Need help making a timer

[ZETSUEI1]

What I would like to make is a circuit that will time a car going down a pine wood derby track. What I was thinking is a micro-switch that will start the circuit and the timer when the car leaves the starting post and then stops the clock when the car finishs the track. I would like the stopper to be a sensor that registers when the car is under it. I'm just not sure where I should start. Sorry if it doesn't make a whole lot of sense just can't seem to think straight tonite.

 

[dknguyen]

I'd start with the switches...the timing seems pretty easy since I think you can just hack a stopwatch and run some wires out (maybe through an intermediary circuit to interface with the buttons on the stop watch).

Maybe the sensor could just be something that detects when it's dark (ie the car is over it) assuming that the sky doesn't change brightness very quickly like the sun burning out or something like that, it should work.

 

[philba]

I would use photo transistors to start and stop the timer(s). Drill holes in each track at the start and end lines. Place photo transistors in each hole, pull the collector high with a 10K or so resistor. Make a structure that goes above the starting and ending lines which holds LEDs, one per track. The LED would cause the PT's collector to be low when no car is over it and high when the car blocks the LED. The signal from the PT at the starting line starts the timer running, the PT signal at the end stops it.

I would use a smallish uC like a PIC to time them. I'd use a notebook computer and send the results to it for automatic tabulation. You could, of course, use an LCD display driven by the uC to display the results stand-alone.