Method of and circuitry for detecting, at a telephone exchange, the

Abstract

In order to detect at an electronic telephone exchange the lifting of a subscriber's handset in response to ringing current sent over the line loop from a generator at that exchange, a charging current proportional to the incoming line current is fed to a capacitor over a recurrent integration period encompassing one or more whole ringing-current cycles. When the handset has been taken off the hook, the capacitor charge present at the end of such a period differs from zero on account of a d-c component superimposed upon the alternating ringing signal. In order to enable the use of a relatively small storage capacitor, the charging current is periodically interrupted by a train of chopping pulses. When the line current is measured at the exchange by a sensor responsive only to absolute magnitude, the polarity of the charging current is reversed in alternate nonzero-amplitude periods of a cycle.