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 circuitry for producing a unidirectional current from the combined alternating and direct voltages. When the line is open no direct current...http://www.google.fr/patents/US4356355?utm_source=gb-gplus-shareBrevet US4356355 - Apparatus for detecting the closure of a subscriber's line during ringing
Apparatus for detecting the closure of a subscriber's line during ringing
