Sensor Sense: Capacitive Slide Sensors

April 7, 2009
Capacitive touch switches combine to simulate an analog control.

In the last Sensor Sense we looked at capacitive touch switches used for on/off control. But these switches are also found in capacitive touch sliders that can control various levels, such as volume and light, or act as a rotary selector switch for channels on a TV or songs in an MP3 player.

A common capacitive slider appears as a circular ring on devices. As your finger slides clockwise around the ring it cranks up the volume of your player or runs down a selection of music choices. Slide your finger counterclockwise and the volume drops or the list scans upwards. While these actions may appear to be an analog response, the action of the slider is all digital.

Ring sliders typically have eight capacitive touch switches spaced at 45° intervals under the ring. As your finger moves around the ring, it slides from one touch plate to the other. The previous column showed how a finger “touching” a touch plate adds body capacitance to the plate producing a longer charge time. That, in turn, lets a counter reach a higher count value to trigger the output of the touch circuit. An application-specific integrated circuit monitors the count for each touch plate, then feeds that signal to its internal microprocessor for decoding direction and value.

As the system internally monitors the count for each touch plate, it’s possible to add additional levels of resolution. A finger placed between two plates produces a similar long count for each plate. If two plates show longer than normal counts, with one plate count higher than the other, it can be assumed that the finger is closer to the plate with the higher count. This can boost an eight-point touch ring to resolutions of 32 or even 64 points.

Linear sliders perform the same function with the touch plates laid out in a straight line, or two triangle-shaped touchpads can work together, providing different plate areas to the finger as it slides over the plate. In both cases special actions can be taken when reaching either end of travel, such as triggering full turn-off or turn-on of the device.

Edited by Robert Repas

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