A Minor Project Report

on

HOME AUTOMATION

USING MOBILE PHONES

Submitted

In partial fulfillment

For the award of the degree of

Bachelor of Technology

in

Department of Electrical and Electronics Engineering

Supervisor: Submitted by:Miss Himadri Singh Raghav Monika Sevda(090491)

Neha Choudhary (090495) Pragya Rohatgi (090501)

November, 2012

Mody Institute of Technology & Science(Deemed University u/s 3 of UGC Act 1956)Faculty of Engineering & Technology

Lakshmangarh, Sikar – 332311 (Rajasthan)

Mody Institute of Technology and Science (A deemed University Under Section 3 of UGC Act 1956)

Lakshmangarh-332311, Dist. Sikar (Rajasthan) Phones :( 01573)225001 to 225012 (12 lines) Fax :( 01573)225042

___________________________________________________________________________________

EXAMINER CERTIFICATE

The minor project entitled “HOME AUTOMATION USING MOBILE PHONES” by Ms. Neha Choudhary(090495), Ms. Monika Sevda(090491), Ms. Pragya Rohatgi (090501) is approved in partial fulfillment of the requirement of the Degree of Bachelor of Technology in Electrical and Electronics Engineering of Mody Institute of Technology and Science (a Deemed University), Lakshmangarh.

Prof. B.P Singh Ms. Himadri Singh Raghav

H.O.D (ECE) Supervisor Examiner

Mody Institute of Technology and Science (A deemed University Under Section3 of UGC Act 1956)

Lakshmangarh-332311, Distt. Sikar (Rajasthan) Phones :( 01573)225001 to 225012 (12 lines) Fax :( 01573)225042

___________________________________________________________________________________

CERTIFICATE

This is to certify that the minor project report entitled “HOME AUTOMATION

USING MOBILE PHONES” submitted by Ms. Neha Choudhary (090495), Ms.

Pragya Rohatgi (090501), Ms. Monika Sevda (090491) for the partial fulfillment of

requirements for the degree of Bachelor of Technology in Electrical and

Electronics Engineering to be awarded by Mody Institute of Technology and

Science (Deemed University), Lakshmangarh, is a record of their/her work under

my supervision and guidance.

Date: Ms. Himadri Singh Raghav

(Project Guide)

ACKNOWLEDGEMENT

“Perseverance, inspiration and motivation have always played a key role in any venture. It is not

just the brain that matters most, but that which guides them. The character, the heart, generous

qualities and progressive forces. What was conceived just as an idea materialized slowly into

concrete facts. The metamorphosis took endless hours of toil, had its moments of frustration, but

in the end everything seemed to have sense”.

At this level of understanding it is often difficult to understand the wide spectrum of knowledge without proper guidance and advice. Hence, we take this opportunity to express our heartfelt gratitude to our project guide Ms. Himadri Singh Raghav who had faith in us and allowed us to work on this project.

We would like to thanks Prof. B.P. Singh (H.O.D Electrical) for his immense interest, valuable guidance, constant inspiration and kind co-operation throughout the period of word undertaken, which has been instrumented in the success of our project.

We would like to pay our sincere gratitude to our respected Dean Prof DESAI for providing us opportunity to work in computer lab as a part of the major part.

We also acknowledge our profound sense of gratitude to all the teachers who have been instrumental for providing us the technical knowledge and moral support to complete the project with full understanding.

We thank our friends and family for their moral support to carve out this project and above all GOD for removing all hurdles in the way.

