Door intercom with door-opening button - centrally generates call signal and feeds to individual internal intercoms using circuit including two series-connected Zener diodes and parallel decoupling diode

摘要

The intercom system uses door units connected to the inside units through separate wires, one for calling and one for answering for each door. The door units and the inside units have a common reference line, to which the speaking/hearing circuits of the units are connected through one pole and all the units are fed through this line from the common direct voltage source. To call an inside unit a generator produces a call signal when a call button on the door unit is pressed and the call signal is passed to the inside unit in question. Each inside unit has a key for switching on the circuits, otherwise blocked, and another key for opening the outside door. USE/ADVANTAGE - Esp. for large apartment blocks. Reduced wiring.