TT, Lets see how this sounds to you.
1- single lane
2- start and stop time detected with photo cell
3- 4 digit resolution.
4- AC powered (wall wart)
You will need to be pretty handy. It will be a lot of wires to hook up. Do you have any equipment for troubleshooting?
If not it might be better to get most of it off the shelf.
Pinewood Derby Track, Pinewood Derby Timer Headquarters
I would have to do it with hardware cause I'm a Microprocessor illiterate. Maybe a u guy will come up. It would be better.