1 February. Allthe giants of the industry such A T & T,Siemens, Ericson, British Telecom, Telet-tra, Plessey. France Telecom. Aerospatial.Alcatel. Telecom Australia International,and others, will be there. Further informa-tion from Overseas Exhibition ServicesLtd, 11 Manchester Square, LONDONW1M 5AB, Telephone 01-486 1951.


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THE DIGITAL MODEL TRAINPART 11 - THE MONITOR UNIT

by T. Wigmore

The monitor unit enables the system to read signals from, for in-stance, the rails. This makes it possible to determine the positionof any train at any one instant. Monitor units are indispensablewhere track sections are protected via a host computer or if the

running schedule is controlled by the computer.

The monitor units have eight digital inputsand 62 of them may be used, so that a totalof 496 inputs can currently be provided inthe system, but this number may be dou-bled in the future.

Note that the monitor units are not ex-changeable with decoder Type s88 fromGoppinger. Those decoders have 16 inputsand are interconnected via a 6 -way insteadof a 5 -way connector.

The circuit of the monitor unit is fairlysimple as may be seen in Fig. 70. It is basedon an 8 -bit shift register Type 4014 (ICI)that reads the parallel data at the eight in-puts serially. The inputs are not connecteddirect to the parallel load pins of the regis-ter but via eight R -S bistables (IC2 andIC3). This is done in this manner because

the serial reading of the monitor units canonly take place when the host computerhas issued a status request instruction viathe RS232 bus.

The bistables in the monitor units alsoensure that even very short input signalsto the system are acted upon.

Resistors RI-Rs form some protectionfor the inputs and also, in conjunction withCI-Cs, suppress noise pulses.

Self diagnosisWhen requesting the status of the monitorunits, the system loads the position of theeight bistables in IC2 and 1C3 into the shiftregister, la via a pulse at pin I of Kt. Thebistables are reset automatically by the

130

10

20

30

40

50

6d

70

K2DIN S

5V

+55 ilto Ril RI

g.

513 RI 515

0515,

'5171

o 0n

518

3

52;.9

1

RI e R10

Si9t®7

1252

EMI "02

53

53 4

1C2

4044EMI 3

Ro

SO

1311

54 14 0 SEA

ECM 1 o _1150

"16 51 a''3

F OFICI

5V7.

CG 120 4014

CH 315 5 8 C

e 5 '0tS755

7

RP51

'LI

WOi2

...1...'R6

"R2 0252

108 9 10E311

4

1C3

404457 1 PoSG

Oo3

14Rli03

t553'

CI C2 C3 CI ciiiCCI C7 Ca CRMIN MIN MIN NM MIN MS MN NMI

TieI 7707070470nTIOTICto

87591-515- 11x vv

DI? 5 -1,/,

Fig. 70. Circuit diagram of the monitor unit.

trailing edge of this pulse. After the datahave been loaded, the system reads themserially from the shift register. If more thanone monitor unit is used, the shift registersare cascaded so that they may be read asone large shift register.

The number of data bits that the systemhas to read does, of course, depend on thenumber of monitor units in use. To preventtime being wasted in reading data of non -connected monitors units, the system car-ries out a self -diagnosis at power -up andreset, when the number of connected mon-itor units is determined automatically. Allbistables are then reset and read immedi-ately.

The serial input of the last shift registeris shorted to the positive supply line viaRis, which causes the system to read eighttimes '1' immediately after the data of thelast monitor unit have been received. Inthis way, the system knows that the lastunit has been read.

At each subsequent status request, thesystem takes account of the number of ac-tually connected units. This self -diagnosiscauses a small limitation: on power -up, itis not permissible for all eight inputs of amonitor unit to be active simultaneously,because that would indicate that the unitwas the last in the row.

If the status of a non -connected unit isrequested, the error LED lights. The num-ber of monitor units may also be deter-mined via a separate RS232 instruction(see Table 8 in last month's instalment).

ConstructionThe construction of the monitor units issimple and is best carried out on the PCBshown in Fig. 71.

Before the board is fitted in the enclo-sure specified in the components list, twocorners (indicated on the board) should becut away as shown in Fig. 73.

The monitor units are interconnectedvia fairly inexpensive 5 -way DIN connec-tors_ If they are going to be used in fixedlocations, soldered connections, with orwithout the use of soldering pins, are, ofcourse, perfectly all right.

Connexions to the inputs may also be


54 GENERAL INTEREST

soldered, but if the board is fitted in an en-closure, preference should be given to theuse of 2.5 mm plugs and sockets.

Connecting up andactuating the inputsMonitor units are interconnected via Ktand K2. The arrow shown near these con-nectors in Fig. 70 must always point to themother board. If this is not observed, thesystem will not work, although nothingmore disastrous will happen.

All connector pins must be connectedto the same numbered pin on the connec-tors in other units, preferably by the use ofready-made non -twisted, 5 -core DIN cable.The total length of cable between the unitsis of no importance to the correct operationof the system.

In principle, a monitor unit is actuatedwhen the brown wire or rail of the systemis connected to earth. For this purpose, twoearthing points are provided on the PCB. Itis also possible to actuate it by connectingan input to the red wire of the system, butnot if the system is in the STOP mode.

Electronic train detectionSignalling to and from locomotives andcoaches is often performed with the aid ofreed relays. Both from a technical and anaesthetic point of view, this is not a goodway of doing it: electronic means are invis-ible and seldom fail (which can not be saidof reed relays).

If Marklin rails are used, M -sections areavailable in which one rail is isolated fromthe 'metal gravel' base. This isolated sec-tion is simply connected to one of the in-puts of a monitor unit.

Where K -rails (which are isolated fromeach other) are used, a section of one railmay be isolated from that rail by two sawcuts as shown in Fig. 72. The length of this

section should be at least as long asthe largest wheel base of locomotivesand coaches used: this results in onelong signal instead of a series ofshorter ones. Cut the rail at an anglerather than straight across: in thatway trains will have a smoother rideover the section.

Where a standard rail system isused, the detection methods shown inFig. 74 or Fig. 75 should be used.

The isolated section of rail inFig. 74 is powered via two anti -paral-lel -connected diodes. It does not mat-ter whether the diodes are connectedto the brown or the red terminal.Rails sections from which no signal isrequired may be connected direct tothe booster unit.

A disadvantage of the method inFig. 74 is that only powered rollingstock can be detected. If a train losesone or more coaches, these will be 'in-visible' to the system Therefore, thecircuit in Fig. 75 may be preferred.The output of the comparator in thatcircuit is actuated when a relativelyhigh resistance exists between thetwo rails. By providing coaches with10 k resistor as shown in Fig. 76, they canalso be detected by monitor units.

Fig. 71. Printed circuit board for the monitor unit.

a

Status requestsThe status of monitor units can be re -gusted only via the RS232 interface. Afterpower -up or a reset, one or more groups oftwo bytes from two successive monitorunits (that is, groups of 16 inputs) are sentin response to each status request instruc-tion. This is done in this way to ensure fullcompatibility with the NIarklin Digital Sys-tem. However, as explained in Part 10, ifdesired, the monitor units may be made toreact individually.

COMPONENTS LIST

Resistors:R1-RB=10kR9 -F117 =100 k

1418.47k

Capacitors:=

=

Semiconductors:ICI = 4

IC2: iC3 =

Miscellaneous:Kt: K2 = 5.way DIU connect)! for PCS imourm-ng

En.osurei OKV/ Type A4407111

10 pairs of 2_5 mm plugs and sockets (see text)PCB Type 87291-8

Fig. 72. How to connect an isolated section of the track to a monitor unit (black arrow points to inputon monitor unit). In the illustration. use is made of Marklin rails.

Fig. 73. The finished board fitted in the specifiedenclosure; note the cut -away corners.

