Hi:
I forgot to tell you more about the visible light sensors: The robo ball is a 102 Hz red light source, so at first, I need a active band pass filter.

NOW I'M USING A AT89C2051 INSTEAD OF THE BP. But can anyone tell me more the advantages and the disadvanges (some "unseen" problems) of using this method? (check the time delay of two positive edge of the signal received)

the 2051 is affected by the motor.