[Al Skierkiewicz]

b,
What you are asking for is not a simple circuit. You need something that is programmable, capable of taking sensor input, using it for a program that controls a motor driver of some type. Lego Mindstorms is just such a product. This controller is capable of several sensor inputs and several motor outputs. It is easily programmed via supplied language with the kit. Unfortunately, it is designed for special motors that are also supplied in the kit. Actually, some of the motors also contain tachometer sensors that allow you to actually count rotations and use that to feedback into the program.