Neha Choudhary

Monika Sevda

Pragya Rohatgi

CONTENTS

Abstract 1

CHAPTER-1 Dual-tone multi-frequency 2

1.1 Audio sample

1.2 Theory of operation

1.3 Construction

1.4 Construction highlights

CHAPTER-2 Various devices used 6

2.1 Resistors

2.2 Capacitors

2.3 Crystal oscillator

2.4 Voltage Regulator

2.5 Soldering

CHAPTER-3 MT 8870 IC 10

3.1 Features

3.2 Applications

3.3 Description

3.4 Functional Description

3.5 Pin Description

3.6 Steering Circuit

3.7 Filter Section

3.8 Guard Time Adjustment

3.9 Differential Input Configuration

3.10 Crystal Oscillator

3.11 Electrical Characteristics

3.12 Operating Characteristics

CHAPTER-4 CONCLUSION 33

REFERENCES 34

LIST OF FIGURES

1.1 A DTMF telephone keypad 2

1.2 Tone frequency for "1" key 6

1.3 Tone Generator 7

1.4 Tone Decoder 8

2.1 Resistor 11

2.2 Resistor 11

2.3 Capacitor 12

2.4 Capacitor 12

2.5 Capacitor 13

2.6 Crystal Oscillator 13

2.7 Crystal Oscillator 14

2.8 Voltage Regulator 15

2.9 Voltage Regulator 15

2.10 Soldering 16

2.11 PCB 17

3.1 Functional Description of MT 8870 IC 21

3.2 18 PIN CERDIP/PLASTIC DIP/SOIC 22

3.3 20 PIN SSOP/TSSOP 22

3.4 Basic steering circuit 25

3.5 Filter response 26

3.6 Guard time adjustment 27

3.7 Oscillator connections 28

LIST OF TABLES

1.1 DTMF keypad frequencies 4

1.2 DTMF event frequencies 4

1.3 DTMF row/column frequencies 6

1.4 Dip switch positions 10

3.1 Pin descriptions 23

3.2 Absolute maximum conditions 25

3.3 DC electrical characteristics 30

3.4 Operating characteristics 31

ABSTRACT

Home automation refers to the use of computer and information technology to control home appliances and features (such as windows or lighting). Systems can range from simple remote control of lighting through to complex computer/micro-controller based networks with varying degrees of intelligence and automation. Home automation is adopted for reasons of ease, security and energy efficiency.

In modern construction in industrialized nations, most homes have been wired for electrical power, telephones, TV outlets (cable or antenna), and a doorbell. Many household tasks were automated by the development of specialized appliances. For instance, automatic washing machines were developed to reduce the manual labor of cleaning clothes, and water heaters reduced the labor necessary for bathing.

Other traditional household tasks, like food preservation and preparation have been automated in large extent by moving them into factory settings, with the development of pre-made, pre-packaged foods, and in some countries, such as the United States, increased reliance on commercial food preparation services, such as fast food restaurants. Volume production and the factory setting allows forms of automation that would be impractical or too costly in a home setting. Standardized foods enable possible further automation of handling the food within the home.

The use of gaseous or liquid fuels, and later the use of electricity enabled increased automation in heating, reducing the labor necessary to manually refuel heaters and stoves. Development of thermostats allowed more automated control of heating, and later cooling.

As the number of controllable devices in the home rises, interconnection and communication becomes a useful and desirable feature. For example, a furnace can send an alert message when it needs cleaning, or a refrigerator when it needs service. Rooms will become "intelligent" and will send signals to the controller when someone enters. If no one is supposed to be home and the alarm system is set, the system could call the owner, or the neighbors, or an emergency number.

In simple installations, domotics may be as straightforward as turning on the lights when a person enters the room. In advanced installations, rooms can sense not only the presence of a person inside but know who that person is and perhaps set appropriate lighting, temperature, music levels or television channels, taking into account the day of the week, the time of day, and other factors.

Introduction

Here is a circuit that lets you operate your home appliances like lights and water pump from your

office or any other remote place. So if you forgot to switch off the lights or other appliances

while going out, it helps you to turn off the appliance with your cell phone. Your cell phone

works as remote control to your home appliances. You can control the desired appliance by

presetting the corresponding key. The system also gives you voice acknowledgement of the

appliance status.

