Intercom. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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In one of the magazines "Young Technician" about a decade ago, I found a diagram of an intercom, which I tested in action. True, I had to pick up the operating modes of transistors a little. Until now, I use this device while talking with a school friend.
One channel of the intercom (Fig. 1) is made on two transistors - this is an audio frequency amplifier (AF), the input of which receives a signal from the microphone VM 1. The output signal of the amplifier is fed through the communication line to the subscriber's headphones BF1. Of course, the subscriber has the same amplifier connected via a communication line to the head phones located at my place.
To call a subscriber, just touch the E1 sensor - a small metal plate. A low-pitched sound is heard in the subscriber's phones - an alternating current background (of course, when the contacts of the SA1 switch are closed). If the sound volume is not enough, it can be recommended to connect the plate E1 through a capacitor with a capacity of several thousand picofarads to the collector of the transistor VT2.
Microphone - TA-56 capsule or other low-resistance capsule from head phones or telephone set, head phones - TON-1, TON-2 or similar high-resistance ones. Transistors - any of the MP39 - MP42 series with the highest possible base current transfer coefficient. Power source - battery "Krona" or series-connected galvanic cells with a total voltage of 8 ... 9 V. Switch - any design.
Some parts of the amplifier are mounted on a board (Fig. 2) made of one-sided foil fiberglass.
Establishing the design begins with checking the voltage at the collector of the transistor VT2 - it should be equal to about half the voltage of the power source. You can set it this way by selecting a resistor R3. Set the same voltage on the collector of transistor VT1 by selecting resistor R1. After that, the device is checked in operation, and by more accurate selection of these resistors, the highest sound volume and the least distortion are achieved.
Author: D. Almukhametov, Ufa, Bashkortostan
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