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.

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Diagram 1
Diagram 2