The Project “Home Automation using mobile communication” has different sections such as:

1.Microcontroller

2.DTMF decoder

3. Voice recording and playback device

Chapter-1

Dual-tone multi-frequency

Dual-tone multi-frequency (DTMF) signaling is used for telephone signaling over the line in

the voice-frequency band to the call switching center. The version of DTMF used for telephone

tone dialing is known by the trademarked term Touch-Tone and is standardized by ITU-T

Recommendation Other multi-frequency systems are used for signaling internal to the telephone

network.

As a method of in-band signaling, DTMF tones were also used by cable television broadcasters

to indicate the start and stop times of local commercial insertion points during station breaks for

the benefit of cable companies. Until better out-of-band signaling equipment was developed in

the 1990s, fast, unacknowledged, and loud DTMF tone sequences could be heard during the

commercial breaks of cable channels in the United States and elsewhere.

Fig 1.1

A DTMF telephone keypad

1.1 AUDIO SAMPLE

1.1.1 Dtmf push to talk id

Present-day uses of the A, B, C and D keys on telephone networks are few, and exclusive to

network control. For example, the A key is used on some networks to cycle through different

carriers at will (thereby listening in on calls). Their use is probably prohibited by most carriers.

The A, B, C and D tones are used in amateur radio phone patch and repeater operations to allow,

among other uses, control of the repeater while connected to an active phone line.DTMF tones

are also sometimes used in caller ID systems to transfer the caller ID information, however in the

USA only Bell 202 modulated FSK signaling is used to transfer the data.

1.1.2 Keypad

The DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, and

each column representing a high frequency. Pressing a single key (such as '1' ) will send a

sinusoidal tone of the two frequencies (697 and 1209 hertz (Hz)). The original keypads had

levers inside, so each button activated two contacts. The multiple tones are the reason for calling

the system multifrequency. These tones are then decoded by the switching center to determine

which key was pressed.

Table 1.1

DTMF event frequencies

Event Low frequency High frequency

Busy signal 480 Hz 620 Hz

Dial tone 350 Hz 440 Hz

Ringback tone 440 Hz 480 Hz

DTMF keypad frequencies (with sound clips)

1209 Hz 1336 Hz 1477 Hz 1633 Hz

697 Hz 1 2 3 A

770 Hz 4 5 6 B

852 Hz 7 8 9 C

941 Hz * 0 # D

(US)

Table 1.2

The tone frequencies, as defined by the Precise Tone Plan, are selected such that harmonics and

intermodulation products will not cause an unreliable signal. No frequency is a multiple of

another, the difference between any two frequencies does not equal any of the frequencies, and

the sum of any two frequencies does not equal any of the frequencies. The frequencies were

initially designed with a ratio of 21/19, which is slightly less than a whole tone. The frequencies

may not vary more than ±1.8% from their nominal frequency, or the switching center will ignore

the signal. The high frequencies may be the same volume or louder as the low frequencies when

sent across the line. The loudness difference between the high and low frequencies can be as

large as 3 decibels (dB) and is referred to as "twist". The minimum duration of the tone should

be at least 70 msec, although in some countries and applications DTMF receivers must be able to

reliably detect DTMF tones as short as 45ms.DTMF can be decoded using the Goertzel

algorithm.

1.2 Theory of Operation

1.2.1 So what are these tones?

In DTMF there are 16 distinct tones. Each tone is the sum of two frequencies: one from a low

and one from a high frequency group. There are four different frequencies in each group.

Your phone only uses 12 of the possible 16 tones. If you look at your phone, there are only 4

rows (R1, R2, R3 and R4) and 3 columns (C1, C2 and C3). The rows and columns select

frequencies from the low and high frequency group respectively. The exact value of the

frequencies are listed in Table 3 below:

TABLE 1.3: DTMF Row/Column Frequencies

LOW-FREQUENCIES

ROW # FREQUENCY (HZ)

R1: ROW 0 697

R2: ROW 1 770

R3: ROW 2 852

R4: ROW 3 941

HIGH-FREQUENCIES

COL # FREQUENCY (HZ)

C1: COL 0 1209

C2: COL 1 1336

C3: COL 2 1477

C4: COL 3 1633

C4 not used in phones

Thus to decipher what tone frequency is associated with a particular key, look at your phone

again. Each key is specified by its row and column locations. For example the "2" key is row 0

(R1) and column 1 (C2). Thus using the above table, "2" has a frequency of 770 + 1336 = 2106

Hz The "9" is row 2 (R3) and column 2 (C3) and has a frequency of 852 + 1477 = 2329 Hz.

