Versatile Intercom System

Most intercoms use up to five wires for hooking up to other units,while the one presented here needs just two.There are several features that make this design unique.

• Two wire line systems.
• Single chip design
• Touch to call facility
• Hands free conversation which allows conferencing
• Compact design

The unit uses a type 1895 amplifier IC which forms the heart of  the unit.The amplifier takes the input from a biased microphone M1.The output is given to the line.Point A and E are connected to communication line.The basic design is conventional.When the touch plate is touched,a buzzing sound is sent to other unit to give the call signal.This is due to the ambient mains hum pickup and its harmonics.

The 8 ohm speakers is used if two units are used.For four,each speaker must be a 16 ohm type.The microphone is the usual tape recorder type.A suitable power supply is given in schematic b.

Schematic a :This intercom needs just two outgoing wires 

while most others would need at least five.

Schematic b :The power supply set up

The final wiring is done as shown in Picture As for enclosures,even cases will do the job well enough.

 Picture : Wiring Schematic for two units.