APPARATUS FOR DETECTING THE CLOSURE OF A SUBSCRIBER'S LINE DURING RINGING

摘要

PHF. 79-589 15 Apparatus for detecting the closure of a subscriber's telephone line in the presence of an alternating ringing voltage superimposed on a direct voltage comprises device for producing a unidirectional current from the combined alternating and direct voltages. When the line is open no direct current will flow and hence the peak value of successive half cycles will be equal whereas when the loop is closed and direct current flows the peak values or successive peaks will be different. A capacitor is charged by a voltage U1, which is proportional to the combined voltage, through a transistor provided that the voltage U1 is greater than the voltage across the capacitor and discharged through a resistor. The time constant of the discharge circuit is selected so that when the line is looped the smaller peak is lower than the voltage to which the capacitor is discharged and hence charging current flows into capacitor on alternate peaks of the voltage U1 only. The intervals between pulses of charging current are determined by comparator which provides an output S which indicates whether the subscriber's line is closed.