by ohneschuh @ instructables.com:
Capacitive sensors are an elegant way to control an Arduino using the Capacitive Sensing Library. But the sensitivity and error tolerance depend strongly on the hardware (sensor) design. I found a design guideline here and tested different setups which mostly work well if the Arduino was powered by battery. But the sensor signal changes dramatically if I connect the Arduino to a power supply.
Actually I found a design for five (and more) sensors which works well powered with battery and power supply.
Capacitive Sensor Design – [Link]