ELEKTOR ELECTRONICS FEBRUARY .1990

THE DIGITAL. MODEL 'TRAIN - PART II: TIIE MONITOR t.-Nrr

Fig. 74. This circuit for electronic train detection can locate only poweredrolling stock (locomotives and coaches with lighting).

rt, n n n n n 1-1 n n 1-1 n n rl

EEDTSbooster

R 2

Mala

1 6

01 02 03IB ATS5

A I = 113 IC 1 = i4 LM339 ( 1/2 LM393)

LM339

C1101

WIPE""

1-30n

5V

LM393

CiPt77- metaTeR

17201 - - 14

2--.,ax01377.MR, $

Fig.Fig. 75. Circuit of a sensitive detector system that is actuated by a rela-tively high resistance between the two rails.

System miscellanyLocomotive decoders (Part 2).It appears that loc decoders can be affectedby switching instructions intended forpoints and signals. This can be preventedby increasing the value of RI from 12 kS2 to33 kfl or even 39 ka

Switching decoders (Part 4).An error has occurred in the circuit dia-gram and components list. Circuits ICsand IC6 should be Type ULN2003 and notULN2001 as stated. The decoder will work

Fig. 76. Providing non -powered rolling stock with a 10 ki2 resistor enablesit to be detected with the aid of the circuit in Fig. 75.

Request instruction <192+n> in = 1-311

1St AN BYTE

D7 06 S Dd D3 02 01 D0

74

2odANSWER BYTE

D7 06 OS 06 D3 D2 01 00

-0 r

(2n - 1) unit

4

2n unit

2nd answer byte only with NIiirklin emulation(default at power -up and reset)

Fig. 77. Composition of bytes sent by the system in response to a statusrequest via the RS232 interface.

Fig. 78. Additional circuit for obtaining proper overload protection whenmore than one booster is used.

with the ULN2001 hut not entirely to spec-ification.

Parts published so far.

Part 1. Marklin system (description).Using more than one booster (Part 6). Part 2. Loc decoder (description).If the size of the track makes it necessary Part 3. Loc decoder (construction).for several booster units to be used (when Part 4. Universal signal and switchingeach booster powers a section of the track), decoder.a small addition is necessary to provide ['art 5. Description of Elektor Electronicsadquate overload protection. The circuit system.for this is shown in Fig. 78. It is basically a Part 6. Booster unit.composite OR gate that detects overloads ['art 7. Mother board (description).in up to five booster units. If any one Part 8. Mother board (construction).booster unit becomes overloaded, the en- Part 9. Keyboards.tire system is placed in the STOP mode. Part 10. RS232 Interface.


56

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PLATE AMPLIFIERSBipolar and Mosfet modules with the sameelectronics as above amplifiers housed in adifferent extrusion without heatsink.HY6060P 3044' Stereo 9 ioc'er amp. £20.10 HY3/344, 1601. 0 _... = = = 4 - V..5.91HYf24P 60W Bipolar o-ro (4 ohml £14.65 t83265P 180W 6 ;,. .- a - , = , - - E-25.99HY128P EON Bipolar rm.° 18 ohm) £14.46 Y051221 670i V.V.. E-7. 9.10.50HY244P 120W Broolar ampl4ollm! £220.10 0524.61,120W M: -.1,s.. =_= r.-39.93"

1:1,6 (5 9-2-0.10 v.:9192P I .9-?..., V.:1,.. _ .. . . £95.96

Note: These modules require additional heatsinks

POWER SUPPLIESComprising toroidal transformer and DC board topower the ILP amplifier modules.P5U2.0 Pre.am.; 7 e- £10.50 i'5..5. --I2 kiY24E £27.40PSU2I2 I or 2 HY30 £19.25 P50552 0.0524,7. £29.60PSU412 HY6060.F11124.1or2 iiY60 £21.45 51.1712 HY24.1 £31.75PSU472 HY128 £23.70 .50722 HY246 .2. £32.60P50432 M06128 £24.75 050732 i4Y354 £32.60P.S.U512 HY244. 11V128 121 £26.35 F50737 HY39.5 £34.95PSU522 H. 12412( £26.35 .50752 M09-304 ''05249 .12 £33.95P5U532 005128 (2! £27.40

PRE -AMP and MIXER MODULESThese encapsulated modules are supplied within -line connectors but require potentiometers,switches etc.

HY6HY66HY8386866

Mono pfeamp with bass anal ueb!eStereo are -amp with bass and treVeGuitar pre -amp with so-mial effects .

No.cyJntina board for HY6 .Mo.ntiro board for HY66 or iiè83

£9.30£15.50£18.65

£1.15£1.75

POWER SLAVESThese cased amplifiers are supplied assembled andtested in 60 and 120 watt Bipolar or Mosfet versions.0612 80 wan 13,Wai 14v' n £77.50 0532 60 au Mosfet £105.950672 120 Watt Bipolar Kohm £85.95 0542 120 watt !Arafat £115.95

Pripet ,ncl.ds VAT and car; 570

Quantity prices available on requestWrite or phone for free Data Pack

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Telephone: (0227)375254 Fax: 0227 365104

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Vie have ust received a batch of Assorted Mcdu!es ALL AT PRICE

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FOR PRINTED CIRCUIT BOARDS. OJanily aya:iaose from . £10

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INTERMEDIATE PROJECTA series of projects for the not -so -experienced constructor. Although each article

will describe in detail the operation, use, construction and, where relevant, theunderlying theory of the project, constructors will, none the less, require an

elementary knowledge of electronic engineering. Each project in the series will bebased on inexpensive and commonly available parts.

8. Reflex MW AM receiver

Radio techniques even at the most fun-damental level will continue to fascinateelectronics enthusiasts because a handfulof components work the miracle of wire-less reception. This month we save a oncepopular circuit from oblivion. The radiofascination catches on with this reflex AMreceiver, sporting one tuned circuit formedium -wave reception.The so-called direct conversion receiverforms the basis of all radio receivers. In itsmost rudimentary form, it provides anaerial input, a tuned circuit to select thewanted RF signal from many others trans-mitting in the same band, and a detectorto extract the modulation signal (i.e., theinformation) from the carrier. In a moreelaborate design, an RF- and an AF- am-plifier may be used before and after thedetector respectively. The direct conver-sion receiver differs from the super -he-terodyne receiver in that it does not usefrequency conversion of the received RFsignal at any stage. Although the directconversion receiver can not stand corn -

J. Bareford

parison with the selectivity of thesuperhet, it is none the less interesting tobuild as a first ever electronics project.Again making the comparison with thesuperheterodyne receiver, the direct con-version type is inexpensive, uncritical,and free of interference effects. What ismore, it provides quite acceptable soundquality.

Flashback or startThe circuit diagram of the simplest of re-ceivers, the direct conversion type with-out RF or AF amplification, is shown inFig. I. The combination of an inductor inparallel with a variable capacitor forms atuned circuit that resonates at the transmitfrequency of the station to be received.One side if the tuned circuit is connectedto ground, the other side to a small capa-citor that takes the RF signals from a long-wire aerial. The detector, a semiconductordiode, is connected to a tap on the induc-tor. This is done to reduce the loading(damping) effect on the tuned circuit,which would degrade the selectivity. TheRF component, i.e., the carrier, is shortedto ground by a capacitor at the anode ofthe diode. The resulting AF componentcan be heard in the high -impedance ear-piece connected across the capacitor. Theone advantage of the circuit in Fig. I isthat it does not require a supply voltage.The disadvantages, however, are moreimportant: sensitivity is poor, and the AFoutput signal is too small in many cases.

The addition of a FET (field-effect tran-sistor) and a resistor as shown in Fig. 2results in a better receiver. The FET pro-vides considerable AF amplification andat the same time, by virtue of its highinput impedance, allows the detector to beconnected to the 'top' of the tuned circuit.This makes this type of receiver fairly sen-sitive.

