Circuit diagram of intercom made with LM386

The circuit diagram below illustrates the circuit's principle. This circuit only supports half-duplex intercom, meaning only one party can speak while the other listens, not both simultaneously. Speaking and listening are controlled by switch S1, located at the base station. With S1 positioned as shown, the base station listens while the extension speaks. Flipping S1 downward allows the base station to speak while the extension listens. The extensions have only one speaker, BL2, which serves as both a microphone and receiver. The same applies to the base station's speaker, BL1. The LM386 acts as an amplifier, with input signal at pin 3 and output signal at pin 5. Capacitors connected to pins 1 and 8 adjust the circuit's gain and are optional. The power supply voltage can range from 4.5 to 9V.

It is best to use a twisted pair cable for the BL2 lead to reduce interference. If necessary, a power switch can be added to avoid wasting energy when not in use. The disadvantage of this circuit is that the extension cannot call the host.