My original telephone intercom project was published in Silicon Chip magazine here in Oz in May 1992. Photocopies of that original article are still available from Silicon Chip, and they kindly let me re-publish the circuit here.
The intercom unit powered two telephones, allowing you to talk between them. Lifting one handset automatically started the other ringing, and cradling both devices resets it.
Applications were the obvious “batphone” intercom between two locations, whether those be neighbours down a country road, or from the main house to the granny-flat. Service people could use it to test and repair telephones, telephone collectors used it to make static displays more interactive, and it saw use on stage and screen where a phone was required to ring on set. In a corporate environment it could be used to couple two phone systems or telephony gateway devices that only had FXO interfaces, or turn an FXO port into an FXS one. I’ve used it personally to make a “phone call” between my modem and our home burglar alarm, allowing me to reprogram the latter from within the building.
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