For the last 2 days I have experimented with capacitive sensing using the method described in Atmel’s Qtouch ADC guide: charge division between external capacitor and internal sample and hold. Things are a little bit trickier that with buttons: normally a capacitive sensor drifts, therefore it needs to be constantly adjusted. For a button it is not an issue, as you expect it to be not pressed most of the time. So, you just compare the current readout with some average over the past and you are done. For the Sit.Up sensor it needs to be able to detect a “button press” for longer periods of time, so the button methods don’t work. I believe I solved this problem differently, but more on that later.
The new sensor can be a lot smaller than before, this new board sits on top of 2 AA batteries, and will be hidden under the chair anyway.