The following graph is a captured screen from an oscilloscope. It is a plot of the tone

frequency for the "1" key:

Fig 1.2

You can see that the DTMF generated signal is very distinct and clear. The horizontal axis is in

samples. The frequency of the tone is about 1900 Hz - close to the 1906 Hz predicted by Table 3

(697+1209).

1.3 Construction

This section is organized as follows:

o Schematics

o Construction Hightlights

1.3.1 Schematics

The schematic in the figure below is relatively straightforward. I recommend that you use a combination

of soldering and wirewrapping using sockets for all IC component placement.

1.3.2 Tone Generator

Fig. 1.3

1.3.3 Tone Decoder

The schematic for the DTMF decoder in the figure below. Again you can use a combination of

wirewrapping and soldering. Part placement is not critical.

Fig 1.4

1.4 Construction Highlights

1.4.1 Tone Generator

The DTMF generator circuit is straight forward to construct. Only 3 of the 5089's 4 column pins

(3,4,5) and all 4 row pins (11 to 14) were used. Thus it uses only 12 of the 16 touch tones (just

like your phone). In this schematic you'll note the "/" in front of column and row pin labels

(e.g. /C1). This means that these pins are active low. In other words, a pin is enabled when it is

grounded. When the circuit is powered on, these pins normally high (+5V). C1-C3 and R1-R4

are wired to an 8-position DIP switch. In a single-package this DIP contains 8 single-pole-single-

throw (SPST) switches. It is much cheaper to use than 8 real SPST switches. You slide a DIP

position to open or close its switch. When closed that particular switch connects its associated

column or row pin to ground and makes it active.

You could use a 12-key keypad available from many surplus or electronics mail-order

companies. But you must be aware of what you buy. Not all keypads can be used with the 5089.

I think the proper keypad will have 9 pins: 8 (for 4 rows plus 4 columns) plus 1 for a common

which you'd connect to ground. Often surplus keypads do not come with techsheets, and you will

have to manually figure out which pin is associated with which row or column. I found to my

surprise that my particular surplus 12-key keypad (from Electronic Goldmine) did not have this

common pin and so I resorted to using a DIP.

In this photo DIP positions 1 and 4 (C1 and R1 respectively) are in their ON positions. C1 and

R1 is "1" on your phone's keypad. The speaker will emit the touch-tone associated with the "1"

key (see Table 4)

The speaker is driven through the TIP31 transistor. Note: the labels 1, 2 and 3 that refer to the

base, collector and emitter pins respectively in the schematic are not standard. Be sure to check

your spec sheet for your TIP31.

TABLE 1.4: DIP SWITCH POSITIONS

(1) DIP: 1+4 (2) DIP: 2+4 (3) DIP: 3+4

(4) DIP: 1+5 (5) DIP: 2+5 (6) DIP: 3+5

(7) DIP: 1+6 (8) DIP: 2+6 (9) DIP: 3+6

(*) DIP: 1+7 (0) DIP: 2+7 (#) DIP: 3+7

Table 4 shows the DIP positions that will activate the tone associated with the key. The numbers

in bold and parenthesis are your desired key tone (like your phone). Thus if you wanted to dial a

"0", you would slide only positions 2 and 7 on the DIP switch.

1.4.2 Tone Decoder

The decoder circuit is also easy to construct. You will have to physically wire (using alligator

clips for instance) the TONE OUT pinout from the generator to the TONE IN pinout of the

decoder.

