Sensor Sense: Capacitive Touch Sensors

Feb. 2, 2009
Touch sensors uses body capacitance to change the frequency of an oscillator

Many people mistake the growth in capacitive touch sensors as the adoption of new technology. But the fact is advances in mixed-signal programmable devices, those that combine analog and digital into a single package, have brought back this quite old technology and given it new life. The commercial application of capacitive touch sensors can be traced back to 1919 when Leonard Theremin built the first electronic musical instrument.

The Theremin uses body capacitance to change the frequency of two oscillator pairs in heterodyne. Musicians play the Theremin by waving their hands in front of two antennas, shifting the frequency of one of the oscillators in each pair to create beat frequencies. One beat frequency produces the output tone of the Theremin while the other controls its volume.

Modern capacitive touch sensors also use oscillator pairs, but in a slightly different way. As with the Theremin, no actual touching of the sensor “antenna” or touch plate occurs. The plate is isolated from direct touch by a thin layer of insulating material. It forms one side of a capacitor. The ground or a ground plane serves as the other plate of the capacitor.

The capacitor formed by the two plates becomes the timing capacitor in a relaxation oscillator fed by a constant-current source. As charge pumps into the capacitor, voltage rises to a threshold level monitored by a voltage comparator. At the threshold, the comparator discharges the capacitor, restarting the charge cycle.

A finger in proximity to the touch plate capacitively links to the plate to form a larger plate area. The bigger plate raises the capacitance value, so the timing capacitor charges longer to reach the comparator threshold voltage. A counter tracks charging times by counting the number of square waves generated by a second fixed-frequency oscillator. Counts are low without a finger present because charging times are fast. However, the presence of a finger lengthens charging times, creating a higher count. Counts that exceed a specific value indicate a touched sensor.

Edited by Robert Repas

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