What about an array of photodiodes like this: o o o o X o o o o This is in an enclosed box with a hole over the center. Here, "o" represents a photodiod

What about an array of photodiodes like this: o o o o X o o o o This is in an enclosed box with a hole over the center. Here, "o" represents a photodiode and "X" represents the center of the hole. When the solar panel is pointed directly at the sun, the sun is at position X and no photodiode is lit up. As the sun changes position, eventually it will light up one of the photodiodes. That completes a circuit which uses power from the panel to drive a motor, which is attached to a worm gear and rotates the panel towards the sun. Eventually the sun will be pointed at the X position again which breaks the circuit and stops the rotation. Each diode completes a circuit, driving one or both motors (in the case of the corner diodes) rotating them appropriately. It'd be interesting as well to have some sort of mechanism which, when the panel receives power, keeps it locked in place. When the panel has no power for a period of time (ie: after sunset) the mechanism disengages and the panel falls back to its default position pointing towards sunrise.