GB633520A - Improvements in or relating to telephone circuits - Google Patents

Abstract

633,520. Telephone extension circuits. ERICSSON TELEPHONES, Ltd., and GUNSON, G. R. Aug. 25, 1947, No. 23407. [Class 40 (iv)] In a circuit in which a main instrument and an extension instrument are connected to the same two-wire exchange line, the extension subscriber can recall the main instrument by means of a vibratory relay or like device after the exchange line has been switched through to the extension. The main instrument TI, which has the usual bell BM, 'communiates with the exchange in the normal manner through line L1, L2. The subscriber at the main instrument operates key KE to connect the extension instrument TE to the exchange line, and an -indicator relay R then gives visual indication of all extension calls and also short-circuitsbell BM at contact rl so that outgoing calls from the extension do not affect the bell BM. Speech currents follow a low impedance path through a condenser QD. Ringing current from the exchange rings both bell BM and the bell at the extension, but does not produce sufficient voltage to operate relay R or a relay A in a rectifier bridge circuit MRB. To recall the main instrument, the extension subscriber operates a key KN, which has make-beforebreak contacts, thus connecting up a vibratory relay RR whose make-before-break contact rrl is provided with a non-linear resistance SS as a spark suppressor. The vibratory relay RR superimposes upon the direct current in the exchange line an alternating component which is rectified by the rectifier bridge MRB and operates the relay A whose contact al completes a circuit to ring a bell MC in the main instrument. The subscriber at the main instrument may then join in the conversation. The subscriber at the other end of the line is prevented from receiving undue noise during the signalling by a shunt circuit comprising rectifiers MRA and a condenser QB.