Fifty Shades of Gray Code

Some years back, a museum asked me to help them with an exhibit a contractor had built for them. It was a wheel like the one on Wheel of Fortune, but smaller and mounted on the wall instead of the floor. You would spin the wheel, it would stop on some item, and a computer would play a short video about the item. Physically and mechanically, it was a beautifully built exhibit. The electronics, though, left something to be desired.

In principle, this is pretty simple computer task. Measure the position of the wheel, and when it stops moving, play a video based on the position. The problem was the folks who created the artistic mechanics didn’t think hard about the electronics behind it. Sometimes–but not often–the wheel would play the wrong video. Sometimes it wouldn’t play at all.

The Prime Suspect

My immediate suspicion turned out to be correct. I took the wheel off its mount to discover copper foil tape on the back of it. Each pie wedge had foil in different areas and there were two brushes in each area. When the wheel stopped, two of the brushes would be shorted together and the rest were open. The way they detected that was bizarre, but that wasn’t the problem. (It involved a cannibalized PS/2 keyboard.)
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