Simple Door Intercom

This intercom circuit suitable for accompanying  door bell. This circuit uses the speakers both as microphones and as loudspeaker. This intercom is made simple by using only a single amplifier IC LM390 to serve two-way communication. This circuit is based on master controlled half-duplex operation.  With LM390, a 6V battery supply would produce loud enough volume. Here is the schematic diagram of the circuit:

The switch for this intercom circuit is ideally a push button PTT switch, but any double SPDT should work. The interaction scenario between the guest (remote)  and the host (master) can be like this:

• The guest press the door bell
• The host press the intercom switch to talk position and then say: “who’s that?”,  then release the switch to return to the listen position.
• The guest then answer: “This is me bla bla bla.. “
• The host press the button and say: “OK, wellcome bla bla bla..”

The simplicity of this intercom circuit is achieved by using half-duplex method, so we can use only a single amplifier for two way communication. This simplicity is further improved by utilizing the loudspeaker to act both as a microphone and as speaker. [Circuit’s schematic diagram source: National Semiconductor Application Notes]