Time-out circuit for key telephone systems

Claim 1

In a telephone system comprising a plurality of key telephone line circuits each including signalling means and means responsive to ringing current from a line for operating said signalling means, the improvement comprising a timeout circuit common to all of said line circuits, said timeout circuit comprising a neon lamp for each line circuit, a photoconductive cell coupled to all of said neon lamps, and a time delay circuit energized by the photoconductive cell in response to ringing current in any of said line circuits, said time-delay circuit comprising a timing circuit including a resistor and a capacitor and a solid-state switching circuit operated by said timing circuit, said switching circuit comprising a first normally conductive transistor connected in series with a normally nonconductive second transistor, the base of the second transistor being connected to the collector of the first transistor and the base emitter junction of the first transistor being connected in series with a Zener diode across said capacitor, the capacitor being adapted to discharge through the photoconductive cell and render the first transistor nonconductive and the second transistor conductive when ringing current is present in any of the lines, and to charge to the breakdown voltage of the Zener diode and render the first transistor conductive and the second transistor nonconductive within a predetermined time interval after ringing current has stopped in all of the lines.