INTRODUCTION

Intercom is the abbreviation of “Intercommunication” and refers to the

communication by means of wires/cables between various individuals situated at

comparatively distant location within a certain premises. The various locations are

known as stations. The difference between an intercom and public telephone

system lies in the fact that wires connecting the various stations do not cross any

public place.

The intercom circuit will let us talk with another person in a distant room. The kit

will be in one room and speaker will be in other room. When a person speaks, his

voice signals will be changed into electrical signal by a condenser mic and then

the electrical signals generated will be transmitted through the circuit. The

electrical signals at the other end will be converted into analog or voice signals by

the speaker used and hence the person at the other end can hear the message. The

unit having all components is called master unit and the unit having only the

speaker is called slave unit.

WORKING

The circuit described throughout this report, uses three transistors. The circuit

comprises of a 3 stage RC coupled amplifier. The circuit comprises of a condenser

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mic through which audio or voice signal is produced. Condenser mic works on the

principle of piezoelectric effect. This phenomena state that under the influence of

mechanical pressure, the voltage gets generated across the opposite faces of

piezoelectric crystal and vice versa. Thus the analog or voice signal is converted

into electrical signal. The mic is connected to the 3 stage RC coupled

configuration. The two transistors, T1 and T2(BC 548)are used for amplification

purposes. The third transistor T3(BEL 187) is a power amplifier which is used to

drive the speaker. The circuit works on 9 volt dc supply and the current

consumption of the circuit is 10µA to 15 µA only.

When the switch S2 is pressed the two transistors T1 and T2 get converted into an

astable multivibrator, thus generating ring signals which alerts the receiver on the

other side. The ring signals are amplified by transistor T3. The absence of switch

S2 will not affect the functionality of circuit. As soon as the switch S2 is released,

T1 and T2 function as normal amplifiers, amplifying the electrical signals. The

amplified electrical signals are then transmitted to the speaker via the power

amplifier T3. Two circuits like this can be used, one of which will be the

transmitter and the other will be receiver.

COMPONENTS USED

1. T1, T2 and T3 NPN Transistors

(T1 & T2= BC548 & T3= BEL187)

2. C1, C2, C3 0.1µF Ceramic

Capacitors

3. C4 and C5 10µF 16 V Capacitor

4. C6 100µF 16 V capacitor

5. R1 and R3 10kΩ Resistance

6. R2 1MΩ Resistance

7. R4 220kΩ Resistance

8. R5 2.2kΩ Resistance

9. R6 100kΩ Resistance

10. R7 150 Resistance

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11. Speakers 8Ω

12. Battery 9V

13. Mic Condenser Microphone

14. S2 PUSH to ON Button

15. S1 Slide Switch

DESCRIPTION OF COMPONENTS USED

1. TRANSISTOR(BC548)

The BC548 is a general purpose silicon NPN bipolar junction transistors found

commonly in European electronic equipment.

If the TO-92 package is held in front of one's face with the flat side facing toward

us and the leads downward the order of the leads, from left to right is collector,

base and emitter. In the intercom circuit two BC548 transistors, T1 and T2 have

been used.

SPECIFICATIONS

The exact specification of a given device depends on the manufacturer. The data

sheets can be used to gain this information. Philips and Telefunken are two

manufacturers of the BC548. Below are some basic specifications of BC548.

Vcbo = 30 V

Ic = 100 mA

Ptotal = 50 mW

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ft = 300 MHz

2. TRANSISTOR(BEL187)

Now a days the transistors of BEL company are usually used. Transistors

which were available of Ge material, now also available of Si material, for

example AC 187 and AC 1800 are Ge transistor, now equivalent Si transistors

of BEL are available in market whose numbers are BEL 187 and BEL 188.

The identification of terminals of that type and other Si transistors, we do

according to following figures : these transistor are - BEL 188, BEL187,

BEL147, BEL148, BEL158, BEL157etc,the shapes of all that transistor are

semicircular and terminal.'; are in straight line. To identify the terminals, we

take transistors in hand in a way that the portion of transistor on which

numbers written, remain towards us and terminals remain lower side. Then the

left-most terminal is collector and right most is emitter and middle one is base.

These transistors are called Si planer transistors.

In the intercom circuit, BEL187 works as a class A power amplifier and is

used to drive the speakers at the receiving end.

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CERAMIC CAPACITOR

A ceramic capacitor is a two-terminal, non-polar device. The classical ceramic

capacitor is the "disc capacitor". This device pre-dates the transistor and was

used extensively in vacuum-tube equipment (e.g., radio receivers) from about

1930 through the 1950s, and in discrete transistor equipment from the 1950s

through the 1980s. As of 2007, ceramic disc capacitors are in widespread use

in electronic equipment, providing high capacity and small size at low price

compared to other low value capacitor types.

