Indicating circuit for telephone system - indicates subscriber loop closure as result of call by using two voltage dividers and comparator

摘要

The indicating circuit has a supply resistor (Rs2) in the grounded wire (b) of the telephone line (AL) serving the call-current circuit and the supply-current circuit. The voltage drop across this resistor is passed to a comparator (K) via two voltage dividers (R1,R2,R3 and R4, C, R5, R6,R3). The first divider contains a series capacitor (C). The comparator produces an output when a call occurs whilst the subscriber loop is open and a second output when a call occurs whilst the loop is closed. The advantage lies in fast response.