drag racing timer circuit

[johns1588]

I would like to build a drag racing timer for pinewood derby cars. I planned to use 2 Banner photo sensors per lane, one at the start, and the other at the finish. I want to have a display similar to that of 1/4 mile drags; winner and mph. Showing the winner is easy, my problem is how to show mph.

 

[fat-tony]

You'd probably need radar to measure max MPH. Or a whole string of optical sensors, but that'd probably be considerably more hassle than necessary.

 

[BartSimpson]

Drag racing time-slips usually show what is called the "trap speed", or the speed at which the vehicle crossed the finish line. One way to do this easily would be to have two sensors at the finish line. One actually on the line, (which will double as your "winner" detector ) the other say 12 inches past the line. Simply time how long the derby-car takes to travel the last foot and calculate it from there.