Intercom

The circuit of a two-position intercom is presented here. This circuit is very simple yet it functions quite satisfactorily. The circuit does not involve any complicated switching. The switches S1/S2 must be fixed in such a way that when the handset is resting on the cradle, the switch is OFF and when it is taken off the cradle, the switch turns on. Both the sets used are identical in construction.

When one set (say, party 1) is switched on, the other set's (party 2's) bell energises. When party 2 turns on his own set, his bell automatically stops and he can talk to party 1 via his microphone. One can substitute the BEL 1895 IC based amplifier and bell circuit with any other low power amplifier and bell circuit.

The block diagram clarifies the connection of the two sets. Only three wires are required to connect the two sets if separate battery is used in each set. However, if the battery is common for the two sets, it requires four wires for interconnections. The circuit can be easily assembled on a general purpose PCB. Intercom cases are also available in the market which may be used for giving it a professional outlook.