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