Telephone Call Counter

PROJECT REPORT

ON

Design and Implementation of Telephone Call Counter with Live Demo

For the partial fulfillment of the Diploma of associated membership awarded by

Submitted BY KM AKANSHA

SD-19332 DIPIETE (ET)

UNDER THE GUIDANCE OF Mr. Vikas Parasar

THE INSTITUTION OF ELECTRONICS AND

TELECOMMUNICATION ENGINEERS, NEW DELHI

CERTIFICATE

This is certified that KM AKANKSHA has carried out project work

presented in this thesis entitled “DESIGN AND IMPLEMENTATION OF

TELEPHONE CALL COUNTER WITH LIVE DEMO” for the award of

IETE, under my supervision. The report embodies result of original

work and studies carried out by students herself and the contents of

the thesis do not form the basis for the award of any other degree or

diploma to the candidate or to anybody else.

Signature of the project Guide

MR. VIKAS PARASAR

Place:- Date :-

ACKNOWLEDGEMENT

I am very much thankful from the core of my heart for the precious

contribution of my guide who provided his possible help the

successful completion of this project has been possible due to sincere

co operation guidance, inspiration moral support and timely advice of

my guide who devoted his utmost co-operation in this project work. I

also give special thanks to my colleagues for that endless flow of

ideas and all those who helped in this project in some way or the

other.

KM AKANKSHA SD – 193221

DECLARATION BY THE CANDIDATE I, KM AKANKSHA, hereby declares that the project work entitled

“DESIGN AND IMPLEMENTATION OF TELEPHONE CALL COUNTER WITH LIVE DEMO” under the partial fulfillment and award of the diploma or degree

and this has been submitted anywhere else for the award of any other degree.

I have not submitted the matter embodied in this project for the award of any

other degree or diploma. and all the content given in this project and can be believed.

Signature of the student KM AKANKSHA SD – 193221

Introduction to Telephone

An Olivetti rotary dial telephone, c.1940s

The telephone (from the Greek: τῆλε, tēle, "far" and φωνή, phōnē,

"voice"), colloquially referred to as a phone, is a telecommunications device

that transmits and receives sounds, usually the human voice. Telephones are

a point-to-point communication system whose most basic function is to allow

two people separated by large distances to talk to each other. Developed in the

mid-1870s by Alexander Graham Bell and others, the telephone has long been

considered indispensable to businesses, households and governments, is now

one of the most commonappliances in the developed world. The word

"telephone" has been adapted to many languages and is now recognized

around the world.

All modern telephones have a microphone to speak into, an earphone (or

'speaker') which reproduces the voice of the other person, a ringerwhich makes

a sound to alert the owner when a call is coming in, and a keypad (or on older

phones a telephone dial) to enter the telephone number of the telephone to be

called. The microphone and earphone are usually built into a handset which is

held up to the face to talk. The keypad may be part of the handset or of a base

unit to which the handset would be connected. A landline telephone is

connected by a pair of wires to the telephone network, while a mobile

phone (also called a cell phone) is portable and communicates with the

telephone network byradio. A cordless telephone has a portable handset which

communicates by radio with a base station connected by wire to the telephone

network, and can only be used within a limited range of the base station.

The microphone converts the sound waves to electrical signals and then these

are sent through the telephone network to the other phone and there converted

by an earphone, or speaker, back into sound waves. Telephones are

a duplex communications medium, meaning they allow the people on both ends

to talk simultaneously. The telephone network, consisting of a worldwide net

of telephone lines, fiberoptic cables,microwave transmission, cellular

networks, communications satellites, and undersea telephone cables connected

by switching centers, allows any telephone in the world to communicate with

any other. Each telephone line has an identifying number called its telephone

number. To initiate a telephone call the user enters the other telephone's

number into a numeric keypad on the phone. Graphic symbols used to

designate telephone service or phone-related information in print, signage, and

other media include.

Although originally designed for simple simultaneous voice communications,

modern telephones, particularly 'smart' mobile phones, have many other

capabilities besides that, and may be able torecord spoken messages, send

and receive text messages, take and display photographs or video and surf the

Internet.

History

Main articles: History of the telephone and Timeline of the telephone Further

information: Invention of the telephone, Elisha Gray and Alexander Bell

telephone controversy, and Canadian Parliamentary Motion on Alexander

Graham Bell

Alexander Graham Bell's telephone patent drawing, 7 March 1876. Credit for

the invention of the electric telephone is frequently disputed, and new

controversies over the issue have arisen from time to time. As with other

influential inventions such as radio, television, the light bulb, and the computer,

there were several inventors who did pioneering experimental work on voice

transmission over a wire and improved on each other's ideas. Innocenzo

Manzetti, Antonio Meucci, Johann Philipp Reis, Elisha Gray, Alexander Graham

Bell, and Thomas Edison, among others, have all been credited with pioneering

work on the telephone. An undisputed fact is that Alexander Graham Bell was

thefirst to be awarded a patent for the electric telephone by the United States

Patent and Trademark Office (USPTO) in March 1876.[2] That first patent by Bell

was the master patent of the telephone, from which other patents for electric

telephone devices and features flowed.

The early history of the telephone became and still remains a confusing morass

of claims and counterclaims, which were not clarified by the large number of

lawsuits that hoped to resolve the patent claims of many individuals and

commercial competitors. The Bell and Edison patents, however, were

forensically victorious and commercially decisive.

A Hungarian engineer, Tivadar Puskás, quickly invented the telephone

switchboard in 1876, which allowed for the formation of telephone exchanges,

and eventually networks.

Basic principles

Figure 1: Schematic of a landline telephone installation.

A traditional landline telephone system, also known as "plain old telephone

service" (POTS), commonly carries both control and audio signals on the

sametwisted pair (C) of insulated wires: the telephone line. The signaling

equipment, or ringer, (see figure 1) consists of a bell, beeper, light or other

device (A7) to alert the user to incoming calls, and number buttons or a rotary

dial (A4) to enter a telephone number for outgoing calls. Most of the expense of

wire-line telephone service is the wires, so telephones transmit both the

incoming and outgoing voice channels on a single pair of wires. A twisted pair

line rejectselectromagnetic interference (EMI) and crosstalk better than a single

wire or an untwisted pair. The strong outgoing voice signal from the microphone

does not overpower the weaker incoming speaker signal with

a sidetone because a hybrid coil (A3) subtracts the microphone's signal from

the signal sent to the local speaker. The junction box (B) arrests lightning (B2)

and adjusts the line's resistance (B1) to maximize the signal power for the line's

length. Telephones have similar adjustments for inside line lengths (A8). The

wire's voltages are negative compared to earth, to reduce galvanic corrosion.

Negative voltage attracts positive metal ions toward the wires.

Details of operation

The landline telephone contains a switchhook (A4) and an alerting device,

usually a ringer (A7), that remains connected to the phone line whenever the

phone is "on hook" (i.e. the switch (A4) is open), and other components which

are connected when the phone is "off hook". The off-hook components include a

transmitter (microphone, A2), a receiver (speaker, A1), and other circuits for

dialing, filtering (A3), and amplification.

