Key telephone system line circuit

Abstract From the Canadian patent

A line card circuit providing supervisory functions in a key telephone system is disclosed. Two multi-contact relays perform all of the switching functions required to operate the audible and visual signaling devices used to indicate incoming calls, busy lines, and held calls, and to switch a shunt impedance across the line conductors to maintain line current when a call on that line is being held. Ringing voltage and line current are sensed by semiconductors which comprise the combination of lightemitting diodes and an adjacent light-responsive semiconductor switch. These optical semiconductors are interconnected with other semiconductor devices to form a logic circuit for actuating the …