TELEPHONE CIRCUITS

1318858 Substation circuits STANDARD TELEPHONES & CABLES Ltd 13 Sept 1971 42550/71 Heading H4K A circuit for a push-button subscriber's instrument having a latching electromagnetic relay to provide loop disconnect dialling impulses includes a circuit, additional to that for driving the relay during impulsing, for closing the relay contacts when the instrument goes "off hook". In the circuit shown, capacitor C2 is charged up from the exchange battery through lines L1, L2 and when the cradle switch contacts GS1, GS2 are closed, it discharges through relay winding LR into capacitor C1, thereby closing the loop through contacts lr1 which are latched. The MOST circuitry M controls the operation of the off-normal contacts ONC and the release of relay LR to provide impulsing. If contacts lr1 inadvertently open, e.g. due to shock when the instrument is dropped, capacitor C2 will charge due to the resistance imposed by T1, T2 and relay LR will be re-operated. In a modification, the whole circuit is isolated from the lines by the cradle switch contact and a relay operating circuit is formed when this contact is closed, Fig. 2 (not shown).