Ceramic capacitors come in various shapes and styles, including:

disc, resin coated, with through-holes leads

multilayer rectangular block, surface mount

bare leadless disc, sits in a slot in the PCB and is soldered in place, used

for UHF applications

tube shape, not popular now

Ceramic capacitors of different shape and sizes.

CLASSES OF CERAMIC CAPACITORS

Three classes of ceramic capacitors are commonly available:

CLASS I CAPACITORS

Accurate, temperature-compensating capacitors. They are the most stable over

voltage, temperature, and to some extent, frequency. They also have the lowest

losses. On the other hand, they have the lowest volumetric efficiency. A typical

class I capacitor will have a temperature coefficient of 30 ppm/°C. This will

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typically be fairly linear with temperature. These are used in filters. A typical

class I capacitor will have a dissipation factor of 0.15%. Very high accuracy class

I capacitors are available.

CLASS II CAPACITORS

Better volumetric efficiency, but lower accuracy and stability. A typical class II

capacitor may change capacitance by 15% over a −55 °C to 85 °C temperature

range. A typical class II capacitor will have a dissipation factor of 2.5%. It will

have average to poor accuracy.

CLASS III CAPACITORS

High volumetric efficiency, but poor accuracy and stability. A typical class III

capacitor will change capacitance by -22% to +56% over a temperature range of

10 °C to 55 °C. It will have a dissipation factor of 4%. It will have fairly poor

accuracy. These are typically used for power supply applications.

At one point, Class IV capacitors were also available, with worse electrical

characteristics than Class III, but even better volumetric efficiency. They are now

rather rare and considered obsolete, as modern multilayer ceramics can offer

better performance in a compact package.

None of the classes are better than any others—the relative performance depends

on application. Class I capacitors are physically larger than class III capacitors,

and for bypassing and other non-filtering applications, the accuracy, stability, and

loss factor may be unimportant, while cost and volumetric efficiency may be. As

such, Class I capacitors are primarily used in filtering applications, where the

main competition is from film capacitors in low frequency applications, and more

esoteric capacitors in RF applications. Class III capacitors are typically used in

power supply applications. Traditionally, they had no competition in this niche, as

they were limited to small sizes. As ceramic technology has improved, ceramic

capacitors are now commonly available in values of up to 100 µF, and they are

increasingly starting to compete with electrolytic capacitors, where ceramics offer

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much better electrical performance at prices that, while still much higher than

electrolytic, are becoming increasingly reasonable as the technology improves.

CODING

There is a three digit code printed on a ceramic capacitor specifying its value. The

first two digits are the two significant figures and the third digit is a base 10

multiplier. The value is given in picofarads(pF). A letter suffix indicates the

tolerance.

C ± 0.25 pF M ±20%

D ± 0.5 pF P +100 −0%

J ± 5% Y −20 +50%

K ±10% Z −20 + 80%

A label of "104K" indicates 10×104 pF = 100,000 pF = 100 nF = 0.1 µF ±10%

There is also an EIA three character code that indicates temperature coefficient.

For non-temperature-compensating capacitor, the code consists of three letters.

The first character is a letter that gives the low-end operating temprature. The

second is a digit gives the high-end operating temperature. The final letter gives

capacitance change over that temperature range:

LETTER (LOW TEMP) DIGIT (HIGH TEMP) LETTER (CHANGE)

X= −55 °C (−67 °F) 2= +45 °C (+113 °F) D= ±3.3%

Y= −30 °C (−22 °F) 4= +65 °C (+149 °F) E= ±4.7%

Z= +10 °C (+50 °F) 5= +85 °C (+185 °F) F= ±7.5%

6=+105 °C (+221 °F) P= ±10%

7=+125 °C (+257 °F) R= ±15%

8=+150 °C (+302 °F) S= ±22%

T= +22 to −33%

U= +22 to −56%

V= +22 to −82%

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For instance, a Z5U capacitor will operate from +10 °C to +85 °C with a

capacitance change of at most +22% to −56%. An X7R capacitor will operate

from −55 °C to +125 °C with a capacitance change of at most ±15%.

Temperature-compensated capacitors use a different EIA code. Here, the first

letter gives the significant figure of the change in capacitance over temperature in

ppm/°C. The second character gives the multiplier. The third character gives the

maximum error from that in ppm/°C. All ratings are from 25 to 85 °C:

SIGNIFICANT FIGURE MULTIPLIER TOLERANCE

C: 0.0 0: -1 G: ±30

B: 0.3 1: -10 H: ±60

L: 0.8 2: -100 J: ±120

A: 0.9 3: -1000 K: ±250

M: 1.0 4: +1 L: ±500

P: 1.5 6: +10 M: ±1000

R: 2.2 7: +100 N: ±2500

S: 3.3 8: +1000

T: 4.7

V: 5.6

U: 7.5

For instance, a C0G will have 0 drift, with an error of ±30 ppm/°C, while a P3K

will have −1500 ppm/°C drift, with a maximum error of ±250 ppm/°.