A calling party wishing to speak to another party will pick up the

telephone's handset, thereby operating a lever which closes the switchhook

(A4), which powers the telephone by connecting the transmitter (microphone),

receiver (speaker), and related audio components to the line. The off-hook

circuitry has a low resistance (less than 300 ohms) which causes a direct

current(DC), which comes down the line (C) from the telephone exchange. The

exchange detects this current, attaches a digit receiver circuit to the line, and

sends a dial tone to indicate readiness. On a modern push-button telephone,

the caller then presses the number keys to send the telephone number of

the called party. The keys control a tone generator circuit (not shown) that

makes DTMF tones that the exchange receives. A rotary-dial

telephone uses pulse dialing, sending electrical pulses, that the exchange can

count to get the telephone number (as of 2010 many exchanges were still

equipped to handle pulse dialing). If the called party's line is available, the

exchange sends an intermittent ringing signal (about 90 volts alternating

current (AC) in North America and UK and 60 volts in Germany) to alert the

called party to an incoming call. If the called party's line is in use, the exchange

returns a busy signal to the calling party. However, if the called party's line is in

use but has call waiting installed, the exchange sends an intermittent audible

tone to the called party to indicate an incoming call.

The phone's ringer (A7) is connected to the line through a capacitor (A6), a

device which blocks direct current but passes alternating current. So, the phone

draws no current when it is on hook (a DC voltage is continually connected to

the line), but exchange circuitry (D2) can send an AC voltage down the line to

ring for an incoming call. (When there is no exchange, telephones often have

hand-cranked magnetos to make the ringing voltage.) When a landline phone is

inactive or "on hook", the circuitry at the telephone exchange detects the

absence of direct current and therefore "knows" that the phone is on hook

(therefore, only AC current will go through) with only the alerting device

electrically connected to the line. When a party initiates a call to this line, the

exchange sends the ringing signal. When the called party picks up the handset,

they actuate a double-circuit switchhook (not shown) which simultaneously

disconnects the alerting device and connects the audio circuitry to the line. This,

in turn, draws direct current through the line, confirming that the called phone is

now active. The exchange circuitry turns off the ring signal, and both phones are

now active and connected through the exchange. The parties may now

converse as long as both phones remain off hook. When a party "hangs up",

placing the handset back on the cradle or hook, direct current ceases in that

line, signaling the exchange to disconnect the call.

Calls to parties beyond the local exchange are carried over "trunk" lines which

establish connections between exchanges. In modern telephone

networks, fiber-optic cable and digital technology are often employed in such

connections. Satellite technology may be used for communication over very

long distances.

In most landline telephones, the transmitter and receiver (microphone and

speaker) are located in the handset, although in a speakerphone these

components may be located in the base or in a separate enclosure. Powered by

the line, the microphone (A2) produces a modulated electrical current which

varies its frequency and amplitude in response to the sound waves arriving at

its diaphragm. The resulting current is transmitted along the telephone line to

the local exchange then on to the other phone (via the local exchange or via a

larger network), where it passes through the coil of the receiver (A3). The

varying current in the coil produces a corresponding movement of the receiver's

diaphragm, reproducing the original sound waves present at the transmitter.

Along with the microphone and speaker, additional circuitry is incorporated to

prevent the incoming speaker signal and the outgoing microphone signal from

interfering with each other. This is accomplished through a hybrid coil (A3). The

incoming audio signal passes through a resistor (A8) and the primary winding of

the coil (A3) which passes it to the speaker (A1). Since the current path A8 - A3

has a far lower impedance than the microphone (A2), virtually all of the

incoming signal passes through it and bypasses the microphone.

At the same time the DC voltage across the line causes a DC current which is

split between the resistor-coil (A8-A3) branch and the microphone-coil (A2-A3)

branch. The DC current through the resistor-coil branch has no effect on the

incoming audio signal. But the DC current passing through the microphone is

turned into AC current (in response to voice sounds) which then passes through

only the upper branch of the coil's (A3) primary winding, which has far fewer

turns than the lower primary winding. This causes a small portion of the

microphone output to be fed back to the speaker, while the rest of the AC

current goes out through the phone line.

A Lineman's handset is a telephone designed for testing the telephone network,

and may be attached directly to aerial lines and other infrastructure

components.

Early development

Main article: Timeline of the telephone

1896 Telephone from Sweden.

Wooden wall telephone with a hand-cranked magneto generator.

A candlestick phone

Modern sound-powered emergency telephone.

A modern mobile phone, also called a cell phone.

1844 — Innocenzo Manzetti first mooted the idea of a “speaking

telegraph” or telephone. Use of the 'speaking telegraph' and 'sound

telegraph' monikers would eventually be replaced by the newer, distinct

name, 'telephone'.

26 August 1854 — Charles Bourseul published an article in the

magazine L'Illustration (Paris): "Transmission électrique de la parole"

(electric transmission of speech), describing a 'make-and-break' type

telephone transmitter later created by Johann Reis.

26 October 1861 — Johann Philipp Reis (1834–1874) publicly

demonstrated the Reis telephone before the Physical Society of Frankfurt.

22 August 1865, La Feuille d'Aoste reported “It is rumored that English

technicians to whom Mr. Manzetti illustrated his method for transmitting

spoken words on the telegraph wire intend to apply said invention in England

on several private telegraph lines". However telephones would not be

demonstrated there until after Alexander Graham Bell received his patent in

the United States in 1876. The first attempted telephone demonstration in

the United Kingdom was later that year, by the scientist Lord Kelvin, who

obtained a set of telephones from Bell after the 1876 Philadelphia Centennial

Exposition.

28 December 1871 — Antonio Meucci files patent caveat No. 3335 in the

U.S. Patent Office titled "Sound Telegraph", describing communication of

voice between two people by wire. A 'patent caveat' was not an

invention patent award, but only an unverified notice filed by an individual

that he or she intends to file a regular patent application in the future.

1874 — Meucci, after having renewed the caveat for two years does not

renew it again, and the caveat lapses.

6 April 1875 — Bell's U.S. Patent 161,739 "Transmitters and Receivers for

Electric Telegraphs" is granted. This uses multiple vibrating steel reeds in

make-break circuits.

11 February 1876 — Gray invents a liquid transmitter for use with a

telephone but does not build one.

14 February 1876 — Elisha Gray files a patent caveat for transmitting the

human voice through a telegraphic circuit.

14 February 1876 — Alexander Bell applies for the patent "Improvements

in Telegraphy", for electromagnetic telephones using undulating currents.

19 February 1876 — Gray is notified by the U.S. Patent Office of an

interference between his caveat and Bell's patent application. Gray decides

to abandon his caveat.

7 March 1876 — Bell's U.S. patent 174,465 "Improvement in Telegraphy"

is granted, covering "the method of, and apparatus for, transmitting vocal or

other sounds telegraphically … by causing electrical undulations, similar in

form to the vibrations of the air accompanying the said vocal or other sound."

10 March 1876 — The first successful telephone transmission of clear

speech using a liquid transmitter when Bell spoke into his device, “Mr.

Watson, come here, I want to see you.” and Watson heard each word

distinctly.

