Two Line Intercom circuit

This project is used as an electronic private exchange. It has two telephones, which have the intercom facility, and they can be connected to the telephone line. All the functions are controlled by the 8-bit microcontroller AT89C2051 which has an Program Memory (ROM) of 2 kilobytes.
Two Line Intercom circuit - schematic

The DTMF (Dual tone multiple frequency) signals are decoded by the DTMF decoder IC 8870 and its output will be in decimal of the pressed key. The switching functiond are done by 12v relays.

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