Once physically wired together, the 7-segment display will light up the number associated with

the touch-tone you activate with the DIP switch.

Note: A "0" tone lights up as "[" and not zero. This is because, "0" key's tone is actually a ten in

binary (1010). Because the 7-segment displays only a single digit, ten is displayed as a "[".

Similarly, the "*" (binary 1011) and "#" (binary 1100) light up as "]" and "U" respectively.

CHAPTER 2

VARIOUS DEVICES USED

2.1 RESISTOR :

A resistor is a two-terminal electrical or electronic component that opposes an electric current

by producing a voltage drop between its terminals in accordance with Ohm's law: The electrical

resistance is equal to the voltage drop across the resistor divided by the current through the

resistor while the temperature remains the same. Resistors are used as part of electrical networks

and electronic circuits.

Fig 2.1

Axial-lead resistors on tape. The tape is removed during assembly before the leads are formed and the

part is inserted into the board.

Fig 2.2

Three carbon composition resistors in a 1960s valve radio.

.

.

2.2 CAPACITOR

A capacitor is an electrical/electronic device that can store energy in the electric field between a

pair of conductors (called "plates"). The process of storing energy in the capacitor is known as

"charging", and involves electric charges of equal magnitude, but opposite polarity, building up

on each plate.

Capacitors are often used in electric and electronic circuits as energy-storage devices. They can

also be used to differentiate between high-frequency and low-frequency signals. This property

makes them useful in electronic filters.

2.2.1 Capacitor types

Fig 2.3

Capacitors: SMD ceramic at top left; SMD tantalum at bottom left; through-hole tantalum at top right;

through-hole electrolytic at bottom right. Major scale divisions are cm.

Fig 2.4

Various types of capacitors. From left: multilayer ceramic, ceramic disc, multilayer polyester film, tubular

ceramic, polystyrene, metallized polyester film, aluminium electrolytic. Major scale divisions are cm.

Fig 2.5

2.3 CRYSTAL OSCILLATOR

Fig 2.6

A miniature 4 MHz quartz crystal enclosed in a hermetically sealed HC-49/US package, used as the

resonator in a crystal oscillator.

A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating

crystal of piezoelectric material to create an electrical signal with a very precise frequency. This

frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable

clock signal for digital integrated circuits, and to stabilize frequencies for radio

transmitters/receivers.

Fig 2.7

Inside construction of a modern high performance HC-49 package quartz crystal

2.4 VOLTAGE REGULATOR

A voltage regulator is an electrical regulator designed to automatically maintain a constant

voltage level.

It may use an electromechanical mechanism, or passive or active electronic components.

Depending on the design, it may be used to regulate one or more AC or DC voltages.

2.4.1 Voltage Regulator 7805

The Digilab board can use any power supply that creates a DC voltage between 6 and 12 volts. A

5V voltage regulator (7805) is used to ensure that no more than 5V is delivered to the Digilab

board regardless of the voltage present at the J12 connector (provided that voltage is less than

12VDC). The regulator functions by using a diode to clamp the output voltage at 5VDC

regardless of the input voltage - excess voltage is converted to heat and dissipated through the

body of the regulator. If a DC supply of greater than 12V is used, excessive heat will be

generated, and the board may be damaged. If a DC supply of less than 5V is used, insufficient

voltage will be present at the regulators output.

Fig 2.8

If a power supply provides a voltage higher than 7 or 8 volts, the regulator must dissipate

significant heat. The "fin" on the regulator body (the side that protrudes upward beyond the main

body of the part) helps to dissipate excess heat more efficiently. If the board requires higher

currents (due to the use of peripheral devices or larger breadboard circuits), then the regulator

may need to dissipate more heat. In this case, the regulator can be secured to the circuit board by

fastening it with a screw and nut (see below). By securing the regulator tightly to the circuit

board, excess heat can be passed to the board and then radiated away.

Fig 2.9

2.5 Soldering

Fig 2.10

(De)soldering a contact from a wire.

