Contactless capacitive finger operated switch - depends on change in capacitive impedance rather than fixed threshold levels

摘要

The switch is especially directed at telephone systems. A capacitance accessible to an operator in an unspecified way, is part of an a.c. excited circuit linked to the base of a transistor. In the emitter of this transistor there is a chain of resistors and capacitances across some of which other transistors are connected. A d.c. voltage is established in this chain in quiescent conditions, but if a capacitance change occurs - when a finger is applied - one of these transistors in the chain switches on and by activating a feedback network changes the chain impedance so that the feedback amplifier remains activated until another considerable capacitance change occurs - on the removal of the finger. The output of the feedback amplifier also serves as the switch output.