30 January 1877 — Bell's U.S. patent 186,787 is granted for an

electromagnetic telephone using permanent magnets, iron diaphragms, and

a call bell.

27 April 1877 — Edison files for a patent on a carbon (graphite)

transmitter. The patent 474,230 was granted 3 May 1892, after a 15 year

delay because of litigation. Edison was granted patent 222,390 for a carbon

granules transmitter in 1879.

Early commercial instruments

Early telephones were technically diverse. Some used a liquid transmitter, some

had a metal diaphragm that induced current in an electromagnet wound around

a permanent magnet, and some were "dynamic" - their diaphragm vibrated a

coil of wire in the field of a permanent magnet or the coil vibrated the

diaphragm. The sound-powered dynamic kind survived in small numbers

through the 20th century in military and maritime applications, where its ability to

create its own electrical power was crucial. Most, however, used the

Edison/Berliner carbon transmitter, which was much louder than the other kinds,

even though it required an induction coil which was an impedance

matching transformer to make it compatible with the impedance of the line. The

Edison patents kept the Bell monopoly viable into the 20th century, by which

time the network was more important than the instrument.

Early telephones were locally powered, using either a dynamic transmitter or by

the powering of a transmitter with a local battery. One of the jobs of outside

plantpersonnel was to visit each telephone periodically to inspect the battery.

During the 20th century, "common battery" operation came to dominate,

powered by "talk battery" from the telephone exchange over the same wires that

carried the voice signals.

Early telephones used a single wire for the subscriber's line, with ground

return used to complete the circuit (as used in telegraphs). The earliest dynamic

telephones also had only one port opening for sound, with the user alternately

listening and speaking (or rather, shouting) into the same hole. Sometimes the

instruments were operated in pairs at each end, making conversation more

convenient but also more expensive.

At first, the benefits of a telephone exchange were not exploited. Instead

telephones were leased in pairs to a subscriber, who had to arrange for a

telegraph contractor to construct a line between them, for example between a

home and a shop. Users who wanted the ability to speak to several different

locations would need to obtain and set up three or four pairs of

telephones. Western Union, already using telegraph exchanges, quickly

extended the principle to its telephones in New York City andSan Francisco,

and Bell was not slow in appreciating the potential.

Signalling began in an appropriately primitive manner. The user alerted the

other end, or the exchange operator, by whistling into the transmitter. Exchange

operation soon resulted in telephones being equipped with a bell in a ringer box,

first operated over a second wire, and later over the same wire, but with a

condenser (capacitor) in series with the bell coil to allow the AC ringer signal

through while still blocking DC (keeping the phone "on hook"). Telephones

connected to the earliest Strowgerautomatic exchanges had seven wires, one

for the knife switch, one for each telegraph key, one for the bell, one for

the push-button and two for speaking. Large wall telephones in the early 20th

century usually incorporated the bell, and separate bell boxes for desk phones

dwindled away in the middle of the century.

Rural and other telephones that were not on a common battery exchange had

a magneto or hand-cranked generator to produce a high voltage alternating

signal to ring the bells of other telephones on the line and to alert the operator.

In the 1890s a new smaller style of telephone was introduced, packaged in

three parts. The transmitter stood on a stand, known as a "candlestick" for its

shape. When not in use, the receiver hung on a hook with a switch in it, known

as a "switchhook." Previous telephones required the user to operate a separate

switch to connect either the voice or the bell. With the new kind, the user was

less likely to leave the phone "off the hook". In phones connected to magneto

exchanges, the bell, induction coil, battery and magneto were in a separate bell

box or "ringer box". [4] In phones connected to common battery exchanges, the

ringer box was installed under a desk, or other out of the way place, since it did

not need a battery or magneto.

Cradle designs were also used at this time, having a handle with the receiver

and transmitter attached, now called a handset, separate from the cradle base

that housed the magneto crank and other parts. They were larger than the

"candlestick" and more popular.

Disadvantages of single wire operation such as crosstalk and hum from nearby

AC power wires had already led to the use of twisted pairs and, for long

distance telephones, four-wire circuits. Users at the beginning of the 20th

century did not place long distance calls from their own telephones but made an

appointment to use a special sound proofed long distance telephone

booth furnished with the latest technology.

A Swedish Ericsson 1001 model, from 1939.

What turned out to be the most popular and longest lasting physical style of

telephone was introduced in the early 20th century, including Bell's Model 102.

A carbon granule transmitter and electromagnetic receiver were united in a

single molded plastic handle, which when not in use sat in a cradle in the base

unit. The circuit diagram of the Model 102 shows the direct connection of the

receiver to the line, while the transmitter was induction coupled, with energy

supplied by a local battery.[5] The coupling transformer, battery, and ringer were

in a separate enclosure. The dial switch in the base interrupted the line current

by repeatedly but very briefly disconnecting the line 1–10 times for each digit,

and the hook switch (in the center of the circuit diagram) disconnected the line

and the transmitter battery while the handset was on the cradle.

After the 1930s, the base also enclosed the bell and induction coil, obviating the

old separate ringer box. Power was supplied to each subscriber line by central

office batteries instead of a local battery, which required periodic service. For

the next half century, the network behind the telephone became progressively

larger and much more efficient, but after the telephone dial was added the

instrument itself changed little until American Telephone & Telegraph

Company (AT&T) introduced touch-tone dialing in the 1960s.

Digital telephony

Main article: Digital Telephony

The Public Switched Telephone Network (PSTN) has gradually evolved towards

digital telephony which has improved the capacity and quality of the network.

End-to-end analog telephone networks were first modified in the early 1960s by

upgrading transmission networks with T1 carrier systems, designed to support

the basic 3 kHz voice channel by sampling the bandwidth-limited analog voice

signal and encoding using PCM. While digitization allows wideband voice on the

same channel, the improved quality of a wider analog voice channel did not find

a large market in the PSTN.

Later transmission methods such as SONET and fiber optic transmission further

advanced digital transmission. Although analog carrier systems existed that

multiplexed multiple analog voice channels onto a single transmission medium,

digital transmission allowed lower cost and more channels multiplexed on the

transmission medium. Today the end instrument often remains analog but the

analog signals are typically converted to digital signals at the (serving area

interface (SAI), central office (CO), or other aggregation point. Digital loop

carriers (DLC) place the digital network ever closer to the customer premises,

relegating the analog local loop to legacy status.

IP telephony

Main article: Voice over Internet Protocol

Hardware-based IP phone

Internet Protocol (IP) telephony (also known as Voice over Internet Protocol,

VoIP), is a disruptive technology that is rapidly gaining ground against traditional

telephone network technologies. As of January 2005, up to 10% of telephone

subscribers in Japan and South Korea have switched to this digital telephone

service. A January 2005 Newsweek article suggested that Internet telephony

may be "the next big thing."[6] As of 2006 many VoIP companies offer service

toconsumers and businesses.

IP telephony uses an Internet connection and hardware IP

Phones or softphones installed on personal computers to transmit

conversations encoded as data packets. In addition to replacing POTS (plain

old telephone service), IP telephony services are also competing with mobile

phone services by offering free or lower cost connections via WiFi hotspots.