Soldering is a process in which two or more metal items are joined together by melting and

flowing a filler metal into the joint, the filler metal having a relatively low melting point. Soft

soldering is characterized by the melting point of the filler metal, which is below 400 °C. The

filler metal used in the process is called solder.

Soldering is distinguished from brazing by use of a lower melting-temperature filler metal; it is

distinguished from welding by the base metals not being melted during the joining process. In a

soldering process, heat is applied to the parts to be joined, causing the solder to melt and be

drawn into the joint by capillary action and to bond to the materials to be joined by wetting

action. After the metal cools, the resulting joints are not as strong as the base metal, but have

adequate strength, electrical conductivity, and water-tightness for many uses. Soldering is an

ancient technique mentioned in the Bible and there is evidence that it was employed up to 5000

years ago in Mesopotamia.

2.5.1 Applications

The most frequent application of soldering is assembling electronic components to printed circuit

boards (PCBs). Another common application is making permanent but reversible connections

between copper pipes in plumbing systems. Joints in sheetmetal objects such as food cans, roof

flashing, rain gutters and automobile radiators have also historically been soldered, and

occasionally still are. Jewelry and small mechanical parts are often assembled by soldering.

Soldering is also used to join lead came and copper foil in stained glass work. Soldering can also

be used to effect a semi-permanent patch for a leak in a container cooking vessel.

2.5.2 Desoldering and resoldering

Used solder contains some of the dissolved base metals and is unsuitable for reuse in making

new joints. Once the solder's capacity for the base metal has been achieved it will no longer

properly bond with the base metal, usually resulting in a brittle cold solder joint with a

crystalline appearance.

It is good practice to remove solder from a joint prior to resoldering—desoldering braids or

vacuum desoldering equipment (solder suckers) can be used. Desoldering wicks contain plenty

of flux that will lift the contamination from the copper trace and any device leads that are

present. This will leave a bright, shiny, clean junction to be resoldered.

The lower melting point of solder means it can be melted away from the base metal, leaving it

mostly intact though the outer layer will be "tinned" with solder. Flux will remain which can

easily be removed by abrasive or chemical processes. This tinned layer will allow solder to flow

into a new joint, resulting in a new joint, as well as making the new solder flow very quickly and

easily.

2.6 Printed circuit board

Fig 2.11

Part of a 1983 Sinclair ZX Spectrum computer board; a populated PCB, showing the conductive traces,

vias (the through-hole paths to the other surface), and some mounted electrical components

Fig 2.11

PCB Layout Program

A printed circuit board, or PCB, is used to mechanically support and electrically connect

electronic components using conductive pathways, or traces, etched from copper sheets

laminated onto a non-conductive substrate. Alternative names are printed wiring board

(PWB),and etched wiring board. A PCB populated with electronic components is a printed

circuit assembly (PCA), also known as a printed circuit board assembly (PCBA).

PCBs are rugged, inexpensive, and can be highly reliable. They require much more layout effort

and higher initial cost than either wire-wrapped or point-to-point constructed circuits, but are

much cheaper and faster for high-volume production. Much of the electronics industry's PCB

design, assembly, and quality control needs are set by standards that are published by the IPC

organization.

Chapter 3

MT 8870 IC

3.1 Features

• Complete DTMF Receiver

• Low power consumption

• Internal gain setting amplifier

• Adjustable guard time

• Central office quality

• Power-down mode

• Inhibit mode

• Backward compatible with MT8870C/MT8870C-1

3.2 Applications

• Receiver system for British Telecom (BT) or CEPT Spec (MT8870D-1)

• Paging systems

• Repeater systems/mobile radio

• Credit card systems

• Remote control

• Personal computers

• Telephone answering machine

3.3 Description

The MT8870D/MT8870D-1 is a complete DTMF integrating both the bandspl it f i l ter

and Digital decoder functions. The f i l ter section uses witched capacitor techniques

for high and lowgroup f i l ters; the decoder uses digital counting techniques to detect

and decode al l 16 DTMF tonepairs into a 4-bit code. External component count is

minimized by on chip provis ion of a differential input amplif ier, c lock osci l lator and

latched three-state bus interface

3.4 Functional Description

The MT8870D/MT8870D-1 monolithic DTMF receiver offers small size, low power consumption and high

performance. Its architecture consists of a bandsplit filter section, which separates the high and low

group tones, followed by a digital counting section which verifies the frequency and duration of the

received tones before passing the corresponding code to the output bus.

