Solar panel voltage detection triggers motor to run off solar powered batteries. For use with gearbox to spin crystals that refract rainbows.  

When do you want your crystals spinning? When the sun is shining through them - Therefore, why complicate things with sensors, microcontrollers etc. Why not drive a motor directly off the solar panel? I don't know if it'll work, but I can't see why not - so I'll be giving it a try. (currently the voltage from the solar panel is not enough to drive the MOSFET I'm using reliably - I'll try alternatives and adjustments)

How fast do you want them spinning? Probably not that fast. I'm talking a few RPM. Motors spin at an enormously higher rate than that. (PWM is one way to slow down your motor, but I tried it and it's got a limit on how slow it can go. It was still too fast.) So we're going to build a gearbox, and your crystals will hang from the gears as they spin in a dreamy slow manner.