VoIP is also used on private networks which may or may not have a connection

to the global telephone network.

IP telephones have two notable disadvantages compared to traditional

telephones. Unless the IP telephone's components are backed up with

anuninterruptible power supply or other emergency power source, the phone

will cease to function during a power outage as can occur during an emergency

or disaster, exactly when the phone is most needed. Traditional phones

connected to the older PSTN network do not experience that problem since they

are powered by the telephone company's battery supply, which will continue to

function even if there's a prolonged power black-out. A second distinct problem

for an IP phone is the lack of a 'fixed address' which can impact the provision of

emergency services such as police, fire or ambulance, should someone call for

them. Unless the registered user updates the IP phone's physical address

location after moving to a new residence, emergency services can be, and have

been, dispatched to the wrong location.

Telephone call

"Phone Call" redirects here. For the OmPuff album, see Phone Call (album).

An early 20th century Candlestick telephone used for a phone call.

Photo illustrative of a man suffering from telephone anxiety on an mid-seventies

rotary dial model.

A telephone call is a connection over a telephone network between the

calling party and the called party.

Information transmission

A telephone call may carry ordinary voice transmission using a telephone,

data transmission when the calling party and called party are using modems,

or facsimile transmission when they are using fax machines. The call may

use land line, mobile phone, satellite phone or any combination thereof.

Where a telephone call has more than one called party it is referred to as a

conference call. When two or more users of the network are sharing the

same physical line, it is called a party line or Rural phone line.

If the caller's wireline phone is directly connected to the calling party, when

the caller takes their telephone off-hook, the calling party's phone will ring.

This is called a hot line or ringdown. Otherwise, the calling party is usually

given a tone to indicate they should begin dialing the desired number. In

some (now very rare) cases, the calling party cannot dial calls directly, and is

connected to an operator who places the call for them.

Calls may be placed through a public network (such as the Public Switched

Telephone Network) provided by a commercial telephone company or a

private network called a PBX. In most cases a pivate network is connected to

the public network in order to allow PBX users to dial the outside world.

Incoming calls to a private network arrive at the PBX in two ways: either

directly to a users phone using a DDI number or indirectly via a receptionist

who will answer the call first and then manually put the caller through to the

desired user on the PBX.

Most telephone calls through the PSTN are set up using ISUP signalling

messages or one of its variants between telephone exchanges to establish

the end to end connection. Calls through PBX networks are set up using

QSIG, DPNSS or variants.

Costs

Some types of calls are not charged, such as local calls (and Internal calls)

dialed directly by a telephone subscriber in Canada, the United States, Hong

Kong, United Kingdom, Ireland or New Zealand (Residential subscribers

only). In most other areas, all telephone calls are charged a fee for the

connection. Fees depend on the provider of the service, the type of service

being used (a call placed from a landline or wired telephone will have one

rate, and a call placed from a mobile telephone will have a different rate) and

the distance between the calling and the called parties. In most

circumstances, the calling party pays this fee. However, in some

circumstances such as a reverse charge or collect call, the called party pays

the cost of the call. In some circumstances, the caller pays a flat rate charge

for the telephone connection and does not pay any additional charge for all

calls made. Telecommunication liberalization has been established in

several countries to allows customers to keep their local phone provider and

use an alternate provider for a certain call in order to save money.

An early 21st century mobile phone used for a phone call.

Placing a call

A typical phone call using a traditional phone is placed by picking the phone

handset up off the base and holding the handset so that the hearing end is

next to the user's ear and the speaking end is within range of the mouth. The

caller would then rotary dial or press buttons for the phone numbers needed

to complete the call.

In addition to the traditional method of placing a telephone call, new

technologies allow different methods for initiating a telephone call, such as

voice dialing. Voice over IP technology allows calls to be made through a

PC, using a service like Skype. Other services enable callers to initiate a

telephone call through a third party without exchanging phone numbers.

The use of headsets is becoming more common for placing or receiving a

call. Headsets can either come with a cord or be wireless.

Tones

Preceding, during, and after a traditional telephone call is placed, certain

tones signify the progress and status of the telephone call:

• a dial tone signifying that the system is ready to accept a telephone number and

connect the call

• either:

o a ringing tone signifying that the calling party has yet to answer the

telephone

o a busy signal (or engaged tone) signifying that the calling party's telephone

is being used in a telephone call to another person (or is "off the hook" though

no number has been dialled, i.e. the customer does not want to be disturbed)

o a fast busy signal (also called reorder tone or overflow busy tone)

signifying that there is congestion in the telephone network, or possibly that

the calling subscriber has delayed too long in dialling all the necessary digits.

The fast busy signal is generally twice as fast as the normal busy signal.

• status tones such as STD notification tones (to inform the caller that the telephone

call is being trunk dialled at a greater cost to the calling party), minute minder

beeps (to inform the caller of the relative duration of the telephone call on calls

that are charged on a time basis), and others

• a tone (sometimes the busy signal, often the dial tone) to signify that the called

party has hung up.

• tones used by earlier inband telephone switching systems were simulated by a

Red box or a blue box used by "phone phreaks" to illegally make or receive free

trunk/toll calls.

Unwanted calls

Unsolicited telephone calls are a modern nuisance. Common kinds of

unwanted calls include prank calls, telemarketing calls, and obscene phone

calls.

Caller ID provides some protection against unwanted calls, but can still be

turned off by the calling party. Even where end-user Caller ID is not

available, calls are still logged, both in billing records at the originating telco

and via automatic number identification, so the perpetrator's phone number

can still be discovered in many cases. However, this does not provide

complete protection: harassers can use payphones, in some cases,

automatic number identification itself can be spoofed or blocked, and mobile

telephone abusers can (at some cost) use "throwaway" phones or SIMs

PROJECT DESCRIPTION The circuit presented here is a very useful add-on device to be

connected to the telephone lines to count and display the number of

incoming calls received in the absence of the subscriber. The circuit

can be fabricated using low-cost and easily available components. It

uses a popular decade counter IC, a timer IC, and a few other

discrete components. The circuit may be divided into three sections:

(a) ring detector, (b) call counter, and (c) timer and relay controller.

The ring detector circuit detects the incoming ring signals. The timer

circuit is used to control a relay, which controls the switching of ring

signals to the ring detector section. The call counter is used to count

the number of incoming calls. To count the number of calls (not the

number of rings), the response of the counter is limited to the first

ring pulse only. In other words, it responds to the initial ring pulse

and ignores subsequent ring pulses which repeat till the calling

subscriber or the telephone exchange cuts them off. When power

switch S1 is turned on, the circuit gets 9V supply from the battery

and the reset indicator LED1 lights up. If LED1 does not glow initially,

press S2 until it glows. Now, in this standby mode, transistor T1 is in

nonconducting state. But transistor T2 is forward biased via resistor

R3. As a result pin 14 of IC1 is at a low potential. When the telephone

rings, ac voltage of about 75 volts appears across the input terminals

of the circuit. During positive half cycles of the ring signal, D1 starts

conducting and transistor T1 gets forward biased. When transistor T1

conducts, transistor T2 gets reverse biased. Consequently, pin 14 of

IC1 gets a short positive pulse through R4 and LED2 starts glowing,

indicating call number 1. Simultaneously IC2 (which is wired as a

monostable flip-flop) is triggered by the ring detector circuit and the

relay gets energised. As a result the circuit is disconnected from the

telephone lines for a predetermined period (decided by resistor R7

and capacitor C5 values). At the end of the mono time period, the

circuit automatically returns to monitor the telephone line for the next

call. This operation cycle is repeated with each succeeding call. This

circuit is capable of indicating up to 9 calls which are received in the

absence of subscriber. It can be extended for more calls by cascading

the required number of CD4017B Ic.

