timing an RC car

I want to make a timing gate for an RC car. I don't want to spend more than $50. My thoughts would be to have a light beam sensor so it would start and stop a timer when the car went through each of 2 timing gates. What would be nice is if I could find a cheap stop watch that has an external trigger attachment or one that I could open and hack into the button with out much difficulty. Any one have any suggestions on this or a better way to time an RC car? The goal is to find out how fast they are going by measuring the distance between the 2 gates and then doing the calculation. But I am also thinking of hooking up one of the timing gates up to my camera to take a photo right as the car passes by, since my camera (Nikon N90S) already has an connector for an electronic remote control I don't think that will be hard once I get the timing gates figured out.

Hi, You have some sound ideas but as you progress you may go over your $50.00 limit. If you use a basic start/stop timer with external switches you may do it. Check these guys out for some ideas :

I would love a schematic, thanks. The speeds will mostly be in the 15 to 40 mph range but some go over 60 so so up to about 70 mph would be good. I don't want to put the gates more than about 50 ft apart. I would like the reading to be accurate within +/- 1/2 of a mph. I can easily put the gates closer if that doesn't effect the accuracy too much. I could move them a little further apart but that would make the whole thing much less useful. If the accuracy falls off to say +/- 2 mph at the high end of 70 mph I can live with that, I mostly want it more accurate in the middle range.

Long time ago when my son was in high school I made some of these timers for his science competition. I used laser pointers for a light source and photo diodes for the receivers. With the laser source I could trigger cmos circuits with just a 20k pull up resistor. The clock was a crystal IC and all of the counters were cmos 4518 dual BCD counters and displays were BCD led displays. Start/Stop/reset latches were cmos NAND gates. The LED displays came from my junk box and the rest of the circuits were under $20. Power was a 1000ma 5VDC was a wall wort. 12V battery and 7805 regulator was used for outside operation. The counter would display up to 99.999 seconds or five stages of counters. Dave

Thanks very much for the schematic John. Eric R Snow