Fig 3.1

3.5 MT8870D/MT8870D-1 ISO2-CMOS

18 PIN CERDIP/PLASTIC DIP/SOIC

Fig 3.2

20 PIN SSOP/TSSOP

Fig 3.3

3.6 PIN DESCRIPTIONS:-

Table 3.1

3.7 Functional Description

The MT8870D/MT8870D-1 monolithic DTMF receiver offers small size, low power consumption and high

performance. Its architecture consists of a bandsplit filter section, which separates the high and low

group tones, followed by a digital counting section which verifies the frequency and duration of the

received tones before passing the corresponding code to the output bus.

3.8 Steering Circuit

Before registration of a decoded tone pair, the receiver checks for a valid signal duration (referred to as

character recognition condition). This check is performed by an external RC time constant driven by ESt.

A logic high on ESt causes vc to rise as the capacitor discharges. Provided signal condition is maintained

(ESt remains high) for the validation period (tGTP), vc reaches the threshold (VTSt) of the steering logic

to register the tone pair, latching its corresponding 4-bit code into the output latch. At this point the GT

output is activated and drives vc to VDD. GT continues to drive high as long as ESt remains high. Finally,

after a short delay to allow the output latch to settle, the delayed steering output flag (StD) goes high,

signalling that a received tone pair has been registered. The contents of the output latch are made

available on the 4-bit output bus by raising the three state control input (TOE) to a logic high. The

steering circuit works in reverse to validate the interdigit pause between signals. Thus, as well as

rejecting signals too short to be considered valid, the receiver will tolerate signal interruptions (dropout)

too short to be considered a valid pause. This facility, together with the capability of selecting the

steering time constants externally, allows the designer to tailor performance to meet a wide variety of

system requirements.

Basic steering circuit

Fig 3.4

3.9 Filter Section

Separation of the low-group and high group tones is achieved by applying the DTMF signal to the inputs

of two sixth-order switched capacitor bandpass filters, the bandwidths of which correspond to the low

and high group frequencies. The filter section also incorporates notches at 350 and 440 Hz for

exceptional dial tone rejection (see Figure 3). Each filter output is followed by a single order switched

capacitor filter section which smooths the signals prior to limiting. Limiting is performed by high-gain

comparators which are provided with hysteresis to prevent detection of unwanted low-level signals. The

outputs of the comparators provide full rail logic swings at the frequencies of the incoming DTMF

signals.

Filter response

Fig 3.5

3.10 Guard Time Adjustment

In many situations not requiring selection of tone duration and interdigital pause, the simple steering

circuit shown in Figure is applicable. Component values are chosen according to the formula:

tREC=tDP+tGTP

tID=tDA+tGTA

The value of tDP is a device parameter (see Figure 11) and tREC is the minimum signal duration to be

recognized by the receiver. A value for C of 0.1 μF is recommended for most applications, leaving R to be

selected by the designer.

Different steering arrangements may be used to select independently the guard times for tone present

(tGTP) and tone absent (tGTA). This may be necessary to meet system specifications which place both

accept and reject limits on both tone duration

and interdigital pause. Guard time adjustment also allows the designer to tailor system parameters such

as talk off and noise immunity. Increasing tREC improves talk-off performance since it reduces the

probability that tones simulated by speech will

maintain signal condition long enough to be registered. Alternatively, a relatively short tREC with a long

tDO would be appropriate for extremely noisy environments where fast acquisition time and immunity

to tone drop-outs are required. Design information for guard time adjustment is shown in Figure .