It is definitely worth while to build thecircuit of Fig. 2. A few hints: use a wireaerial of at least 5 metres, and connectground of the circuit to the water supply

Fig. 1. The one advantage of the diode re-ceiver is that it does not need a power sup-ply. The circuit is. however. useless withouta large aerial and an earth connection.

Fig. 2. The amplifier in this modified diodereceiver raises the sensitivity of the basicdesign considerably.


58 INTERMEDIATE PROJECT

Fig. 3. Extending the direct -conversionreceiver with an amplifier for RF signals only(3a) and one that provides gain for RF as wellas AF signals (3b).

or the central heating system. The induc-tor is made by winding 85 turns of 0.2 mmdiameter (SWG36) enamelled copper wireon a 10 -cm long ferrite rod. The receiverdraws about 4 mA from the 9 V battery.

RF amplificationIt will be clear that an RF amplifier asdrawn in Fig. 3 is required if the receiverin Fig. 2 is to work with a much smaller(shorter) aerial, and without the connec-tion to earth via the water supply or thedomestic CH system. Also note that thegermanium diode has a threshold voltageof about 200 my, which makes receptionof signals below this level impossible -weaker signals are simply not detected. Ifthe RF amplification is high enough, thetuned circuit may double as an aerial, ob-viating the long wire. A second tuned cir-cuit, identical to the one already used,may be added to improve the selectivity.A further improvement that requires noextra components is shown in Fig. 3b. Achoke is connected to the output of the RFamplifier, and the detector is not con-nected to the headphones, but fed back tothe 'cold' side of the tuned circuit. Theresult of this configuration is that the am-

plified RF signal is 'forced' towards thedetector because it is blocked by thechoke. After detection, it is applied to theinput of the amplifier. Since the chokedoes not present a high reactance to AFfrequencies, the amplifier raises the de-modulated signal to a level suitable fordriving earphones. In other words: RFand AF amplification are provided by asingle amplifier. This forms the principleof the reflex receiver.

Enter two MOSFETs

Transistors are available today that im-prove the performance of the good oldreflex receiver considerably. Figure 4shows the circuit diagram of a receiverbased on the reflex principle and de-veloped as a result of many experiments.Dual -gate NIOSFETs Type BF981 are used.One functions as an AF amplifier that ob-viates passive components, and the otheras an AF/RF amplifier with very highinput impedance.

The heart of the receiver is formed byTi. Resistors R3 and R5 provide the gate -2bias voltage, which is decoupled by Ci.Capacitor C5 similarly decouples thedrain voltage. The tuned circuit of the re-ceiver is formed by Li-Ci. The inductor iswound on a small ferrite rod which formsthe aerial. The RF signal at the 'top' of thetuned circuit is fed to gate -1 of the MOS-FET via Ci. The extremely high gate im-pedance of the transistor eliminates alllikelihood of damping, and obviates a tapconnection or a coupled winding. All thissimplifies the construction of the inductorand increases the sensitivity of the re-ceiver because at the same (high) load re-sistance, the voltage at the top of the tuned

circuit is higher than at a tap towards theearthy side. The RF signal amplified by Tiis blocked by L2 and consequently arrivesat detector Di -D2 -C3 -R2 via coupling capa-citor C3. The detector also functions as avoltage doubler circuit. Resistor Ri feedsthe rectified signal back to gate -1 of theMOSFET, which functions as an AF ampli-fier. The amplified AF signal passesthrough L2 and is coupled out via C7 to thevolume control, Pi. TransistorT2 raises theAF signal to a level suitable for drivinglightweight headphones.

CompactThe low number of components and theuse of a small ferrite rod enables the reflex

Fig. 4. Circuit diagram of the reflex receiver based on dual -gate MOSFETs. The L -C tunedcircuit at the input doubles as a compact aerial for medium -wave reception.


REFLEX AM MW RECEIVER 59

receiver and the 9 V battery to be built intoa miniature enclosure of the size of, say,two match boxes.

If you consider size less important andat the same time want to go round con-structional problems, build the circuit onUniversal Prototyping Board Size -1 asshown in Fig. 5. Pay attention to the orien-tation of the NIOSFETs: when in doubtlook at the pinning given in the circuitdiagram (transistor viewed from the top).Choke L2 is a ready-made type. The aerialis made from 70 turns of 0.2 mm diameter(36S%VG) enamelled copper wire woundon a 4 -cm long, 10 -mm diameter ferriterod. The rod is secured on to the PCB withthe aid of plastic ties as shown in thephotograph. The two diodes may be re-placed by almost any other small -signalgermanium type such as the 0A90 or0A95. Do not use silicon types: their thre-shold voltage is too high for this applica-tion.

The receiver is connected to a pair ofheadphones with a minimum impedanceof 64 O. This perhaps less usual value issimple to achieve by connecting the two32 SI earpieces of the headphones inseries.

esa-vis

aoPIll,1Ecm,PI

P1

C

0-1Fo

.1

C7 T2921.' d

91 *qr.

cue( d*

Li

9v

Lsi

C4

L2

UM!

Fig. 5. Suggested construction of the receiver on ready-made PCB Type UPBS-1.

Parts list

Resistors:1:11;1:12 = MAO

Rs = 47kR4 = 470c.1

Rs = 56kRS= ik0Pi = 100k logarithmic potentiometer

Capacitors:Ci = 500p mica foil tuning capacitor -C2= 47pC3 = 100pC4 = 100nCs= 4n7Ce = 470pC? = 47n

Semiconductors:Di:D2 = AA119Tt ;T2 = BF981

Inductors:LZ = wound on ferrite rod - (see text)L2= 4mH7 choke. e.g.. Toko 181LY-472 -

Miscellaneous:St = miniature orVoff switch.Headphones; impedance 6452 (2x320)PCB Type UPBS-1 (see Readers Servicespage).

Suggested supplier: Cirkit, Park Lane,BROXBOURNE EN10 7N0. Telephone(0992) 441306. Fax (0992) 464457.

Hard disk monitorDecember 1989, p. 54The parts list is not in agreement with thecircuit diagram, and should be modifiedto read:

RI -R7;Rii;R13-1116;R2o = 100k= 22n

Simple transmission lineexperimentsSeptember 1989, p. 38The value of R2 in the experiment shouldbe 225 52, not 43 O.

Logic analyser with Atari STOctober 1989, p. 30Please add to the parts list:

R3:114;Rs - R12 =100kCs = 100n

The control program (ESS111) is suitablefor monochrome systems only.

LF/HF signal tracerDecember 1989, p. 22The resistor next to preset P3 on theprinted -circuit board should be labelled

CORRECTIONS

R23, not

I/O extension forArchimedesNovember 1989, p. 14-15The parts list should be modified to read:

IC3 = 6522-2The circuit diagram in Fig. 2 contains anerror: R4 should be connected betweenground and the +input of ICi. The printed -circuit board is all right and requires nomodification.

Intruder alarmNovember 1989, p. 45-47The references to Cs in lines 6 and 7 of thelast full paragraph on page 45 should readCi .

The reference to Rs in the third para-graph under Alarm sensing should readR9.

The first full sentence in the centre col-umn on page 47 should read

'Two assurance bleeps are generated:

one at the instant of switch -on, and one asthe exit door is opened for departure be-fore the end of the 15-s switch -on delay.'

In the circuit diagram, Fig. 2, the diodeacross Bzi should be numbered Die, notD12. Capacitor C2 is missing: it is an 10 uFelectrolytic type connected between the+12 V and ground rail. Finally, the insettext 'D1...D8 = 1N4148' should read Dl,D2,173,D5,D6,D7,D8,D10,D12= I N3148.

