I had to get the Arduino timer mounted on the track so Liam can run some tests for his science expo before this weekend. Wednesday, I drilled holes in the sides of the track at 25cm from the top and 30cm from the bottom to mount the 'light beam' LEDs on the right side and the photoresistor sensors on the left side (as you look from the top of the track to the bottom.) Used hot glue to position the LEDs in the holes. Then I powered the LEDs so I could see exactly where the beam of light hit the opposite rail. That's when I drilled the holes for the sensors. Then I hot glued the sensors in place.
Since I need to get Liam to do his measurements soon, I just wired the LEDs into the breadboard to feed the Arduino inputs using jury rigged wires for now. It's not pretty, but it works. I ran his pinewood derby car down the track and took the first measurement at about midnight last night! Yay! :-)
I'm getting approximately 1.036 seconds for it to run with no extra weight, plus or minus about 0.006 seconds. I repeated the run about 15 times and got readings from around 1.029 to 1.042 seconds. Not too shabby.