Any experiences with touch sensing?

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I've got a small AVR project that would benefit from benefit from allowing a user to control more/less via simple tapping. If I implement this it has to be simple and cheap (and small), since the device is simple and cheap (and small) to start with. I just need to detect touch/no touch, nothing more.

The input pin would be the MOSI pin on the ISP connector, which can also be configured as an analog input (tiny24). I'd really like to limit myself to just one additional series resistor to minimize potential ESD damage.

I'll be giving this a try in the next few days, but I wanted to find out if anybody had done anything similar.

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I haven't got around at using the touch library ie can't get my head around getting it to work :( .

I just use single touch sensor chips. The new ones are SOT23-6

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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I am unsure how the actual touch library works, but the general procedure needs two IO pins per button. Some other MCU manufacturers use a bit different methods, that enable you to use two IO pins for two buttons.

But what kind of touch sensing you need? Do you need to have it detected through glass/plastic panel or does it suffice if user touches some metal electrode?

It might be best to use separate touch sensor chips, but there are software tricks that might work.

For example you could have a 1Mohm pull-up and have your finger connect that pin to GND. Use ADC for measuring voltage.

Or you could use the pin as output to charge or discharge any capacitance to known voltage, and then turn it to input, and measure how long it takes for the capacitance to charge through a high value pull-up.

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Jepael wrote:
But what kind of touch sensing you need? Do you need to have it detected through glass/plastic panel or does it suffice if user touches some metal electrode?

Metal electrode would be fine.

Quote:
Or you could use the pin as output to charge or discharge any capacitance to known voltage, and then turn it to input, and measure how long it takes for the capacitance to charge through a high value pull-up.

I was thinking along these lines as well. Periodically setting the pin to a 0 output, then perhaps switching to ADC and detecting whatever difference there is between a touched pin and untouched pin.