Touch switch needs no dc return path

Common designs for touch switches detect a decrease in resistance when a user's fingertip either connects a contact to the circuit's common ground or supplies an injection of 60-Hz ac voltage, resulting from immersion in the electrostatic field that nearby power lines radiate. But what if no nearby power lines exist and the equipment operates from a battery source, such as in an automotive application, or if a galvanic contact to circuit common is unavailable?

The circuit shown in Figure 1 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.

In Figure 1, IC_1A operates as a square-wave 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 square-wave signal to its gate repetitively drive 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 IC_1B's output to a logic low.