This very simple sketch shows how to implement a so called QTouch ADC one wire touch button. I always meant to turn it into a more user-friendly library but never did. Fortunately there are now two libraries that seem to work very well and even have some advantages over my approach. You can find these libs here or here.
Please use one of these libraries and concider this code as a relic.
QTouch ADC is a technology by Atmel Corporation offered in form of a free precompiled library but unfortunately currently only for the attiny 40. QTouchADC touch measurement is extremely fast, robust to EMI, reliable touch detection, very very easy hardware setup and most importantly: only one pin per touch button or three per small slider or wheel. The pin needs to be an analog input pin. So, on the Uno you have 6 channels and eight on a Mega (I believe), leaving all the digital pins for other cool stuff!!! This is in contrast to the standard QTouch where two (digital) pins are needed per channel.
Another alternative is the well known Capacitive Sensing Library (http://playground.arduino.cc//Main/CapacitiveSensor?from=Main.CapSense) which uses one pin per button plus one extra pin that can be used for all buttons together. The hardware setup for this method is more complicated and I didn't have much luck with the accuracy of the measurements.
So, in summary, QTouch ADC IMHO is very cool!
I took inspiration and hints on the working principle and programming techniques from these pages:
http://www.mikrocontroller.net/topic/260841
http://tuomasnylund.fi/drupal6/content/capacitive-touch-sensing-avr-and-single-adc-pin
http://www.atmel.com/Images/doc8497.pdf
But it basically follows the code given here:
http://www.mikrocontroller.net/topic/156809#2975291
At first I thought it would be possible to charge/discharge the sample&hold capacitor of the ADC unit via an internal reference. But then I realized that this is not done by the original QTouchADC library either: Application Note AVR3001 states that if only one touch button is needed it needs a "partner" adjacent pin that cannot be used for anything else. This pin is then used to provide the GND and Vcc input to the s&h cap. When more than one touch channels are used any channel that is not currently being probed can provide this function.
To make the code work you will have to solder a two cent coin to a 1k resistor. The other end of the resistor goes into Arduino port A0. You can watch the raw touch values at the serial monitor or make them visible with an LED connected to pin 9. Make sure the LED has an appropriate resistor to limit the current (resistance= (5V - LED_voltage)/LED_current). This basic setup can be seen in this video: http://youtu.be/-30wSuzNkvg
The working principle and programming techniques are explained in the comments in the source code. The code is a simple sketch (no library yet, sorry). For efficiency it makes direct use of atmega registers and ports. So, it's not directly portable. But porting, changing or extending it should be no big problem as the principle is very very simple indeed. I think the ADC registers and constants are the same for every atmega used in Arduino boards. Only the digital port C associated with the ADC ports might need to be changed.