I can see fitting a small magnet to the inside of the plastic roller drum, or drilling a hole for it in the end cap (bearing housing) of my rollers, and then gluing it in place. Sorry to not be more clear the first time around. I agree that it would eventually fling off the outside of the drum.

I am totally interested in checking out the VI you wrote for this. I didn't really think a parallel port could sample that fast.

Can you send it to me @ lyledriver at gmail?