31/2 -digit SMD voltmeterNovember 1989, p. 37-41The obsolete half -digit LED display TypeHD1108 from Siemens may be replaced byTelefunken's Type D29xPK, where 'x' in-dicates the colour: 0 for red, I for amber,2 for green and 3 for yellow. These dis-plays are also available under new typeindications as TDSR3120, TDS03120,TDSG3120 and TDSY3120 respectively.Telefunken also manufactures equival-ents for the HD1105: the type numbers areTDSR3150, TDS03150, TDSG3150 andTDSY3150.Distributor information on Telefunkencomponents from AEG (UK) Ltd. 217Bath Road SLOUGH SL1 4AW. Tele-phone: (0753) 872101. Fax: (0753) 872176.


READERS' FORUM

LETTERSDear Sir - With reference to the Decem-ber 1989 issue of Elektor Electronics, Iwould like to comment on two of the pro-jects.1. "Solid-state preamplifier. This projectis very good as far as it goes. The virtualearth CMOS switches have been used forsome years on audio mixing consoles togreat effect.

CMOS switches

and their functional equivalent

Bri(coci)

CTL

AOL/T(Ti/C)

AOtillPV0)

Bounti/C)

BOUT(WO)

9,-71.1111

An enhancement that I would person-ally like to see is the inclusion of active.symmetrical balanced inputs and outputs -I have for some time been toying with atheoretical design based on a circuit ele-ment called a 'superbal'.

I can lay no claim to originating thiscircuit (in fact, it came from Messrs Stan -coil, the manufacturers of Alice mixers,who are no longer in business), but I haveseveral circuits roughed out that incorpo-rate this element.

I enclose the following sketch of the el-ement together with the proposed CNIOSswitch circuit.2. "IC tester for the IBM PC". I have re-cently constructed this project with noproblems at all. A few hardware enhance-ments were incorporated as follov. :

( i) the address selection links w ere re-placed by a four -pole DIP switch to facili-tate later changing of the base address:

i the ZIF socket and LED were builtinto a small plastic box alone with another25 -pin D -connector to tidy up the testPCB:

(iii) the ribbon cable from the mainboard to the test PCB was discarded infavour of a standard 25 -way D -connectorcomputer cable.

While the hardware cannot he faulted.the software supplied is not quite so bugfree.

As far as I can tell, there are no prob-lems with the interpreter, but the libraryfiles contain several errors. I have sincehad two further copies of the software: al-though these correct some omissions,other errors still exist.

Perhaps you might consider an occa-sional article on listings for test routinesfor ICs other than those contained in mylist attached.A.G. Crane. Kings Lynn.

Your sketches are reproduced here, since Ifeel that these will of interest to a numberof other readers (they certainly were tome). Also, all your comments have beenforwarded to the designer of the "All -solid -state preamplifier". Any continentsfrom him will be published in a futureissue.

As regards the software bugs. your listhas been forwarded to WI the de-signers of the IC tester. As soon aswe have their comments. we willpublish your list and their re-sponse. if any. [Ed]

CMOS powered from :=6 V: opamp powered from -20 V.This gives several lOs of dB headroom!

Dear Sir - I have just received theOctober 1989 issue of ElektorElectronics and am surprised tonote that you are charging £2.00 for

a paper copy of a PCB pattern to addressesoutside Europe. This is rather unfair onreaders like me. I like to keep records ofPCBs for future use and have tried to doso from issue 1 of Elektor Electronic -s.

1988 was a bad year for PCB patterns.Many articles did not have a PCB pattern.only a component layout diagram. If Ihave to pay out £2.00 for every PCB pat-tern. I may have to dish out 1:40-£60!

Even copies of articles are £2.50 perarticle, not per page. Why can't a similarstandard be used for PCBs'? Most PCBsdon't even fill half a page!

Again, what you are doing is unfair andpainful - please be more reasonable.K.G. Verais, Likas. Malaysia.

First. it appears as if you think that thebasic prices charged to customers outsideEurope are different from those to cus-tomers in the UK or Europe: this is not so- the differences in prices are caused hrthe different postage rates for these areas.

The processing of a PCB layout onpaper costs about £1.20 (internal han-dling of order. placing order on centralcomputer store, packing, and so on). Sur-face mail postage is the same the worldover and amounts (on average) to £0.35:airmail charges outside Europe amount(on average) to (0.75.

The processing ofa PCB layout on filmcosts about OM (making of the film isaround £4.00. the rest is as for the paperlayout. although the packing is slightlymore evensive). Postage rates are as forthe paper layouts.

We have no desire of making a profiton this service to our readers and othercustomers.

The reason that in general we do notpublish the PCB track layout in the maga-:ine is that it serves no purpose to theconstructor who is only interested in thecomponent layout. It is impossible to pro-duce a PCB from the track layout pub-lished in the maga:ine (because of theprinting on the reverse page).

I do sympathke with your feelings. hutunfortunately we all live in a harsh com-merical world: we wish we could supplythese track layouts free of charge, but it'sjust not feasible without increasing thecost of the inaga:ine. None the less. in.come special cases, such as schools andcolleges. I may be able to make specialarrangements-. ]Ed]

Dear Sir - Regarding "DC -DC powerconverter" (November 1989. p. 54). don'tyou think it is irresponsible to publish apower supply circuit that is not short-cir-cuit protected. A car battery will easily


READERS' FORUM

provide a hundred or more amps, quitesufficient to destroy this circuit and mostof anythine that had the misfortune to beconnected to it.

The author claims that power rises ex-ponentially with current. When I went toschool, power was directly proportional tothe square of the current. I do not believethis has changed.

Is skin effect really significant at theaudio frequencies which the author sug-gests are used? At 20 kHz, the skin depthmight of the order of 0.5 mm. If this is sig-nificant. the author should justify hisclaim.B.A. Jones, Cheltenham.

Air Jones has further pointed out that thepoint °l ids constructive criticism was thatmany of those attempting to construct pro-jects published in Elektor Electronics arebeginners who need everything explainedin detail, a sentiment with which 1 agreewholeheartedly. and which will certainlyhe borne in mind in future.

The author comments: "The articlestates clearly that the converter is not pro-tected against short-circuits and that a16 A fuse must he inserted between it andthe battery. On reading Mr Jones' com-ments, it might perhaps have been betterto have shown this fuse in the circuit dia-gram but. as a rule. parts that are not onthe PCB are not shown (to maintain theproper relation between parts list. boom'and circuit). Moreover. the provision ofshort-circuit protection would haveincreased the cost and complexity of the.converter".

"It is true that a car battery can supplyvery high currents. but long before thesehave risen to unacceptable levels. the 16 Afuse. and possibly resistors R2 and R3.choke LI and diode 1)1. would have givenup the ghost. Moreover. 'anything whichhad the misfortune to be connected to it(the converter)' will not be damaged by ashort-circuit at the output terminals of theconverter. since its input voltage is then=em".

"What you have learned at schoolabout the relationship between currentand power and what 1 sag about it are notin disagreement. I feel you may have con-fused the adjectival and noun uses of theword 'exponential'. An parameter thatvaries in accordance with the power of aquantity does so 'elponentiallv'. whetherthat potter is 2. 5 or e (=2.72). Only whenthe power is 1 do we speak of direct (lin-ear) proportionality". (Note: I agree withMr Jones (in a follow up letter) that thewords 'exponent' and 'exponentially' arewell defined in mathematics and shouldnot be changed 'at the drop of a hat'. Ed).

"As explained in the article. the pulserepetition rate is a junction of the currentdemand of the load and has a 171aXin111111value of 30-50 kik. The reason that theskin effect is significant is that the chargepulses have a fixed duration of 100 p5 orso. which means that they give rise to alarge number of harmonics. These har-monics moist be passed by the inductorwire without attenuation to maintain thepulse rise and fall times. Fourier analysisis required to establish the actual skindepth. but this would be a superfluous lux-ury in this kind of article. The recommen-dation to use a number of thin parallel -connected wires rather than one thick oneensues from a series of practical experi-ments on my prototypes". jEdJ

SWITCHBOARD

Switchboard allows all PRIVATE READERS ofElektor Electronics one FREE advertisement ofup to 108 characters. including spaces. com-mas. numerals, etc. per month.

