Capacitive Touch (etched on PCB)


I am going to be working on a new design and we wanted to add capacitive touch capability to our board.

We wanted to etch a copper pad on the board and then use this as the touch element. After doing some research it looks like this is the approach:

1.) Have a digital output of the micro controller toggle a pin high and low.

2.) This digital output is connected to a resistor that is then connected to the copper touch electrode. This is creating the RC time constant.

3.) The electrode is then connected to an input of the microcontroller.

4.) After I set the digital output high in step 1 I should start the counter.

5.) I then count until I have reached a certain threshold. I will be sampling the input using the ADC.

6.) If the counter is higher than the untouched state to reach the threshold then there has been a touch.

Does this sound correct?

Does anyone have any information on what the geometry of the touch electrode should be?



Reply to
Mike Miller
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Atmel has some pretty good information. We're using several of their chips but my last design used their SAM D20E micro (Cortex M0 with some really nice peripherals, including the touch controller - cheap, too ;). They have recommendations for shapes and dimensions, based on the thickness and material of the cover. I think the app note was named "Buttons, Sliders, and Wheels", or some such. I followed their design guidelines. Worked fine.

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This should be the app note:

Jeff Liebermann 
150 Felker St #D 
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Jeff Liebermann

Another method has the touch sensing pad isolated from the circuit. It goes like this: have a large, finger/thumb sized pad on the top of the board, on the other side or next layer down have two pads (each about half the area of the touch pad) directly underneath the touch sensing pad.

These form capacitors to the touch sensing pad. Toggle one pad and sense the other. You have created two small value capacitors in series with the touch pad at the midpoint. Untouched, the receiving pad sees the toggling edges. A person touching the mid point, that is the touch pad, has a much larger capacity to the rest of the planet and shunts the toggle signal.

There are lots of other possibilities too.

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Yes, that's it. There's a lot of good information in there. The Atmel SAM series of processors is really impressive, too. Less than $.50 at the low end, for an ARM (M0) sorta puts a crimp in the PIC's style.

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If you aren't committed to build-your-own, there's COTS solutions:

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