Circuit Diagram:

RESISTORS

The jobs done by resistors include directing and controlling current,

making changing current produce changing voltage (as in a voltage amplifier)

and obtaining variable voltages from fixed ones (as in a potential divider).

There are two main types of resistor-those with fixed values and those that

are variable.

When choosing a resistor there are three factors which have to be

considered, apart from the stated value.

(i) THE TOLERANCE. Exact values cannot be guaranteed by mass-

production methods but this is not a great disadvantage because in most

electronic circuits the values of resistors are not critical. The tolerance tells us

the minimum and maximum values a resistor might have, e.g. one with a

stated (called nominal) value of 100� and a tolerance of +10% could have

any value between 90� and 110�

(ii) THE POWER RATING. If the rate which a resistor changes

electrical energy into heat exceeds its power rating, it will overheat and be

damaged or destroyed. For most electronic circuit 0.25 Watt or 0.5 Watt

power ratings are adequate. The greater the physical size of a resistor the

greater is its rating.

(iii) THE STABILITY. This is the ability of a component to keep the

same value as it ‘ages’ despite changes of temperature and other physical

conditions. In some circuits this is an important factor.

RESISTOR MARKINGS

The value and tolerance of a fixed resistor is marked on it using codes.

The resistor has four colored bands painted on it towards one end. The first

three from the end give the value and the fourth the tolerance. Sometimes it

is not clear which is the first band but deciding where to start should not be

difficult if you remember that the fourth band (which is not always present) will

be either gold or silver, these being colours not used for the first band.

The first band gives the first number, the second band gives the second

number and the third band tells how many naught (0) come after the first two

numbers.

VALUE CODE NUMBER COLOUR

0 Black

1 Brown

2 Red

3 Orange

4 Yellow

5 Green

6 Blue

7 Violet

8 Gray

9 White

TOLERANCE CODE

PERCENTAGE COLOUR

+-5% Gold

+-10% Silver

+-20% no colour in 4th band

VARIABLE RESISTORS

Description. Variable resistors used as volume and other controls in

radio and TV set are usually called ‘pots’ (short for potential divider- see

below). They consist of an incomplete circular track of either a fixed carbon

resistor for high values and low power (up to 2W) or of a fixed wire-wound

resistor for high powers. Connections to each end of the track are bought out

to two terminal tags. A wiper makes contact with the track and is connected to

a third terminal tag, between the other two. Rotation of the spindle moves the

wiper over the track and changes the resistance between the center tag and

the ones. ‘Slide’ type variable resistors have a straight track.

In a linear track equal changes of resistance occur when the spindle is

rotated through equal angles. In a log track, the change of resistance at one

end of the track is less than at the other for equal angular rotations.

Maximum values range from a few ohms to several mega ohms,

common values are 10k Ohm, 50k Ohm., 100k Ohm., 500k ohm. and 1M

Ohm.

Some circuits use small preset types, the symbol and form of which are

shown in figs. These are adjusted with a screwdriver when necessary and

have tracks of carbon or ceramic (ceramic and metal oxide).

555 TIMER IC

(ASTABLE OPERATION)

The block diagram and pin connections are shown in figure; R1, R2 C1

and C2 are external components. (Note that the circle is omitted from the

transistor symbol in an IC). Threshold (pin 6) is joined to trigger (pin 2).

Initially C1 charges up through R1 and R2 and, when the voltage across it

just exceeds 2\3Vcc, the output from the threshold comparator (with a

reference voltage 2\3 Vcc on its other input form the voltage divider chain

formed by the three equal resistor R in series across Vcc) goes ‘high’ and

resets the flip-flop, i.e. Q goes ‘high’. This has two results. First, the output

from the IC (pin 3) goes ‘low’ (due to the inverting buffer output stage) and

second, Tr1 switches it and R2.

555 TIMER AS AN ASTABLE

When the voltage across C1 has fallen to just below 1\3 Vcc, the output

from the trigger comparator (with a reference voltage of 1\3 Vcc at its other input

form the three-resistor chain) goes ‘high’ and sets the flip-flop. Q therefore goes

‘low’ with two results. First, the output from the IC goes ‘high’ and second, Tr1

turns off (since its base is no longer positive) so letting C1 charge up to 2\3 Vcc

again through R1 and R2, as it did at the start. This cycle is repeated

continuously giving an oscillatory output with a rectangular waveform which is

‘high’ while C1 is charging and ‘low’ while it discharges.

Op-Amp.

Definition of 741-pin functions: (Refer to the internal 741 schematic

of Fig. 3)

Pin 1 (Offset Null): Offset nulling, see Fig. 11. Since the op-amp is the

differential type, input offset voltage must be controlled so as to minimize offset.

Offset voltage is nulled by application of a voltage of opposite polarity to the

offset. An offset null-adjustment potentiometer may be used to compensate for

offset voltage. The null-offset potentiometer also compensates for irregularities

in the operational amplifier manufacturing process, which may cause an offset.

Consequently, the null potentiometer is recommended for critical applications.

See ‘Offset Null Adjustment’ for method.

Pin 2 (Inverted Input): All input signals at this pin will be inverted at output pin

6. Pins 2 and 3 are very important (obviously) to get the correct input signals or

the op amp cannot do its work.

Pin 3 (Non-Inverted Input): All input signals at this pin will be processed

normally without inversion. The rest is the same as pin 2.

Pin 4 (-V): The V- pin (also referred to as Vss) is the negative

supply voltage terminal. Supply-voltage operating range for the 741 is -4.5

volts (minimum) to -18 volts (max), and it is specified for operation between -5

and -15 Vdc. The device will operate essentially the same over this range of

voltages without change in timing period. Sensitivity of time interval to supply

voltage change is low, typically 0.1% per volt. (Note: Do not confuse the -V with

ground).

Pin 5 (Offset Null): See pin 1, and Fig. 11.

Pin 6 (Output): Output signal’s polarity will be the opposite of the inputs when

this signal is applied to the op-amp’s inverting input. For example, a sine-wave

at the inverting input will output a square-wave in the case of an inverting

comparator circuit.

Pin 7 (posV): The V+ pin (also referred to as Vcc) is the positive supply voltage

terminal of the 741 Op-Amp IC. Supply-voltage operating range for the 741 is

+4.5 volts (minimum) to +18 volts

(maximum), and it is specified for operation between +5 and +15 Vdc. The

device will operate essentially the same over this range of voltages without

change in timing period. Actually, the most significant operational difference is

the output drive capability, which increases for both current and voltage range

as the supply voltage is increased. Sensitivity of time interval to supply voltage

change is low, typically 0.1% per volt.