Write the advertisement. which must re-late to electronics. in the coupon on this page:it MUST INCLUDE a private telephone num-ber or name and address: post office boxes arenot acceptable.

Elektor Electronics 'Publishing) can not ac-cept responsibility for any correspondence ortransaction as a result of a free advertisementor of any inaccuracy in the text of such an ad-vertisement_

Advertisements will be placed in the ord,,rin which they are received.

Elektor Electronics i Publishing) reservethe right to refuse advertisements without giv-ing reasons or without returning them.

PEN FRIENDS. I am an electronics engineer andam interested in pen friends on electronics. comput-ers and other allied subjects. Mohammed Granter,

Damavand Ave, 15 Metri 4th, 8 Metri 6th, No. 10Post code 17318 Tehran, Iran.

WANTED. Sweep function generator up to 150 kHzto include triangle to ramp wave output. Phone(0267) 275437.

FOR SALE. New 12 inch black and white videomonitor with sound input; takes 1 V composite; £50.Phone (0704) 214594.

FOR SALE. Tektronix 545 oscilloscope plug-in unitstyle in good working order. £90. Phone (0732)350691 any time.

FOR SALE. Elektor Electronics nos 24. 43. 45, 49.June '81 -Dec '83 (except April '82): 31 copies £20plus post. J. Brown, 74 Humber Ave, South Ock-endon, Essex, RM15 5JN.

FOR SALE. Computers IBM XT £490 ono up toAT386 £1290 ono. Many accessories also available.

Phone (08444) 3953 for details.

WANTED. Circuit diagram for Telequipment Senri-

scope. H.L. Eamshaw, 58 Prices Lane, REIGATERH2 8AY, Telephone (0737) 221480.

WANTED. Elektor Electronics AP Frequency meter.Capacitance meter and Inductance meter. Phone

01-594 7754 after 6 p.m.

WANTED. Eno ,sn manual for NEC PC6001 series:

also exchange software for IBM PS 2 on disks.

Hamidreza Madjdabadi. No 30 Western 194 St.,Tehranpars, TEHRAN 16538. Iran.

FOR SALE. Components owing to giving up hobby.Too numerous to list so ring for details. Offers in-

vitedtotWievihotelot. Gordon Brown. 72 BuryHill. WOODBRIDGE IP12 1JD. Telephone (0394)

385652.

FOR SALE. Tektronix 575 curve tracer. Hartley

scope 13 A. Creed 444 silent cover (free 444). Bestoffers. 13A snare manuals £2.50 inc! R.A. Lord,22 Elizabeth Crescent. EAST GRINSTEADRH19 3JA.

DESIGN SERVICE. _ow -cost PCB CAD arc manu-facture. s or :F:o. to your design.

Small or large quantit,c-.s Phone Dave at (0463)

790006.

WANTED. Manua! c

oscilloscope Type 317. Phone Howard at (0272)

658191.

PEN FRIENDS. lam interested in pe'all over West Germany. Mohammed Etminan. Shi-raz University, School of Engineering. SHIRAZ,Iran.

Send this coupon to: Elektor Electronics(Publishing), Down House, BronmhillRoad, LONDON SIV18 -1.1(2.

Block capitals please -one character to each box

1


,..--E- -7-_-:::::iss MUST ce ,-. ,


62

DAkK-200AT

A. Rigby

CLOCK

Timing is an essential factor in any dark -room. Keeping a constanteye on the clock while the photograph is being developed, however,is a nuisance, and makes it impossible to attend to other activities.

The low-cost timer described here can be programmed to giveaccurately timed audible signals as a reminder to flip the

development tank, add the fixative, remove PE paper, and so on.

The quality of photographic reproduc-tions can be kept constant only if the de-velopment tanks are flipped, and bathsare changed or stirred in good time. Atiming error of a couple of seconds can betolerated in some cases, but longer delays,caused by other activities in the dark-room, may result in an unusable print.

The present clock offers a number ofselectable alarm functions and a maxi-mum time lapse of 30 minutes, which isample for most dark -room activities. Theclock is a compact unit to prevent it takingup too much space in the dark -room.

The block schematic diagram in Fig. 1shows the general structure of the dark-room clock. The 2 Hz clock generator af-fords simple selection of a number ofdifferently timed alarm sounds. The 2 Hzclock signal is applied to a binary counterwhose (non -used) least -significant outputbit counts half -seconds. The 11 other out-put lines of the counter are connected tothe address inputs of an EPROM that con-verts the 11 -bit counter value into controlsignals for a 4 -digit multiplexed LED dis-play. This function requires 7 of the 8

binarycounter

2i-iz

EPROM

buzzer

every after3C5 0 S --.as

buzzer

clockgenerator

multiplex

1:1LI LI 101 LI

1 of 4 decoder

1'694027 11

Fig. 1. Block diagram of the dark -room clock.


GENERAL INTEREST 63EPROM data outputs. The remaining bitis used for the control of an alarmsounder.

EPROM as a decoderThe EPROM used in the dark -room clockis essentially a programmable logic device(PLD), whose combined functions allowthe remainder of the circuit to be keptrelatively simple. The 8 output lines of theEPROM are used for display segment con-trol and sounder control. Each addresslocation in the EPROM is programmed tosupply a bit pattern that determineswhether a display segment is lit or not,and whether the sounder is actuated ornot. Table 1 lists the bit patterns for num-bers 0 through 9. The sounder is actuatedby adding 80H (bit 7 goes high) to thevalues shown. Table 2 shows how thesebit patterns are stored in the EPROM, andillustrates the functions of its address in-puts, which, given the application, mayalso be considered decoder inputs.

The most -significant (MS) addressinput, A13, divides the EPROM addressspace into two blocks. One of these en-sures that the sounder is actuated every30 seconds, which is useful during the de-velopment of films. The other block is pro-

Bit D6 D5 D4 D3 D2 D1 DO

HEXsegmentg f e d C a

read-out

none 0 0 0 0 0 DO

1 i 1 1 1 1 3F

0 00 1 1 0 06

2 i 1 0 1 1 0 1 1 - 1 5B

3 1 0 1 1 1 4F

4 i 1 1 0 1 f 1 0 66

5 1 1 1 1 0 1 6D

6 1 1 1 1 1 0 1 E 7D

0 0 o i o 1 i 1 1 07

1 11 1

1 1 1 1 7F

I !oil 11

1 1 6F

07 = buzzer on. D7 = ''0": buzzer off

Table 1. The databits loaded into the EPROM control the display segments and thetime-lapse buzzer.

St

5V

reset

DI

112T

R2

3

90s

ALARM4

fl5

6

onOM09

C3min oaici 07

1100n OS

rA

RI

4040 04030201

OLXODeon

3 2

7

13

5V

2Ot

26 4132 412

23 41121

12

13

4

2

3

6

6

271

APP PD4

DO

Ds

'Os

Rt9

II

- A iJP 2

13

12

D2 to

03IC4 134 15

De 17te

4 A5 EPROM De

5 45 27128 D7 -""3#2

B

7

7

B 10

9

D JPI

C

3

410AS

2S

DI

22 141 20

3

7

II O1

t2IC5 am

14 uur5 2004 -

15 0617 67

RIB ifbetter off

14 6013 07 06

1C2 04060

12 11R3

C22TOrt ll-412200

T -r32:7614210

18

13

12

11

5V

6

7.476 LD1Re 14

Rte

1? bsaeloe/off

Rai

MML

15

LD2

C

TDZ 14

MIR

0

LD3

= 74HCT139LD1...1.04 = 7751 (Common Anode)Di_D7 = 1N414871...T4 = 8C5578

5V

5V

04

894027.12

Fig. 2. Circuit diagram of the dark -room clock. The heart of the circuit is formed by EPROM IC4.


