I needed some sort of "rotation" encoder, to be able to use some old handles as an interface.
Previous we have been using potmeters, but they tend to get ruined when subjected to public .. pressure and play.
I came up with the idea to use a simple DC motor - With one terminal connected to the 3.3V output of the Arduino, and the other connected to an analog input. (Through a resistor to protect the voltage regulator)
There is a deeper explanation here: http://christian.liljedahl.dk/guides/simple-rotary-encoder
(and thanks for the permission to cross-post) :-)