I swear I've come across a type of tail light that has a coil mounted on the frame and permanent magnets on the spokes. As the wheel turns the magnets pass by the coil and light a red led. I swear I came across this somewhere online but either can't find the correct search term or the product only ever existed in my mind. Either way. I want one. Has anyone else heard of this setup? Link? Any suggestions on where I would start if I wanted to make one?