64 DARK -ROOM CLOCK

buzzer lapsed time digit address data(hex) (hex)

0 minutes. 0 seconds

seconds (units)seconds (tens)minutes (units)minutes (tens)

0000 BF0001 BF0002 BF0003 80

0 minutes, 1 second


0004 060005 3F0006 3F0007 00

17 minutes, 30 seconds

every 30 s


1068 BF1069 CF106A 871065 86



1C18 7F1C19 6D1CiA 6F1C1B 55

29 minutes. 59 seconds


1C1C 6F1C1D 6D1C1E 6F1C1F

248 bytes. not pro-grammed

1C20 FF

IFFF

atter 90 s


seconds (units) 2000 F

seconds (tens) 2001 -,-minutes (units) 2003 PP,..:minutes (tens) 2004 :.:0

1 minute. 30 seconds

seconds (units) 2168 BFseconds (tens) 2169 CFminutes (units) 216A 86minutes (tens) 2165 80


seconds (units)seconds (lens)minutes (units)minutes (tens)

248 bytes, not pro-grammed

3C1C3C1D3C1E3C1F

3C10

3FFF

6F6DSF55

FF

FF

A13 i Al2 A2 Al. AD

Table 1. EPROM organization.

grammed to supply a single alarm after90 seconds, which is useful for the devel-opment of PE (poly -ethylene) paper. Thedevelopment time of PE paper is normallystated as about 60 seconds, but 90 secondsis often found to give better darkeningand quality of reproduction.

The time in seconds is applied as an11 -hit binary number to EPROM inputsA2 through Al2. This arrangement resultsin four bytes that hold the segment pat-terns for the LED displays to be selectedevery second. Address lines AO and Alare driven by two signals that control themultiplexing of the 4 displays. Summariz-ing the above, the EPROM functions as acircuit that decodes binary time informa-tion into a multiplexed 7 -segment controlsignal.

The circuitThe circuit diagram of Fig. 2 shows thatthe clock generator is formed by IC2, aType 4060 14 -bit counter with on -boardoscillator. The oscillator signal is dividedby 2', so that output Q13 supplies a 2 Hzsignal. Outputs Q6 and Q7 supply themultiplex signals that cause each displayto be turned on and off at a rate of 128times per second.

The 2 Hz signal is fed to the clock inputof binary counter ICI. This 12 -bitcounter/divider counts the secondslapsed since the circuit was started. Theuse of binary counting in combinationwith an EPROM allows such a simple sec-onds counter to be used - the perhapsmore conventional alternative wouldhave required at least 8 standard ICs, ortwo or three specialized ones. TheEPROM used in this circuit is availableready -programmed.

The reset circuit of ICI has 3 inputswith an OR function provided by DI -D2 -D3. The counter is cleared (reset to zero)either when RESET key S2 is actuated (Di),or when the circuit is switched to thestand-by mode by Si (D2), or when1800 seconds have lapsed since the lastreset (DA. The last function is realizedwith the aid of D4 -D7 that reset thecounter within a few nanoseconds after itreaches state 1800 (111000010002).

The lapsed time is passed to EPROM1C4, whose function has been detailed ear-lier. The decoded and multiplexed 7 -seg-ment signals on datalines D0 -D6 arebuffered by Darlington array ICs beforebeing applied to the cathodes of the LEDdisplays.

The signal on EPROM dataline D7 con-trols sounder Bzi via driver T5. The soundof the piezoelectric buzzer is made a littleless disagreeable by interrupting it withthe aid of 1-6. A wire link, WI, is fitted toprovide 3 sounder options:

position 'C' for two pips of a quarter of asecond each;position 'D' for a single pip of half asecond;no wire link for a single pip of one sec-ond.


GENERAL INTEREST

Fig. 3. Printed -circuit board for the dark-room clock. The board is double -sided. butnot through -plated.

For reasons of safety, and to keep its over-all cost down, the dark -room clock ispowered by a mains adapter with a direct,unregulated output voltage of 8-12 V.Regulator ICb reduces this voltage to 5 V.A real on /off switch is not provided.Rather, the circuit is switched to stand-bywith Si. The counter is reset to zero, thedisplays and LEDs are turned off, and theEPROM is switched to its low -powerstandby mode. The clock is not re -actu-ated until Si is switched on again.

ConstructionThe dark -room clock is built on thedouble -sided, not through -plated,printed -circuit board shown in Fig. 3. Thisboard is available ready-made.

Figure 4 shows the locations of theholes that must be through -contacted byELEKTOR ELECTRONICS FEBRUARY 1990

Parts list

Resistors:R.= 100kPa= 10k

= 10MR4 - R7:R ti5;RI7= 1k0Re - R15 = 470Rla;Ris.= 4700

Capacitors:CI;C2 = 220pC3 - C5 = 100nC7 = I GOA: 25 V

Semiconductors:Di - D7= 1N4148N:Ds = LED; red; 3 mm

- LD4 = 7751 (common anode)- T4 = BC5575

MTh= BC547BIC, = 4040IC2 = 40601C3 = 74HCT1391C4 = EPROM 27128. Available ready -pro-grammed under order number ESS 58-3(see Readers Services page)ICs = ULN2004ICs = 7805

Miscellaneous:S1 = locking SPOT switch with black keycap. 111,V Type 61-2010400,-.S2 = SPOT switch with red key cap. ITWType 61-1010000-.S3:S4 = locking SPOT switch with integralLED and black key cap. ITV/ Type 61-2030401,.X1 = quartz crystal 32.768 kHz.PCB Type 894027 (see Readers Servicespage).Enclosure: Heddic Type 222'.

ITV/ Switches Division of ITV/ Limited Norway Road Hilsea PORTSMOUTHP03 5HT. Telephone: (0705) 694971.

Emtek Electronic Products Limited Unit19a Industrial Estate Stanton BURY ST.EDMUNDS 1P31 2AR. Telephone: (0359)511155.

inserting a short piece of wire, and solde-ring it at both the component side and thetrack side of the board. This arrangementobviates having to solder the ICs direct onto the board, although this is perfectlypermissible in this case. Note that thewhite overlay print must be removed atthe component side before the spots canbe soldered properly.

The 3 exclamation marks shown inFig. 4 indicate PCB design errors that are,fortunately, simple to correct. The holesnear pins 22 and 28, towards the ÌC4'mark on the overlay, must be connected tothe associated holes that receive the ICpins (or IC socket pins) at both sides of thePCB. In the case of pin 22, the missingconnection is at the track (EPS) side of theboard: a little extra solder tin applied dur-ing the mounting of IC4 will bridge thegap between the spots. The extra connec-

P S BB4027'1 P5 T2 P5 ra T.a

O0 0 xy

0

Lc13 L..

o^1 CV .r)

O 00

0

O

0

00

0

0 0 0 0 0 0

00'

O0 0

0

0000

00O

0 00000

0

O 00

Fig. 4. Showing the locations of the holesthat must be through -plated before any com-ponents can be fitted.

tion for pin 28 is made at the componentside of the board with the aid of a piece ofthin wire that connects the holes. Beforefitting this wire, remove the white overlayprinted on the hole for IC pin 28. Insert thetip of a sharp pencil in the hole to ensurethat it remains open when the wire is sol-dered to the spot.

Make the connection between RI5 andLD2 with a short length of insulated wirefitted at the track (EPS) side of the board.A track is provided at the component side,but this makes fitting LD2 rather difficultif the centre pin in the right-hand row ofthe display is to function as a throughcontact.

