Two-state indicator circuit for subscriber dialling systems - with lamp relay operated by exchange signals to indicate readiness to accept dialling signal

摘要

The switching circuit for indicating a second dialling state in subscriber dialling installations consists of a relay (R1) energised by a contact on the subscriber's dial (nsa). The relay contacts complete the circuit to first and second relay coils (SZ1-II) in one wire of the line to the exchange (a). These relays operate a contact (sz) which controls an indicator lamp (A). - On the first signal from the exchange, the second relay coil is de-energised and the lamp is held off. When the subscriber dials the first digit of the sequence the first relay (R1) drops out with a delay and its contacts isolate the first indicator relay (SZ1). - On receipt of the second signal from the exchange to indicate it is ready, the second indicator relay drops out and the lamp comes on.