ELECTROLYTIC CAPACITOR

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Axial lead (top) and radial lead (bottom) electrolytic capacitors

An electrolytic capacitor is a type of capacitor that uses an electrolyte, an ionic

conducting liquid, as one of its plates, to achieve a larger capacitance per unit

volume than other types. They are often referred to in electronics usage simply as

electrolytics. They are used in relatively high-current and low-frequency electrical

circuits, particularly in power supply filters, where they store charge needed to

moderate output voltage and current fluctuations in rectifier output. They are also

widely used as coupling capacitors in circuits where ac should be conducted but

dc should not. There are two types of electrolytics; aluminum and tantalum.

Electrolytic capacitors are capable of providing the highest capacitance values of

any type of capacitor but they have drawbacks which limit their use. The standard

design requires that the applied voltage must be polarized; one specified terminal

must always have positive potential with respect to the other. Therefore they

cannot be used with ac signals without a dc polarizing bias. However there are

special non-polarized electrolytic capacitors for ac use which do not require a dc

bias. Electrolytic capacitors also have relatively, higher leakage current, poorer

tolerances and temperature range, and shorter lifetimes compared to other types of

capacitors.

CONSTRUCTION

Aluminum electrolytic capacitors are constructed from two conducting aluminum

foils, one of which is coated with an insulating oxide layer, and a paper spacer

soaked in electrolyte. The foil insulated by the oxide layer is the anode while the

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liquid electrolyte and the second foil acts as the cathode. This stack is then rolled

up, fitted with pin connectors and placed in a cylindrical aluminum casing.

Polarity

In aluminum electrolytic capacitors, the layer of insulating aluminum oxide on the

surface of the aluminum plate acts as the dielectric, and it is the thinness of this

layer that allows for a relatively high capacitance in a small volume. This oxide

has a dielectric constant of 10, which is several times higher than most common

polymer insulators. It can withstand an electrical field of strength of the order of

25 megavolts per meter which is an acceptable fraction of that of common

polymers. This combination of high capacitance and reasonably high voltage

result in high energy density.

Most electrolytic capacitors are polarized and require one of the electrodes to be

positive relative to the other; they may catastrophically fail if voltage is reversed.

This is because a reverse-bias voltage above 1 to 1.5V will destroy the center

layer of dielectric material via electrochemical reduction. Following the loss of the

dielectric material, the capacitor will short circuit, and with sufficient short circuit

current, the electrolyte will rapidly heat up and either leak or cause the capacitor

to burst, often in spectacularly dramatic fashion.

To minimize the likelihood of a polarized electrolytic being incorrectly inserted

into a circuit, polarity is very clearly indicated on the case. A bar across the side of

the capacitor is usually used to indicate the negative terminal. Also, the negative

terminal lead of a radial electrolytic is shorter than the positive lead and may be

otherwise distinguishable.

Capacitor Polarized Variable

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Capacitor Capacitor

The above are the most common schematic symbols for electrolytic capacitors.

Some schematic diagrams do not print the "+" adjacent to the symbol. Older

circuit diagrams show electrolytic capacitors as a small positive plate surrounded

below and on the sides by a larger dish-shaped negative electrode, usually without

"+" marking.

Electrolyte

The electrolyte is usually boric acid or sodium borate in aqueous solution,

together with various sugars or ethylene glycol which are added to retard

evaporation. Getting a suitable balance between chemical stability and low

internal electrical resistance is not a simple matter; in fact, the exact compositions

of high-performance electrolytes are closely guarded trade secrets. It took many

years of painstaking research before reliable devices were developed. The

electrolytic solvent has to have high dielectric constant, high dielectric strength,

and low resistivity.

Electrolytes may be toxic or corrosive. Working with the electrolyte requires safe

working practice and appropriate protective equipment such as gloves and safety

glasses. Some very old tantalum electrolytics, often called "Wet-slug", contain

corrosive sulphuric acid; however, most of these are no longer in service due to

corrosion.

Capacitance

The capacitance value of any capacitor is a measure of the amount of electric

charge stored per unit of potential difference between the plates. The basic unit of

capacitance is a farad; however, this unit is too large for general use, so

microfarad(µF), nanofarad (nF) and picofarad(pF) are more commonly used.

