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Transistors Hands off intercom Circuit

2016-05-09 07:05  
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A simple intercom circuit designed based on transistors is shown here. This intercom circuit does not require a changeover switch and you can use it just like a telephone and a ringer circuit is also included.Amplifier A increases the microphone`s output to a usable level. The output signal is fed to op amp B, which inverts the signal. A balance-control potentiometer connects across the outputs of amplifiers A and B. 1f an audio tone is fed into the microphone and the balance potentiometer`s wiper is all the way over to the A output position, the tone will be heard at a high level. As the wiper is rotated toward the B output, the audio level will decrease until it just about disappears near the center of the potentiometer`s range. As you continue to rotate the wiper, the signal will begin to increase once again.With the balance control set for a minimal output, the intercom`s tendency to self-oscillate from acoustical feedback between the microphone and speaker is kept to a minimum. The microphone`s amplified signal at Ns output is fed to the other intercom through the audio in/out cable. Since both intercom units are alike, the audio information coming from one nnit feeds the other at the input of op amp B. The incoming audio is amplified slightly by op amp B and the output signal is sufficiently increased by the power amp to drive the speaker.A very good quality and simple intercom circuit using only three transistors. By pressing the switch S2 the circuit will generate ring signals. For making two way intercom make two identical circuits and combine them as shown in diagram 2. The current consumption of the circuit  is only 10 to 15mA due to which a 9V or 12V battery will last longer. The circuit is very small and can be easily mounted in any small box.