telephone call counter
TRANSCRIPT
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