The fitting of the remainder of the com-ponents should not present problems.

If the Heddic Type 222 enclosure statedin the Parts List is used, the switches mustbe raised by mounting them on IC sockets

66 DARK -ROOM CLOCK

program clock:

vatcount : word;

j, k, 1 : integer:g : file of byte:displayandsound : array(0..9] of byte:

const(The display arraydisplay : array(0..hexff : byte =5ff:hex80 : byte =580:hex0 : byte =10;

beginassign (g,'clockrewrite (g):for i:=0 to 9 do

contains ,iatabyte for each display value)9] ci sif,506,55b,54f,566,56d,57d,5/,571,56f):

::r a blank position in the eprom)t:r a blank display segment with sound output]

i:r a blank display segment without sound output)

.dat'); (Open the desired filename)

displayandsound(i):.-display[i]+580: (Include sound bit)

count:=0:for i:=0 to 2 do

for j:=0 to 9 dofor k:=0 to 5 do

for 1:=0 to 9 doif count mod 30 = 0 (Check if count is a multiple of 30)thenbegin (Calculate the four d_isrlay bytes including the sound bit)

write (g,displayandsound:iwrite (g,displaya:.i2i=d:.:write (g,displayandizuni:::

(Calculate a readout in minutes and seconds)(Reset the displaycount)

(Maximum count is 29 minutes and 59 seconds)

(Leading -zeroif displayandsound:::<>dilayandsound(01

then write (g,'-'7''Ya-'""ndli3)else write (g,r.KE,0):

count:=count+1:

endelsebegin

writewritewrite

(g,display(I]):(g,displaylk1):(g,display(j]):

blanking]

(Calculate the four displaybytes)

(Leading -zero suppression)if displayfi.>display(0] then write (g,display(il)

else write (g,hex0):count:=count+1:

end:

for ii=1800-.4 to Sliff dobegin

write (g,herff):end:

count:=0:for i:=0 to 1 do

for j:=0 to 9 dofor k:=0 to 9 do

for 1:=0 to 9 do

(Fill the remaining part with 5FF)

(Calculate a readout in seconds)(Reset the display count)

{Maximum count is 1999 seconds)

(Check if count is a multiple of 90 and smeller than 91Iif (count =md 90 = 0) and (count<=90)thenbegin (Calculate the four display bytes including the sound bit)

write (g,disolavandscund:::`:if count < 10 then write ,-,hex80) (Lead --_-zero blanking)

if count

if count

< 100 then :7.::: -.g -zero blanking)else wr17.-_: g,d1szlayand5r.d:1):

< 1000 the- g,hex80) (Leading -zero blanking)else ,r::e ,g,displayandsound(i]):

count:=count+1:endelsebegin (Calculate the four display bytes)

write (g,display[l]):if count < 10 then write (e,hex0) (Leading -zero blanking)

else write (g,display[k]):if count < 100 then write (g,hex0) (Leading -zero blanking)

else write (g,displaytj1):if count < 1000 then write (g,hex0) (Leading -zero blanking)

else write (g,display[i]):count:=count+1:

end;

for i:=52000+200.0.4 to $3fff doheoin

write (g,hexff):end;

_se (g);

(Fill the remaining part with 5FF)

(Close the file',

Fig. 5. Listing of the Turbo -Pascal program used to compile the content of the EPROM. TheIF COUUT statements allow you to change the timing intervals to individual requirement. Likethe EPROM content, the program is divided in two parts: the first provides the data for theminutes and seconds read-out, and the second the data for the seconds read-out (0-1800 s).These blocks may be interchanged. EPROM data is written to file =mom'.

(cut off the non -used pins of 14 -way low -profile types). Also note that the LEDs inthe key caps are best removed and re-placed by types with long terminals thatcan be soldered direct to the PCB. Use adrill or a small file to modify the IC socketif a part of it is in the way of the LEDterminals.

The switches are 'Digitast'-like SPDT(single -pole, double -throw) models withan integral LED. With the exception of S2,

they remain locked in both positions. Ifdifficult to obtain locally, the switchesmay, of course, be replaced by suitableequivalents with the same electrical func-tion, mounted on to the front panel of theenclosure, and connected to the circuit byshort wires.


A selection of products fromour BARGAIN LISTS: Ring orwrite for a free copy of ourlatest list.

COIN -OPERATEDMECHANISM2662

Made by Coin Controls, this will accept variot ssize coins by simple adjustment of 4 screwsIncorporates various Security features - ma; -net bent coin rejector. etc. Micros...itch rate::SA 240V. Front panel 115 x 64. depth 130mmCost 010 55Our price E4 00

1

KEYBOARDSSa=e 1".

ir-e --ertir,28852 Keyboard: Superb brand newkeyboard 392 x 181 with LCD displaying 1 lineof 10 characters and a further line with venoussymbols 100 keys. inc. separate numerickeypad. Chips on board are 2 x 74HC05.80C48 LCD - driver chips are easily remov-able from board £15.00

28857 High quality Alphanumeric keyboardon alum:nium frame 314 150mm_ Contact.

.:,sless good for 20 million operations.Origma sold at over 0100 each, they wereuse:. - a Printcom' portable terminal. FullyASC aced output Power supply - 5V and-12v . Hrn/a_ Suppled with comprehensivedata 014.95

28856 Cherry comp :e: Ver, a' mmodel 340 x 130 cy only 14mm deep, in-cluding keys. Matrix output 67 keys in pale/dark brown Et 00

28863 Keyboard. High quality unit made byMicro Switch. 69 pale grey and blue keys. 6 red5mm LED's. 15 various LS chips and socketed08048 by Intel. Output via 7 way plug andthere Ls a 4 way edge connector too. Keyboardframe rs 317 . 128mm_ PCB on which it'smounted is 285 x 1711mm.Price Excellent value at E12.00

TELEPHONE ANSWERINGMACHINE

28874 Superb piece of German equipment_This uncased model looks complete & rsbelieved to be working. Size overall 305 x 163x 57mm. On the PCB is a mains transformer12201/1. relays & associated components.There are 2 mini -cassette decks, 6 positionswitch. Mic - amp circuit to record outgoingmessage am mains lead with 2 pin plug. - a6 core lead for connection to Telecom socket_Excellentquality & value.. E12.50

128 PAGES OF ELECTRONIC COMPONENTS ANDEQUIPMENT. HUGE RANGE! AMAZING VALUE!DON'T MISS OUT - GET YOUR COPY NOW - ONLY

£1.50 POST FREE!!!

DISPLAYS24243 Display panel 152 x 112mm with NEC8 digit display (217311; 8279.5, MC146818, 3uPABOC, & a couple of LS chips crystal, etc.

E2.90

21731 NEC Vacuum Fluorescent DisplayEIP8BIL B digit multiplexed output 10mm high.Heater voltage 2V. grid:anode voltage 24V.

E2.00

24115 8 digit 12.7mm high LCD and holder.These are 14 segment devices allowing alpha.numenc display. Normally costing over 015.03we are offering these forjust E4_50

24148 LCD as 24115 but 6 digit. 50 pins. LikeRS 588-601. Their price 10.86. £300Z1732 Epson LCD 4 digit Eirnm high__ 82.00

21637 LCD Display - Direct drive 31ri digitsimilar to RS 588.572. 12.7mm high digits_ Opvoltage 4-12 RMS r2 3214z typ. Supplied withdata .E200; 10-1E1.75; 25-1E1.50; 100-161.00

21560 7 seg display, 20mm high_ Commonanode..... Only 70p125 /50p; 100-142p

1311416 Alphanumeric 4 character intelligentdisplay 0.16- _ £7.00

013416 4 digit intelligent alphanumeric dis-play with builtin drive and memory. ASCIIROM and multiplexing circuitry. TTL corn'patible inputs. -5V. Supplied with data.Listonce 041 50 £8.00

HIGH QUALITY THITACHI OSCILLOSCOPES

FOR QUAD AND VALUE

V223 DC-70MHz. dual Channel. single time.base delayed sweep. DC offset, alternate mag-nifier. 6in screen. 5mV/div vent- sensitivity0.21slcliv.43.2_Vdiv sweep time. Complete with2 probes. manual, mains lead_ _______ E475Other models from (339 - full details incatalogue. Ask far colour brochure.