Pin 8 (N/C): The ‘N/C’ stands for ‘Not Connected’. There is no other

explanation. There is nothing connected to this pin, it is just there to make it a

standard 8-pin package.

Output Parameters: 1. Output Resistance (Zoi) The resistance seen ‘looking into’ the op-amp’s output.

2. Output Short-Circuit Current (Iosc) This is the maximum output current that the op-amp can deliver to a load.

3. Output Voltage Swing (Vo max) Depending on what the load resistance is, this is the maximum ‘peak’ output

voltage that the op-amp can supply without saturation or clipping.

Dynamic Parameters: 1. Open-Loop Voltage Gain (Aol) The output to input voltage ratio of the op-amp without external feedback.

2. Large-Signal Voltage Gain This is the ratio of the maximum voltage swing to the charge in the input voltage

required to drive the output from zero to a specified voltage (e.g. 10 volts).

3. Slew Rate (SR) The time rate of change of the output voltage with the op-amp circuit having a voltage gain of unity (1.0). Other Parameters: 1. Supply Current This is the current that the op-amp will draw from the power supply.

2. Common-Mode Rejection Ratio (CMRR) A measure of the ability of the op-amp’ to reject signals that are simultaneously

present at both inputs. It is the ratio of the common-mode input voltage to the

generated output voltage, usually

expressed in decibels (dB).

TRANSFORMER

A transformer changes (transforms) an alternating voltage from one value

to another. It consists of two coils, called the primary and secondary windings,

which are not connected electrically. The windings are either one on top of the

other or are side by side on an iron, iron-dust or air core.

A transformer works by electromagnetic induction: AC. is supplied to the

primary and produces a changing magnetic field, which passes through the

secondary, thereby inducing a changing (alternating) voltage in the secondary.

It is important that as much as possible of the magnetic field produced by the

primary passes through the secondary. A practical arrangement designed to

achieve this in an iron-cored transformer in which the secondary is wound on

top of the primary. We should also notice that the induced voltage in the

secondary is always of opposite polarity to the primary voltage.

SYMBOLS

TYPES OF TRANSFORMER (i) MAINS. Mains transformers are used at AC. mains frequency (50

Hz in Britain), their primary coil being connected to the 240V a.c. supply.

Their secondary windings may be step-up or step-down or they may have

one or more of each. They have laminated iron cores and are used in power

supply units. Sometimes the secondary has a center-tap.

Step-down toroidal types are becoming popular. They have virtually no

external magnetic field and a screen between primary and secondary

windings gives safety and electrostatic screening. Their pin connections are

brought out to a 0.1 inch grid, which makes them ideal for printed circuit

board (p.c.b.) mounting.

Isolating transformers have a one-t-one turns ratio (i.e. ns/np = 1/1) and

are safety devices for separating a piece of equipment from the mains supply.

They do not change the voltage.

(ii) AUDIO FREQUENCY. Audio frequency transformer also has

laminated iron cores and are used as output matching transformers to ensure

the maximum transfer of power from the a.f. output stage to the loudspeaker

in , for example, a radio set or amplifier.

(iii) RADIO FREQUENCY. Radio frequency transformers usually have

adjustable iron-dust cores and form part of the tuning circuits in a radio. They

are enclosed in a small aluminum ‘screening’ can to stop them radiating

energy to other parts of the circuit.

CAPACITOR

A capacitor stores electric charge. It does not allow direct current to

flow through it and it behaves as if alternating current does flow through. In its

simplest form it consists of two parallel metal plates separated by an insulator

called the dielectric. The symbols for fixed and variable capacitors are given

in fig. Polarized types must be connected so that conventional current enters

their positive terminal. Non-polarized types can be connected either way

round.

The capacitance (C) of a capacitor measures its ability to store charge

and is stated in farads (F). The farad is sub-divided into smaller, more

convenient units.

1 microfarad (1uf) = 1 millionth of a farad = 10-6 f

1 nanofarad (1 nf) = 1 thousand- millionth of a farad = 10-9 f

1 Pico farad ( 1pf ) = 1 million-millionth of a farad = 10-12 f

In practice, capacitances range from 1 pf to about 150 000 uf: they

depend on the area A of the plates (large A gives large C), the separation d of

the plates (small d gives large C) and the material of the dielectric (e.g.

certain plastics give large C).

When selecting a particular job, the factors to be considered are the

value (again this is not critical in many electronic circuits), the tolerance and

the stability. There are two additional factors.

(i) The working voltage. It is the largest voltage (d.c.or peak a.c.),

which can be applied across the capacitor and is often marked on it, e.g. 30V

wkg. It is exceeded, the dielectric breaks down and permanent damage may

result.

(ii) The leakage current. No dielectric is a perfect insulator but the

loss of charge through it as ‘leakage current’ should be small.

FIXED CAPACITORS

Fixed capacitors can be classified according to the dielectric used; their

properties depend on this. The types described below in (i), (ii) and (iii) are

non-polarized; those in (iv) are polarized.

(i) Polyester. Two strips of polyester film (the plastic dielectric) are

wound between two strips of aluminum foil (the plates). Two connections, one

to each strip of foil, form the capacitor leads. In the metallized version, films of

metal are deposited on the plastic and act as the plates. Their good all-round

properties and small size make them suitable for many applications in

electronics. Values range from 0.01uf to 10mfd. or so and are usually marked

(in pf) using the resistor colour code. Polycarbonate capacitors are similar to

the polyester type; they have smaller leakage currents and better stability but

cost more.

(ii) Mica. Mica is naturally occurring mineral, which splits into very thin

sheets of uniform thickness. Plates are formed by depositing a silver film on

the mica or by using interleaving sheets of aluminum foil. Their tolerance is

low ( +1% ), stability and working voltage is high, leakage current low but they

are used in radio frequency tuned circuits where low loss is important and are

pictured in figs. Polystyrene capacitors have similar though not quite so good

properties as mica types but are cheaper.

(iii) Ceramic. There are several types depending on the ceramic used.

One type has similar properties to mica and is used in radio frequency

circuits. In another type, high capacitance values are obtained with small size,

but stability and tolerance are poor; they are useful where exact values are

not too important. They may be disc, rod- or plate-shaped. A disc-shaped

capacitor is shown in fig. Values range form 10pf to 1uf.

(iv) Electrolytic: In the aluminum type the dielectric is an extremely

thin layer of aluminum oxide, which is formed electrolytically. Their

advantages are high values (up to 150 000uF) in a small volume and

cheapness. Their disadvantages are wide tolerance (-20 to +100% of the

value printed on them), high leakage current and poor stability but they are

used where these factors do not matter and high values are required, e.g. in

power supplies.

Electrolytic are polarized. Usually their positive terminal is

marked with a + or by a groove; often the aluminum can is the negative

terminal. The d.c. Leakage current maintains the oxide layer; otherwise

reversed polarity (or disuse) will cause the layer to deteriorate.

Tantalum electrolytic capacitors can be used instead of aluminum in low

voltage circuits where values do not exceed about 100 uf. They have lower

leakage currents.

