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Intercom is actually an abbreviation of inter-communication, which means a communication of messages or conversations from one room to another or one office to another existing within the same building. Thus, a device used for delivering messages or mutual conversation between one room to the other room or one office to the other office, existing in the same offices, commercial and industrial organizations, and larger houses, is called an intercom, and this entire system consisting of intercommunication is called intercom system. As intercom sets are mostly used in homes or offices, which are located in the same large building, therefore, communication is done from one place to another nearest place, through the application of an ordinary wire. Two handsets are used in an intercom, one of which is placed in one room or office while the other one is placed in the other room or office. For example, one set of the intercom is placed in an officer’s office whereas the second set is placed in his assistant’s office. The source of communication between these two sets tends to be a common wire “transmission media” (i.e., a conversation takes place through an ordinary wire connected between both the sets). Therefore, an intercom is not used over long distances, because wires resistance increases as a result of a lengthy wire, due to which intercom efficiency reduces.
A simple intercom circuit, which basically consists of three parts (i.e., amplifier, toggle switches, and speakers) has been illustrated in figure 4.9. The names and functions of different components used in this intercom circuit are as follows;
It converts normal AC voltage (i.e., 220 volts) to 9 volts.
Variable resistance can be increased or decreased or adjusted according to one’s needs to control the set sound (i.e., increase or decrease).
(3). Operational Amplifier
An amplifier, consisting of a transformer, boosts sound. However, a separate 6-volt DC supply is provided to energize it.
(4). Resistor and Capacitors
The object of using a resistor and capacitor is to reduce or filter the noise level and improve sound quality.
The object of using two speakers in an intercom system is to enable persons on both ends to listen and converse with each other.
(6). Toggle Switch
It is a double pole dual throw switch consisting of three points, which connects one line of the circuit with the other line.
Working and Construction of Intercom System
Suppose that one set of an intercom is in an officer’s office, whereas the other set is in some other of his assistant’s office. When the officer intends to talk to his assistant, he picks up the cradle of his telecom set. Let us suppose that at that moment, toggle switch S1 is on position number 1. It is to be reminded here that when switch S1 is on position 1, the toggle switch S2, tends to be on position 3. This has been illustrated in figure 4.10. In such a situation, the speaker of the officer’s set SP1 functions as a microphone, whereas the assistant’s set speaker SP2, works as a speaker (that’s a person on SP1 will talk while the person on the SP2 will listen to him). Thus, with the toggle switch S1 being on position 1 (that’s a lifting of the cradle by the officer from his set) signal transmits to the amplifier via transformer and rheostat, which amplifies sound and provides it on output. The resistor and capacitor are installed on the amplifier’s output, filter (that’s to improve the voice quality) this signal, and provide it on the second speaker SP2 via point number 3 of the toggle switch. As a result, the bell rings on the set present in the assistant’s office.
As soon the assistant picks up the cradle of his set, the bell stops ringing and the officer and assistant start their conversation. After the end of the mutual chat, the officer places back his cradle on the set, thus switching S1, shifting to point number 2. Remember that the same process is repeated on the assistant’s side. Therefore, when the assistant lifts the cradle to chat with his officer, toggle switch S2 connects to point 4, whereas toggle switch S1 is on point 2 at that time, as has been illustrated vide figure 4.11. Thus, as soon as the assistant lifts his set’s cradle, the circuit of the intercom system completes and the bell rings on the set present in the officer’s room, and which stops on lifting the cradle of this set. Thus, officer and his assistant start a conversation through an intercom.
Secret Intercom System
An intercom system, wherein the conversations between two persons remains secret (i.e., a third person cannot listen to this conversation), is called a secret intercom system. In other words, a system of intercom, the advantage of which is that when two persons chat with each other, a third person cannot listen to their conversation, is known as a secret intercom system (as can be implied from its name). This type of intercom can also be used as an ordinary or simple intercom (the explanation of which has been give above) or a telephone. However, no push-to-talk switch exists on it.
Construction and Working Secret Intercom System
The simplified circuit of a secret intercom system has been illustrated in figure 4.12, which consists of the following components;
|(i). Two resistors R1 and R101 of 100 kilo ohms||(ii). Two resistors R2 and R102 of 100 kilo ohms|
|(iii). Two resistors R3 and R103 of 100 kilo ohms||(iv). Two resistors R4 and R104 of 39 kilo ohms|
|(v). Two resistors R5 and R105 of 2.2 kilo ohms||(vi). Two resistors R6 and R106 of 47 kilo ohms|
|(vii). Two resistors R7 and R107 of 100 kilo ohms||(viii). Two capacitors C1 and C101 of 220 pekoe farad|
|(ix). Two capacitors C2 and C102 of 10 pekoe farad||(x). Two potentio-resistors PR1 and PR101|
|(xi). Two 741 number ICs IC1 and IC101||(xii). Two 741 number ICs IC2 and IC102|
|(xiii). Two microphones M1 and M2||(xiv). Two speakers SP1 and SP101 of 5 ohm|
It must be kept in mind that circuit of set number 1 consists of R1, R2, R3, R4, R5, R6, R7, C1, C2, PR1, IC1, IC2, M1, and SP1. Whereas circuit of set number two consists of R101, R102, R103, R104, R105, R106, R107, C101, C102, C103, M101, SP101.
Figure 4.12- secret intercom circuit
The operational mechanism of the circuit is as follows;
When any one speaks on set number 1’s mike, the audio signal is transmitted on pin number 3 of the IC number 1 through M1 via capacitor C1. The capacitor C1 and resistor R1 existing between IC1 and M1, transmit this signal on IC1 through its filtration (i.e., by smoothing the signal or improving its noise level). This signal received on pin number 3 of IC1, amplifies on IC1 and transmitted on the other set (i.e., set number 2) through its output number 5 via resistor number 5 (R5) by means of a coaxial cable, where its further amplification is done and after its amplification, it is heard through set number 2 speaker SP101. Remember that whenever somebody speaks on set number 1 mike, the speaker can also listen his voice on his own set, because two further ICs are also set one every set, via the resistor R6 and R106, as can be clearly seen in via a circuit diagram illustrated in the figure, which also transmit this audio signal on their respective speakers (i.e., SP1 and SP101). Therefore, voices of both the persons can also be heard on the speakers of the sets on which they converse. An unpleasant and unnecessary noise can also be controlled with the help of potentio resistors PR1 and PR101 (i.e., changing their values).
In fact, the first IC of every set (i.e., IC1 and IC101) buffers the input signals (i.e., these amplifiers work as isolation amplifiers the function of which is to keep the different stages of the circuit mutually separated). Then this signal received from the amplifier’s output feeds on cable via resistor R5 or R105. The second IC (i.e., IC2 and IC102) mounted on every circuit, negates these signals emitted from the buffer amplifier’s output (because both ICs are set reversed to each other on both the circuits). Thus, the difference that is found in signals is the signal that is fed on the other end of the cable. Resultantly, the signal which enters or feeds from one end of the cable reflects on the other end of the cable. The potentio resistor or variable resistor of every circuit helps in eliminating unnecessary signals. However, a rejection of 50 decibels to 55 decibels can be achieved through these resistors if they are of fine quality (i.e., the dB range of these unwanted signals can be abolished). Remember that for a better performance of this intercom set, the number of audio input signals provided to it must be between 100mV to 3V RMS.
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