Full duplex intercom using telephones with tone dialling

摘要

This circuit (Fig.1) enables you to build a 2-station intercom with no need for press-to-talk (PTT) switches. It uses standard telephone handsets and it generates a ring voltage so that the called phone will ring before it is picked up. Each handsel can be used to call the other and it is a full duplex system, meaning that both parties can speak and be heard simultaneously over a 2-wire connection. The handsets are powered from a 24V DC line provided by mains transformer Tl, a bridge rectifier, a 1000μF capacitor and regulator REG1, an LM7824. This is fed to the phones via the primary winding of a small mains transformer, T2, which is being used simply as an inductor, to isolate the audio signals on the phone lines from the low impedance of the DC supply.