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Many conditions determine a capacitor's value, such as the thickness of the

dielectric and the plate area. In the manufacturing process, electrolytic capacitors

are made to conform to a set of preferred numbers. By multiplying these base

numbers by a power of ten, any practical capacitor value can be achieved, which

is suitable for most applications.

The capacitance of aluminum electrolytic capacitors tends to change over time,

and they usually have a tolerance range of 20 Tantalum electrolytics can be

produced to tighter tolerances and are more stable.

CONDENSER MIC

The condenser microphone, invented at Bell Labs in 1916 by E. C. Wente is also

called a capacitor microphone or electrostatic microphone. Here, the diaphragm

acts as one plate of a capacitor, and the vibrations produce changes in the distance

between the plates. There are two types, depending on the method of extracting

the audio signal from the transducer: DC-biased and radio frequency (RF) or high

frequency (HF) condenser microphones. With a DC-biased microphone, the plates

are biased with a fixed charge (Q). The voltage maintained across the capacitor

plates changes with the vibrations in the air, according to the capacitance equation

(C = Q / V), where Q = charge in coulombs, C = capacitance in farads and V =

potential difference in volts. The capacitance of the plates is inversely

proportional to the distance between them for a parallel-plate capacitor. The

assembly of fixed and movable plates is called an "element" or "capsule."

A nearly constant charge is maintained on the capacitor. As the capacitance

changes, the charge across the capacitor does change very slightly, but at audible

frequencies it is sensibly constant. The capacitance of the capsule (around 5 to

100pF) and the value of the bias resistor (100MΩ to 100GΩ) form a filter that is

high-pass for the audio signal, and low-pass for the bias voltage. Note that the

time constant of an RC circuit equals the product of the resistance and

capacitance.

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Within the time-frame of the capacitance change (as much as 50 ms at 20 Hz

audio signal), the charge is practically constant and the voltage across the

capacitor changes instantaneously to reflect the change in capacitance. The

voltage across the capacitor varies above and below the bias voltage. The voltage

difference between the bias and the capacitor is seen across the series resistor. The

voltage across the resistor is amplified for performance or recording. In most

cases, the electronics in the microphone itself contribute no voltage gain as the

voltage differential is quite significant, up to several volts for high sound levels.

Since this is a very high impedance circuit, current gain only is usually needed

with the voltage remaining constant. The circuit is therefore often called an

"impedance converter" or "follower" because no voltage gain is provided.

SPEAKERS

A speaker is an electro acoustic transducer that produces sound in response to an

electrical audio signal input.

The most common type of speaker uses a lightweight diaphragm, or cone,

connected to a rigid basket or frame, via a flexible suspension that constrains a

coil of fine wire to move axially through a cylindrical magnetic gap. When an

electrical signal is applied to it, a magnetic field is created by the electric current

in the voice coil, making it a variable electromagnet. The coil and the magnetic

system interact, generating a mechanical force that causes the coil (and thus, the

attached cone) to move back and forth, thereby reproducing sound under the

control of the applied electrical signal coming from the amplifier..

APPLICATIONS OF INTERCOM

The application of intercom is as follows-

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1. The intercom system are used in office as a second communication link in

order to keep the phone lines open for external calls. This kind of intercom

is also well suited for hospitals, prisons and police stations, banks and

financial institutions, defence and schools.

2. These are cheap and privacy can be maintained on the line, as only those

who need it can use it.

3. The Audio and video handset offers features such as color video monitors

and cameras, elevator control for restricted entry with intercom and code

access for tenants with use of digital keypads.

4. The Bi-Way intercom system has many applications starting from where

there is a danger of robbery or violence like banks, post offices, cashier

windows, ticket offices, gas stations or offices in terms of social services.

CONCLUSION

The intercom circuit is very useful circuit. Other then transistors, it can also be

made by op-amps. Wireless intercoms have also appeared in the market. One

reason to use a wireless intercom system is that the cost of retrofitting a building

for a wired intercom system is high. Another reason is the increased portability of

a wireless system. With battery-powered radio frequency wireless intercom units,

a person can carry a station as they walk around.

Besides wireless intercom system, there are various audio and video intercom

systems, which have revolutionized this field. With video intercom systems , the

system placed outside a building, person inside can talk to the person outside, thus

maintaining the safety and reducing the cases of theft, robbery etc.

Intercom have made our lives simple. Research is still going on in field of

wireless and video intercom systems to make them mch more better.

REFERENCES

http://en.wikipedia.org/wiki/Intercom

http://www.circuitstoday.com/transistor-intercom-circuit

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http://www.electronicsforu.com/efylinux/circuit/Oct-cir/cir2.htm

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