METEX METERS8 different models in our cataloguer* 41/2 digit 12mm LCD

display* 30 ranges inc!

20A ea:do* Frequency

counter* Capacitance test

with zero adjust* Data hold switch* Diode test

fi Transistor testa Continuity test Me.650* Test leads with 4mm plugs E94.00* Rugged yellow case* Carrying case

Battery and instruction manual included.AC volts 0-200m-2-20-200-750Vac =0.5atDC volts 0-200m-2-20-2001000vdcAC current 0.2m.200m-20Aac c 1.0%DC current 0-207µ-2m.200m-20Adc -0 5 -Resist 13200.2k -204-2004-2M-2011411 cCapacitance 0-209200n.20_,F -2.0rFrequency 0-204-200kHz =2Transistor hFE 0.1000 NE1. zr.,Dims 176 92

. -tettkfillth116

FREQUENCY COUNTER

FC5250 7 digit frequency counter for fren,..-:-cies between 10Hz and 1505114z_ Power or. r..".1/ .10 gate time and VHFiHF switches. Invia BNC sockets. Supplied complete withstruction manual and test lead Requires =-external 9Vdc nom 200rnA power supply.

METE

CM3300 High accuracy AUTORANGING3 digit capacitance meter. High resolutionmeasurement in the range 0 1pF to 99900uFwith 10 auto ranges. Range hold switch forbatch testing capacitors. Range zero control.Inputs via spring terminals or test leads (sup,plied). Complete with leads and instructionmanual.Price £65.00

EST EQUIPMENTAF GENERATOR/COUNTER

ISM

AG2603AD A combined audio frequency sig-nal generator and frequency counter. A sixcharacter LED display allows direct reading ofinternally generated signal or signals from anexternal source. The frequency generator hasa range of 10Hz to 1MHz with either square orsine waveforms and adjustable output level.The frequency counter has a range of 10Hz to150MHz. Frequency range controlled by a 5 -step selector and fine control. Adjustableouput level with 012040dB anenuator.AUDIO GENERATORFrequency ran ge _______ 10Hz to 151HzOutputimpedance _ 600flunbalancedOutput cannot_ 0.20-40dB andfineadjusterOutput control:

Sine._.__ ..__..___._..av rms maxsquare __10V p -p max

FREQUENCY COUNTERFrequency range _ 1011z to 1505114z:rputvottage - Less th a n 50mV' ox input voltage......_._-.-..... 3V

impedance:0if enCy

F- 50f1Power_ 240Vac50Hz

715 .. 150 x 200mmE175.00

ill COUNTERSimilar in appearance to above with samefrequency counter. Spec:Frequency range 100kHz to 150MHzF Output ___.____100mV rms lup to 3551Hz)

Output central 0/20cIB and fine adjModulation_ Internal lkHz.

external 50kHz20kHzat less than IV rms

P E179.00

SIGNAL TRACER/INJECTOR

Y133 For fault finding en Audio & RF Equip.VII meter and speaker. Level controls. 0613d0atten switch_ VP. OfP. ext spkr, and injectorsits on Front panel Sae 200.140.96mm.UsasPP3 £5500

LCRY134C A fully transistorises AC bridge whichallows accurate measurement of resistances,capacitances. inductances and transformerturns ratios.Resistance range 0.1 to 1 1 Vail = laeInductancerange. _ tali to 111H ±7i..Caoacitance range 10pE to 1110aF c 1.:Power 9Vdc IPP3 ba tte I

Dims. 2C0 135 90mmPrice E115.00

JUST ARRIVEDBig parcel of FETS,MOSFETSinc. 2N7C00 Series, IRF150!250, JSeries, etc., etc., from 10p each!Ring or write for Price List.

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POWER SUPPLIES(al SWITCH MODE

ASTEC Modal AM7531VP. 115130V ac 5060Hz 0.1E- V1 - 5V 5A.V2 - 12V 0.15A Size: 160 x 104 45mmPartially enclosed panel with fixing holes insteel case on 120 x 125mm centres. Inputsand Outputs are on colour coded leads; thereis also an EEC socket on a flying lead. E6-95

2660 Astac switchedmode PSUType AA7271.

This small PCB.wet 50 50mm willaccept 8.24V input andgive a stable 5V DC at up to 2A output The6 transistor circuit provides current overloadprotection, thermal cut-out and excellent filtering. Offered at the remarkably low price of

ES 00

(b) CONVENTIONAL24215 Siliconix mains input. 4.5V DC 150mAoutput to 3.5mm jack plug on 2m lead. Built-incontinental 2 -pin plug Size 62 x 4635mm £1.5024170 Plug in power supply. Built in 13A plug.Output 6V DC 300mA on 2m long lead termin-ated in a 3mm power plug. British made to135415._ £1.5024208 Ode Power Supply. Moulded oleo,:case with built in 13A plug Output 9Vdc a'600rnA delivered to 2m lead with 2_5m--nowerplug E3 50

FLASH GUN RETURNS(Lots more on latest Bargain List)

Hanirnex electronic flash arts that have bereturned by the cons.: -a- me place wherepurchased These arc :":e: zomplete & ingood condition (man.; ,- 7 ai boxes) buthave not been tested ty .1: are offeredwithout any guarantee 4 tr.:, available. as

Z4259 Type x140. Hot shoe attachment Sze75 60 25mm offfon switch & test tuna,'Takes 2 x HP7. Originally sold at E7-.510 £300Z4260 Type X215. Similar to above. E3.20Z4261 Type CX330 Another with samefeatures. - auta,manual switch, Size 70 65 35mm 63.50

BREADBOARDSFREE, if requested. with every breadboaMsold this month! K574 wire link pact withabout 250 lints for use with breadboard oPC -B'S!PROTOBLOC 10708 Protobloc 1 has a total of 400 tie pointsconsisting of two sets of 30 rows of 5 iraercon-netted sockets plus 4 rows of interconnectedsockets running alongside. suitable for use aspower supply rails. All contact positions arecfewly defined on an alphanumeric grid. ABSpolymer board mounted on an adhesive foambase. Will accommodate up to three 16 pindevices. An ideal introduction to solderlesscircuit development systems. Size 80 6Gm rn

f2 50PROTOBLOC 2G711 Protobloc 2 =as a total of 843 te z:WitiaCCOMMC.1.2! _p to seven 16 pin de. :e5Sire 172 r. 64rr E3 55PROTOBLOC 2AG712 As abo.e mounted onto a rigobase plate coins=, with three 4mm terminasfor power car arts. A mounting bracketwhich clips into r re base is also provided toaccept a var:ety of components includingswitches and potentiometers. etcPnce 6695PROJECT BOARD G124G724 2 of type G711 rr.c_, at onto a rig itnaseplate with 3 coloured t n315, for powerconnections. Overall size 225 ISOrnm_Price__...._.... £13.95PROJECT BOARD GL360736 3 of type G711 and an additional strip of103 be points mounted onto a rigid base platewith 4 coloured terminals. Overall size 242195mm.Price. E19.95


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Documents

THE ELECTRONTC,51VMtAZ/NZ WITH TYIP PRACTiCAL ...ELEKTOR ELECTRONICS FEBRUARY 1990 Please mention ELEKTOR ELECTRONICS when contacting advertisers DEN HAAG HOLLAND SELF-INDUCTANCE METER