In order to add the equivalent of a mouse's scroll wheel to a trackball, Instructables user "hilukasz" has used an Arduino and a Sharp infra-red distance sensor to read the location of a hand above the trackball. The data received by the Arduino from the IR sensor is then sent back to a PC running processing, which takes care of the scroll action. Here it is in action:
So that's a different and somewhat obtuse method of adding another form of an interface to a computer. Click here for the tutorial, and for more, we're on twitter and Google+, so follow us for news and product updates as well.