Here's a little video showing something Finbarr Murphy put together for his classes here at UL. It's based on the light pen ideas of Johnny Chung Lee from CMU, and you can see the thing working below. The basic idea is to use an led light source, tracked by a wiimote, to translate movements of the led pen into images on a computer, displayed on the projector you see. So it's like you're writing on the wall. For about 40 euros.

Light Pen Doo Hickey from Stephen Kinsella on Vimeo.

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