Line circuit for key telephone systems

Abstract

A line circuit for a key telephone system using three relays, a line relay, a ring relay and a combined off-hook and hold relay. To control the energization of the ring relay and the off-hook and hold relay responsive to inputs from the exchange or from a station of the line, an operational amplifier network is used. Two amplifiers are used as individual relay drivers and two are used as comparators. The amplifiers of this network may be identical circuits as part of a quad circuit chip. On a ring signal from the exchange, a ring comparator amplifier is activated to energize the ring relay. On a station of the line going off-hook, the off-hook driver amplifier is activated directly to energize its relay. When a station of the line goes into hold, the off-hook and the hold comparator amplifier is activated to energize both the ring relay and the off-hook relay. The line relay is energized during only the ring and hold conditions, and is shunted against energization during the off-hook condition. The hold condition is sensed by the line relay being energized, and the conventional A lead ground having been open-circuited.

Dependencies are inferred from the claim text.