Solar charger, nice idea. I've been thinking of a POV display for car tires, but still working on a few details. I was considering centrifugal force to activate, the switch would be a small weight attached to a spring, maybe the guts of a ballpoint pen, since I would need a tube to enclose it. Basically, it would only turn on, while the car was in motion, and should turn off, regardless of orientation while stopped. The part I'm stuck on though, for POV, is that I need to detect rotation speed, and position, without using the car body. Maybe something IR and reflective. Been kind of on the shelf lately though, my new car doesn't have hubcaps, which was sort of important.