Wednesday, December 7, 2011

Low cost Capacitive Touch Switch

Common designs for touch switch switches detect a decrease in resistance either when a user's fingertip connects a contact to the circuit's common ground. The circuit shown operates by sensing an increase in capacitance that results from touching a contact. Although a straightforward design might require a complex circuit, the design shown offers a low-cost approach that uses few components.

IC1A operates as a squarewave oscillator at approximately 150 kHz. The oscillator's output gets ac-coupled to potentiometer R2, that sets the drive level and, hence, the sensitivity for the touch pad. Applying negative excursions of several volts of squarewave signal to its gate repetitively drives N-channel JFET Q1 from conduction into cutoff. An approximation of the square wave swinging from 0 to 12V appears at Q1's drain. A peak detector circuit formed by D1, R7, and C4 provides sufficient dc voltage to force IC1B's output to a logic low.

However, if someone touches the touch pad, any added capacitance to ground or circuit common reduces the ac drive at the FET's gate, and Q1 continuously conducts. The square-wave voltage applied to D1 decreases. The voltage on C4 drops below the logic threshold, and IC1B's output goes high. You can adjust R2, to set sensitivity and compensate
for device-to-device variations in the FET's pinch-off voltage.

a low cost touch switch

Components Required:
R1------------------------------6.8k ohm
R2------------------------------10k ohm pot
R3------------------------------100k ohm
R4------------------------------10M ohm
R5,R6--------------------------10k ohm
R7------------------------------1M ohm

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