TRANSISTORS

Transistors are the most important devices in electronics today. Not

only are they made as discrete (separate) components but also integrated

circuits (IC) may contain several thousands on a tiny slice of silicon. They are

three-terminal devices, used as amplifiers and as switches. Non-amplifying

components such as resistors, capacitors, inductors and diodes are said to

be ‘passive’; transistors are ‘active’ components.

The two basic types of transistor are:

(a) The bipolar or junction transistor (usually called the transistor); its

operation depends on the flow of both majority and minority carriers;

(b) The unipolar or field effect transistor (called the FET) in which the

current is due to majority carriers only (either electrons or holes).

JUNCTION TRANSISTOR (i) CONSTRUCTION: The bipolar or junction transistor consists of two

p-n junctions in the same crystal. A very thin slice of lightly doped p-or n-type

semiconductor (the base B) is sand witched between two thicker, heavily

doped materials of the opposite type (the collector C and emitter E).

The two possible arrangements are shown diagrammatically in fig with

their symbols. The arrow gives the direction in which conventional (positive)

current flows; in the n-p-n type it points from B to E and in the p-n-p type it

points from E to B.

As with diodes, silicon transistors are in general preferred to germanium

ones because they withstand with higher temperatures ( up to about 175 0C

compared with 75 0C) and higher voltages, have lower leakage currents and

are better suited to high frequency circuits. Silicon n-p-n types, are more

easily mass-produced than p-n-p type, the opposite is true of germanium.

A simplified section of an n-p-n silicon transistor made by the planar

process in which the transistor is in effect created on one face (plane) of a

piece of semi conducting material; fig. Shows a transistor complete with case

(called the ‘encapsulation’) and three wire leads.

(ii) ACTION. An n-p-n silicon transistor is represented and is connected in a

common emitter circuit; the emitter is joined (via batteries B1 and B2) to both

the base and the collector. For transistor action to occur the base emitter

junction must be forward biased, i.e. positive terminal of B1 to p- type base, and

the collector base junction reverse biased, i.e. positive terminal of B2 to n- type

collector.

When the base emitter bias is about +0.6 V, electrons (the majority

carriers in the heavily doped n type emitter) cross the junction (as they would

in any junction diode) into the base. Their loss is made good by electrons

entering the emitter from the external circuit to form the emitter current. At the

same time holes from the base to the emitter, since the p- type base is lightly

doped, this is small compared with the electron flow in the opposite direction,

i.e. electrons are the majority carriers in an n-p-n transistor.

In the base, only a small proportion (about 1%) of the electrons from

the emitter combine with the holes in the base because the base is very thin

(less than millionth of a meter) and is lightly doped. Most of the electrons are

swept through the base, because they are attracted by the positive voltage on

the collector, and the cross base – collector junction to become the collector

current in the circuit.

The small amount of electron – hole recombination, which occurs in

the base, gives it a momentary negative charge, which is immediately

compensated by battery B1 supplying it with (positive) holes. The flow of

holes to the base from the external circuit creates a small base current. This

keeps the base emitter junction forward biased and so maintains the larger

collector current.

Transistor action is turning on (and controlling) of a large current through the

high resistance (reverse biased) collector – base junction by a small current

through the low – resistance (forward biased) base – emitter junction. The term

transistor refers to this effect and comes from the two words ‘ transfer resistor’.

Physically the collector is larger than the emitter and if one is used in place of

the other the action is inefficient.

The behavior of a p-n-p transistor is similar to that of the n-p-n type

but it is holes that are the majority carriers, which flow from the emitter to the

collector and electrons, are injected into the base to compensate for

recombination. To obtain correct biasing the polarities of both batteries must

be reversed.

RELAY

Relay is a common, application of application of electromagnetism. It

uses an electromagnet made from an iron rod wound with hundreds of fine

copper wire. When electricity is applied to the wire, the rod become magnetic.

A movable contact arm above the rod is then pulled toward; a small spring

pulls the contract arm away from the rod until it closes, a second switch

contact. By means of relay, a current circuit can be broken or closed in one

circuit as a result of a current in another circuit. Relays can have several

poles and contacts. The types of contacts could be normally open and

normally closed. One closure of the relay can turn on the same normally open

contacts; can turn off the other normally closed contacts

A relay is a switch worked by an electromagnet. It is useful if we

want a small current in one circuit to control another circuit containing a

device such as a lamp or electric motor which requires a large current, or if

we wish several different switch contacts to be operated simultaneously.

The structure of relay and its symbol are shown in figure. When the

controlling current flows through the coil, the soft iron core is magnetized and

attracts the L-shaped soft iron armature. This rocks on its pivot and opens,

closes or changes over, the electrical contacts in the circuit being controlled.

DIODE

The simplest semiconductor device is made up of a sandwich of P- and

N type semi conducting material, with contacts provided to connect the P-and

N-type layers to an external circuit, this is a junction Diode. If the positive

terminal of the battery is connected to the p-type material (cathode) and the

negative terminal to the N-type material (Anode), a large current will flow.

This is called forward current or forward biased.

If the connection is reversed, a very little current will flow. This is

because under this condition, the p-type material will accept the electrons

from the negative terminal of the battery and the N-type material will give up

its free electrons to the battery, resulting in the state of electrical equilibrium

since the N-type material has no more electrons. Thus there will be a small

current to flow and the diode is called Reverse biased.

Thus the Diode allows direct current to pass only in one direction while

blocking it in the other direction. Power diodes are used in concerting AC into

DC. In this, current will flow freely during the first half cycle (forward biased)

and practically not at all during the other half cycle (reverse biased). This

makes the diode an effective rectifier, which converts ac into pulsating dc.

Signal diodes are used in radio circuits fro detection, Zener diodes are used

in the circuit to control the voltage.

A diode allows current to flow easily in one direction but not in the other,

i.e. its resistance is low in the conducting or ‘forward’ direction but very high in

the opposing or ‘reverse’ direction. Most semiconductor diodes are made

from silicon or germanium.

A diode has two leads, the anode and the cathode: its symbol is given

in fig (a). The cathode is often marked by a band at one end fig.(b); it is the

lead by which conventional current leaves the diode when forward biased –

as the arrow on the symbol shown. In some cases the arrow is marked on the

diode fig.(c) or the shape is different (d), (e)

There are several kinds of diode, each with features that suit it for a particular

job. Three of the main types are:

(a) The junction diode,

(b) The point-contact diode and

(c) The zener diode

Two identification codes are used for diodes. In the American system

the code always starts with 1N and is followed by a serial number, e.g. IN

4001. in the continental system the first letter gives the semiconductor

material (A=germanium, B= silicon) and the second letter gives the use.

(A=signal diode, Y=rectifier diode, Z=Zener diode.). for example, AA119 is a

germanium signal diode,. To complicate the situation some manufacturers

have their own codes.

ZENER DIODE Zener diodes are very important because they are the key to voltage

regulation. The chapter also includes opt electronic diodes, Scotty diodes,

aviators, and other diodes.

