Literature

"Getting in touch with capacitance sensor algorithms"
An overview of different electrical approaches and detecting algorithms.
http://www.eetimes.com/design/embedded/4008781/Getting-in-touch-with-capacitance-sensor-algorithms

"AVR-based Sensor Keyboard"
DIY dual terminal touch sensor with Cx and Cs in series.
http://sensi.org/~svo/capsensor/index.en.html

"Simple Touch Sensor"
DIY single terminal touch sensor with Cx and Cs in parallel.
http://elm-chan.org/works/capsens/report_e.html

"Klingel"
DIY single terminal touch sensor with Cx and Cs in series.
http://www.das-labor.org/wiki/Klingel

"The art of capacitive touch sensing"
Single terminal touch sensor with Cx and Cs in parallel.
http://www.eetimes.com/design/analog-design/4009622/The-art-of-capacitive-touch-sensing