Not related to robots directly, but useful in many situations, here is a new little project I've been working on yesterday: a simple digital counter, that uses a reed relay and a little magnet placed within a coil winding machine, to count the actual turns.
The circuit diagram is straight forward . I went for 3.3V , to be able to use the Nokia 5110 LCD:
More details, Eagle SCH and code, available here: http://www.pocketmagic.net/2013/05/coil-winding-machine-counter-with-atmega8-and-reed-relay/
And there is also a youtube demo video: