Auxiliary circuit for control of telephone bell - has semiconductor circuit determining activation by code

摘要

An auxiliary circuit for a telephone system where the bell can be selectively activated by using a particular code. This is intended to avoid undesirable calls, while recognising desirable calls immediately. The circuit can be incorporated in a two-wire system. The two wire of the telephone (12, 14) are connected to a bridge rectifier (16, 18, 20, 22). The lines joining the diode contain a capacitor (34) and a resistor (38), which is connected to a flip-flop circuit which operates the switching. The resistance (38) is also coupled to a transistor (42) which is in series with a normally closed relay (44). The circuit is coupled to a detector which picks out a component frequency of a signal by determining the difference between the frequency of the component and the frequency of a local oscillator.