24-01-2011, 02:33 PM
INTRODUCTION
This circuit will let you talk with another person in a distant room.The kit will be in one room and one speaker will be in the other room.
When the switch is in first position speaker No.1 pick up sound and change into electronic signals. The signals passes through the circuit and it amplified and sent to the 2nd speaker where it is changed into a sound you can hear. When the position of switch is changed Electric connection to the speaker change places and there by the speaker through which we were speaking previously will be now used to hear and other from which we were hearing previously will be now used for speaking. The unit having all components PCB is called master unit and the unit consist just a speaker for other room is called slave unit. The two units are connected by means of a special wire called pick up a sleeve wire.
Intercom is the abbreviation of “Intercommunication” and refers to the communication by means of wires/ cables between various individuals situated at comparatively distant locations 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 the wires connecting the various stations do not cross any public place.
Thus, intercom sets are used in office workshops, so as to obviate the necessity of carrying messages through messengers from one office to another, thereby improving work efficiency and enabling quick decisions being taken by the supervisory staff.
TYPES OF INTERCOM
Intercom systems are mainly of two types:
(i) “Master- Slave” or Principal- Subsidiary” system and
(ii) “Independent station” system.
In Master – Slave system, the overall control of the intercom system is rested in one individual, the Chief Supervisor/ Manager/ Director, He can call up and speak to his subordinates denoted as “ substations” who can also call up and speak to their boss. But the subordinates (Substations) cannot call up or speak to one another. In the independent- stations- system, all the stations are able to call up and speak to any station in the intercom set. Many such intercom system use the auto- telephone principle with dialing facility. Such sets use the P & T type of hand set consisting of earphones and microphone. In other intercom sets, stations uses only one loudspeaker which is employed as microphones also.
We shall now consider the type of intercom sets which uses a loudspeaker as a microphone also.
GENERAL DISCRIPTION
The circuit of the intercom shown in the figure. This is a 6 volts D.C amplifier. When talk-listen-switch is at ‘talk’ position, input of the amplifier is connected to the speaker of the ‘slave unit’. When we change the switch to listen position, ‘slave unit speaker is at input of the amplifier and’ maser unit’ speaker is at output of the amplifier. Input speaker changes the voice into electrical signal. Each transistor amplifier this signal and finally passing through the transformer it reaches the speaker which again changes the electrical signal into a sound which we have at other speaker.
THE LOUDSPEAKER AS A MICROPHONE
The loudspeaker is capable of being used as a microphone as well. The sound waves are collected by the impinging on the diaphragm which set it into to-and-fro motion. The form of the voice coil which is attached to the diaphragm commences to move in the same direction i.e. to-and-fro. The magnitude and frequency of the mechanical motionof the voice coil are the same as those of the sound waves impinging on the diaphragm. As the voice moves in a radial magnetic field of the permanent magnet (or electromagnet, as the case may be) e.m.fs.are induced with frequency according to the mechanical movements of the voice coil, which, in turn, are caused by the diaphragm. Thus, sound waves are converted into electrical vibrations which are amplified and fed through wires/cables into a loudspeaker as a distant location.
A loudspeaker, therefore, serves the purpose of a microphone also. The circuit shows that a simple transistor stage where a loudspeaker is used as an effective microphone. The output of this stage is adequate to drive practically any transistor or vaccum tube audio frequency amplifier. Different parts are labeled in the circuit diagram. The circuit shown here is very useful and versatile one. While constructing an intercom set if there arises the problem of matching impedance, of the loudspeaker to that of input satate of the amplifier, make use of the circuit shown in figure here. The output of the amplifier whose impedence is to be matched with that of the loudspeaker when the loudspeaker is to be operated as a microphone.
BASIC INGREDIENTS OF AN INTERCOM SET
1. A loudspeaker
2. An amplifier
3.A switch to change over the loudspeaker from output of amplifier to its input and vice-versa, which is called the Listen-Talk Switch (DPDT)
4.A push switches to select a particular station by means of aural indication.The aural indication for ‘call’ is provided by means of a low frequency tone.
5.Matching device: to match the impedence of the loudspeaker to that of output and input of the amplifier and to the wires/cables connecting the intercom network. Impedence matching is achieved by means of resistance- capacitance network or transformer.
6.Conference facility : this facility is also a function of one of the switching system, as a result of which the substation and the master statin are brought in parallel with one another. It, therefore, involves a rearrangement of impedence matching device.
CIRCUIT DESCRIPTION
In the given circuit of intercom for two, the input connected speaker 1 (works as microphone here) convert the incoming sound signals, Transistor T-1 working under common emitter Configuration as pre amplifier, amplifies the law signal. The connected capacitor C-1 at T-1 base is a fitter capacitor that allows only sound signals to go in T-1 base. Capacitor T-2 discharges the excess signals at T-1 base, the operating voltage for T-1 Is coming from resistance R-1 and R-2. The amplified output from T-1 collector is again fed at T-2 base through C-3 capacitor for second stage amplifier.
Again C-3 and C-4 is doing the same funcyion as in stage 1. The voltage provide to this stay is through R-3 and R-4. The output (amplified) is collected from collector of T-2 (also working as common emitter amplifier). Transistor T-3 ( working as power amplifier under common collector mode) get the input through C-5 capacitor and VC provided through R-5. The output from its emitter is given directly at speaker (through two every switch) to convert electrical signal into sound signal that one can hear at a distance of wire. DPDT switch charge the position of two connected speaker every time. The circuit will work satisfactorily on 3V (dc). One can use 6V using big distance wire.
COMPONENTS USED
1. T-1, T-2 & T-3………………………………..NPN TRANSISTORS.
2. C-1 & C-2 ……………………………………CERAMIC CAPACITORS 0.1 uf.
3. C-3 …………………………………………...0.04 uf CAPACITOR.
4. C-4 ………………………………………… 0.02 uf CAPACITOR.
5. C-5 ………………………………………… 10 uf CAPACITOR.
6. R-1 ………………………………………… 180 K OHM RESISTANCE.
7. R-2 & R-5 ……………………………………10 KILO OHM.
8. R-3 ………………………………………… 100 K OHM.
9. R-4 ………………………………………… 1 K OHMS
10. SPEAKERS …………………………………8 OHMS (Both).
11. SWITCH …………………………………… DOUBLE POLE DOUBLE
DOUBLE THROUGH.(DPDT).
