I gotta give a shout out to the humble potentiometer. We picked up a handful last year, and they proved super usefull this year too. They wire directly to an analog in Port, and can essentially give you abaolute position of a mechanism. Getting some with more than 300deg of rotation help protect it in case of over rotation.
Here's two versions I recommend keeping around:
Continuous rotation:
http://www.digikey.com/product-detai...-103-ND/274005
10turn:
http://www.digikey.com/product-detai...03L-ND/2058386