Capacitive switching circuit for energizing remote controlled device

TLDR: In this article, the body capacitance was added to the circuit to actuate the switching circuit in the presence of only stray capacitance (C E ) so that the circuit remains unactuated.

Abstract: The casing (12A) of a hand-held instrument (12) provides a contact surface for a capacitive switching circuit for energizing a controlled device (63). The switching circuit and device are remote from the contact area. The switching circuit includes an input circuit (80) incorporating the body capacitance (C B ) and a variable capacitor (81) which is adjusted to balance the circuit in the presence of only stray capacitance (C E ) without body capacitance so that the switching circuit remains unactuated. The addition of body capacitance will actuate the switching circuit. In the illustrated embodiment, the input circuit (80) is energized by a periodic signal. An output signal is coupled from the switching circuit to the controlled device through a delay off circuit (88) which has a delay time longer than the period of the periodic signal so that if contact with the instrument is lost momentarily, there is no interruption in the controlled function. This is particularly advantageous in the case of a medical device or dental handpiece where light from a high intensity source is coupled back to the instrument for illuminating the work area.