Fig 3.6

3.11 Crystal Oscillator

The internal clock circuit is completed with the addition of an external 3.579545 MHz crystal and is

normally connected as shown in Figure (Single- Ended Input Configuration). However, it is possible to

configure several MT8870D/MT8870D-1 devices employing only a single oscillator crystal. The oscillator

output of the first device in the chain is coupled through a 30 pF capacitor to the oscillator input (OSC1)

of the next device. Subsequent devices are connected in a similar fashion. Refer to Figure for details. The

problems associated with unbalanced loading are not a concern with the arrangement shown, i.e.,

precision balancing capacitors are not required.

Fig 3.7

oscillator connections

3.12 Absolute maximum condition

Table 3.2

3.13 DC Electrical characteristics

Table 3.3

3.15 Operating characterstics

Table 3.4

3.19 Applications

The purpose of this Application Note is to provide information on the operation and aplication of DTMF

Receivers. The MT8870 Integrated DTMF Receiver will be discussed in detail and its use illustrated in the

application examples which follow.

More than 25 years ago the need for an improved method for transferring dialling information through

the telephone network was recognized. The traditional method, Dial pulse signalling, was not only slow,

suffering severe distortion over long wire loops,but required a DC path through the communications

channel. A signalling scheme was developed utilizing voice frequency tones and implemented as a very

reliable alternative to pulse dialling. This scheme is known as DTMF (Dual Tone Multi- Frequency),

Touch-Tone™ or simply, tone dialling. As its acronym suggests, a valid DTMF signal is the sum of two

tones, one from a low group (697-941Hz) and one from a high group (1209-1633Hz) with each group

containing four individual tones. The tone frequencies were carefully chosen such that they are not

harmonically related and that their intermodulation products result in minimal signaling impairment

(Fig. 1a). This scheme allows for 16 unique combinations. Ten of these codes represent the numerals

zero through nine, the remaining six (*,#,A,B,C,D) being reserved for special signalling.

Most telephone keypads contain ten numeric push buttons plus the asterisk (*) and octothorp (#). The

buttons are arranged in a matrix, each selecting its low group tone from its respective row and its high

group tone from its respective column .

The DTMF coding scheme ensures that each signal contains one and only one component from each of

the high and low groups. This significantly simplifies decoding because the composite DTMF signal may

be separated with bandpass filters, into its two single frequency components each of which may be

handled individually. As a result DTMF coding has proven to provide a flexible signalling scheme of

excellent reliability, hence motivating innovative and competitive decoder design.

CONCLUSION

I learned a lot in the process of writing this article, and I hope it will encourage many of you to consider bringing HA into your own lives. I'll admit that wiring up light switches is not the easiest of tasks for someone who hasn't done it before, but the plug-in devices are a snap to set up and make for an easy entry-point to working with the technology.

Most people are very timid the first time they have to replace a component in their PC and, in the same way, there is a learning curve to HA. And although the components you're dealing with are not as sensitive as those inside a computer, there is a risk anytime you are working near electric circuits (in this case more of a risk to yourself).

Once I finished with the installation, the only signs of the work I'd done was a set of nicer wall switches and a handful of extra adapters plugged into the wall. The only way it could have looked more professional was if I had finished the whole house with Insteon- and Decora-style switches to match the ones I changed. I could have also swapped the wall outlets to Insteon rather than using the lamp adapter, but then I couldn't use the adapter for my Christmas tree when that time of year rolls around. I like having the choice of moving it. 

I won't try claiming that anyone can manage a home automation installation, so if you're uncomfortable around electronics, don't know what you're doing in a breaker box, or are particularly accident-prone, don't even risk it. I would think that if you've soldered wires before, though, then you probably have enough knowledge and common sense to take on a task like this.

REFERENCES

1. www.google.com

2. www.wikipedia.com

3. Books