Clarifying the prior post on this idea. The key is that the motor shaft is pressed own against the outside of the wheel by the weight of the cell, motor, frame, etc... The wheel is something with a rubber outside, like a fat rubber band over cardboard or an o-ring between laser cut acrylic disks. The wheel shaft fits in a vertical slot not a hole, so the shaft slides up until the top of the wheel hits the motor shaft. Motor is driven by a circuit that senses light from blocks displayed on the cell phone screen.