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.
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| Figure 1. | A low-cost touch-switch interface uses three Schmitt trigger hex inverters and a single JFET per channel. |
In Figure 1, IC1A 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 12 V 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. For novelty or nostalgia's sake, you can use one-half of a 12AX7 dual triode as an oscillator and the remaining half in place of Q1.
Selecting plate resistors allows operation with a 12 V plate power supply.
