Telephone exchange connecting line condition detector circuit - uses transient EMF induced in tertiary winding on repeating coil to operate opto-coupler

摘要

Two subscribers on an internal exchange are connected together via selector contacts, and to the battery via a relay and a repeating coil. When one subscriber hangs up, the current drawn from the battery falls, but not sufficiently to release the relay. A tertiary winding on the repeating coil has sufficient transient e.m.f. induced in it to operate on optocoupler. This triggers a bistable which energised a time delay relay with a time delay of approx. 100mS. After the time delay, a further relay is energised, one contact of which releases the two subscribers while another resets the bistable. The subscriber who hung up is now free while the other is still engaged.