On=off switching for telephone subscriber's appts. with local battery - using mercury switch to automatically switch on microphone and delay switch on amplifier when handset is lifted for use

摘要: The telephone subscriber's appts. with local battery supply ensures that the speech circuit is only switched on when hand set lifted for use. To avoid acoustic shock during swichi-on delay included. power disconnected if handset replaced cradle or placed a neighbouring support surface. microphone (11) amplifier (12, 13) off whilst contacts (3, 4) in mercury switch are open circuit. When use switch, adjacent to insert, closes and powers unit. has due an RC base (14, 15, 16) first transistor (13). also housed handset.