A Zener diode is specially designed junction diode, which can operate

continuously without being damaged in the region of reverse breakdown

voltage. One of the most important applications of zener diode is the design

of constant voltage power supply. The zener diode is joined in reverse bias to

D.C. through a resistance of suitable value.

Small signal and rectifier diodes are never intentionally operated in the

breakdown region because this may damage them. A zener diode is different;

it is a silicon diode that the manufacturer has optimized for operation in the

breakdown region, zener diodes work best in the breakdown region.

Sometimes called a breakdown diode, the zener diode is the backbone of

voltage regulators, circuits that hold the load voltage almost constant despite

large changes in line voltage and load resistance.

Figure shows the schematic symbol of a zener diode; another figure is

an alternate symbol. In the either symbol, the lines resemble a “z”, which

stands for zener. By varying the doping level of silicon diodes, a manufacturer

can produce zener diodes with breakdown voltage from about 2 to 200V.

These diodes can operate in any of three regions: forward, leakage, or

breakdown.

Figure shows the V-I graph of a zener diode. In the forward region, it

starts conduction around 0.7V, just like a ordinary silicon diode, In the

leakage region (between zero and breakdown), it has only a small leakage or

reverse current. In a zener diode, the breakdown has a very sharp knee,

followed by an almost vertical Vz over most of breakdown region. Data sheets

usually specify the value of Vz at a particular test current IzT.

L.E.D. (LIGHT EMITTING DIODE)

Light emitting diode (LED) is basically a P-N junction semiconductor

diode particularly designed to emit visible light. There are infrared emitting

LEDs which emit invisible light. The LEDs are now available in many colour

red, green and yellow,. A normal LED at 2.4V and consumes ma of current.

The LEDs are made in the form of flat tiny P-N junction enclosed in a semi-

spherical dome made up of clear coloured epoxy resin. The dome of a LED

acts as a lens and diffuser of light. The diameter of the base is less than a

quarter of an inch. The actual diameter varies somewhat with different makes.

The common circuit symbols for the LED are shown in fig. 1. It is similar to the

conventional rectifier diode symbol with two arrows pointing out. There are

two leads- one for anode and the other for cathode.

LEDs often have leads of dissimilar length and the shorter one is the

cathode. This is not strictly adhered to by all manufacturers. Sometimes the

cathode side has a flat base. If there is doubt, the polarity of the diode should

be identified. A simple bench method is to use the ohmmeter incorporating 3-

volt cells for ohmmeter function. When connected with the ohmmeter: one

way there will be no deflection and when connected the other way round

there will be a large deflection of a pointer. When this occurs the anode lead

is connected to the negative of test lead and cathode to the positive test lead

of the ohmmeter.

(i) Action. An LED consists of a junction diode made from the semi

conducting compound gallium arsenate phosphate. It emits light when

forward biased, the colour depending on the composition and impurity content

of the compound. At present red, yellow and green LEDs are available. When

a p-n junction diode is forward biased, electrons move across the junction

from the n-type side to the p-type side where they recombine with holes near

the junction. The same occurs with holes going across the junction from the

p-type side. Every recombination results in the release of a certain amount of

energy, causing, in most semiconductors, a temperature rise. In gallium

arsenate phosphate some of the energy is emitted as light, which gets out of

the LED because the junction is formed very close to the surface of the

material. An LED does not light when reverse biased and if the bias is 5 V or

more it may be damaged.

(ii) External resistor. Unless an LED is of the ‘constant-current type’

(incorporating an integrated circuit regulator for use on a 2 to 18 V d.c. or a. c.

supply), it must have an external resistor R connected in series to limit the

forward current, which typically, may be 10 mA (0.01 A). Taking the voltage

drop (Vf) across a conducting LED to be about 1.7 V, R can be calculated

approximately from:

(supply voltage – 1.7) V

R = ——————————————————

0.01A

For example, on a 5 V supply, R = 3.3/0.01 = 330 Ohm.

(iii) Decimal display. Many electronic calculators, clocks, cash

registers and measuring instruments have seven-segment red or green LED

displays as numerical indicators (Fig.). Each segment is an LED and

depending on which segments are energized, the display lights up the

numbers 0 to 9 as in Fig.. Such displays are usually designed to work on a 5

V supply. Each segment needs a separate current-limiting resistor and all the

cathodes (or anodes) are joined together to form a common connection.

The advantages of LEDs are small size, reliability, longer life, small

current requirement and high operating speed.

SEVEN SEGMENT DISPLAY

SOLDERING TECHNIQUES

Bad solder joints are often the cause of annoying intermittent faults. They

can often be hard to find an cause circuit failure at the most inappropriate time.

It’s much better to learn to make a good solder joints from day one.

Preparing the soldering iron:

� Wipe the tip clean on the wetted sponge provided.

� Bring the resin cored solder to the iron and ‘tin’ the tip of the iron.

� Wipe the excess solder of the tip using the wet sponge.

� Repeat until the tip is properly ‘tinned’.

SOLDERING COMPONENTS INTO THE PCB

� Bend the component leads at right angles with both bends at the same

distance apart as the PCB pad holes.

� Ensure that both component leads and the copper PCB pads are clean

and free of oxidization.

� Insert component leads into holes and bend leads at about 30 degrees

from vertical.

� Using small angle cutters, cut the leads at about 0.1 - 0.2 of an inch

(about 2 - 4 mm) above copper pad.

� Bring tinned soldering iron tip into contact with both the component lead

and the PCB pad. This ensures that both surfaces undergo the same

temperature rise.

� Bring resin cored solder in contact with the lead and the copper pad.

Feed just enough solder to flow freely over the pad and the lead without a

‘blobbing’ effect. The final solder joint should be shiny and concave

indicating good ‘wetting’ of both the copper pad and the component lead.

If a crack appears at the solder to metal interface then the potential for

forming a dry joint exists. If an unsatisfactory joint is formed, suck all the

solder off the joint using a solder sucker or solder wick (braid) and start

again.

PRECAUTIONS

1. Mount the components at the apron places before soldering. Follow the

circuit description and components details, leads identification etc. Do

not start soldering before making it confirm that all the components are

mounted at the right place.

2. Do not use a spread solder on the board, it may cause short circuit.

3. Do not sit under the fan while soldering.

4. Position the board so that gravity tends to keep the solder where you

want it.

5. Do not over heat the components at the board. Excess heat may

damage the components or board.

6. The board should not vibrate while soldering otherwise you have a dry

or a cold joint.

7. Do not put the kit under or over voltage source. Be sire about the

voltage either is d.c. or a.c. while operating the gadget.

8. Do spare the bare ends of the components leads otherwise it may short

circuit with the other components. To prevent this use sleeves at the

component leads or use sleeved wire for connections.

9. Do not use old dark colour solder. It may give dry joint. Be sure that all

the joints are clean and well shiny.

10. Do make loose wire connections specially with cell holder, speaker,

probes etc. Put knots while connections to the circuit board, otherwise

it may get loose.

REFERENCES

1. BASIC ELECTRONIC CIRCUITS: MALVINO & ZBAR.

2. EFY NOV.- 2004

3. WWW.ELECTROGUGS.COM

4. WWW.GOOGLE.COM