Hi, I am making up a Velocity Counter using a photo diode. I have planned it to be in the following way: I will be having two ends, on both of those ends I will be having photo diodes on one side and laser lights falling on the diodes from exactly opposite sides. Now I will make a toy car move, once the toy car intercepts the first light beam, I want my counter to start the time immediately and as soon as it intercepts the other beam, I want it to stop. Can anyone guide me please. Thanks....!!