[Al Skierkiewicz]

b,
We approach these problems with a little more precision. So we need to know the amount of precision you need. A simple circuit can be developed that will spin the motor in one direction when lit and then spin in the other direction when dark. However, the length of time it will spin (or the number of rotations that it will make) will not be controlled and therefore will not be repeatable from one trigger to the next. The size (output power) of the motor also will determine the complexity of the circuit to some extent. If you are merely turning a small hobby motor like a slot car might have, then a simple circuit may be made using a dual 555 timer, some resistors and capacitors, and two miniature relays in addition to the photocells you have shown. Again, this circuit would very likely vary widely in the action of the motor.