I'm a few months late to this thread, but I've run into the same problem that xavi has. Adding a ground (approx. 30cm of copper pipe) helps with some circuit configurations but not the one I want to use.
However, I've found that the circuit can be used as a filter to determine when the capacitance changes. That is, send a square wave through the circuit at a frequency that is either attenuated or passed, and then when the capacitance changes you can detect a difference in the response. I'm doing the following:
- Set up a pin change interrupt on the receiving pin on CHANGE transitions.
- Use the Arduino tone() function to send a square wave on the sending pin.
- Delay for a few milliseconds while counting the transitions on the receiving pin.
- Turn off the tone and disable the pin change interrupt.
If the frequency is close to the cutoff frequency you can see a dramatic change in the transition counts when the capacitance changes. This works both on mains power and batteries. However, the response of the filter changes, so you have to tune the frequency for each power source and touch object. (I've used a foil plate, a banana, and plant fronds. All work well with high sensitivity.) I'm still working on a calibration routine to perform on power-up.
The circuit I'm using is a little different, because I was trying to detect the discharge rate of the charge. I may try different configurations of the resistor and the capacitor. You can see my circuit and earlier tries at http://ideas-tried.blogspot.com/2015/06/capacitive-sensing-on-battery-power.html.