Identifying and blocking system for telephone party line - uses relays controlled by operational amplifier and reset relay

摘要

The marking circuit indicates if a party line connected to is in use, and blocks the operation of a second subscriber appts. in such a case. An operational amplifier (V1, V2) is connected by its non-inverting input (+) to a positive volt. (U) and, over the marking contact (T1, T2), to a zero or negative ref. The inverting inputs (-) of all the amplifiers (V1, V2) are commoned. This common circuit is fed from the output of a further amplifier (V6) which has three selectable ref. inputs (U, e1, e2). The highest (U) causes the marking and blocking relay (A1, A2) to pick up when the appropriate selector marking contact (T1, T2) is operated. The other volts. (e1, e2) are lower, and represent a hold on for the relay which also prevents any other circuit from breaking in, and a reset for the relay. The last is the lowest volt. The reset is transiently applied when the handset is replaced.