Almost any lapcounter can be scaled down. Here are two circuits for you, but reminding first that scratchbuilding timing sensors requires some basic knowledge of electronics.
First one has been built and tested with "Laptimer" software. I used it with good success for timing Biltema cars that are maybe 1:43. Note that there is no exact value for CAP. This capacitor can be left out if very fast response is desired. If you have false triggerings you should add small capacitance, maybe 47p first. Component SFH213-FE(FA) is the sensor, nice feature being that it has built-in IR filter so it's very tolerant to ambient light disturbance. Components acquired from Farnell Electronics.
Second circuit is not tested but should work. Contains very basic components but is more sensitive for disturbance.
Both circuits should be suitable for HO. I don't recommend them for 1:32 use. Good luck with your project!
I built a lap counter for my (now dismantled) Tyco 1/64 track using Maplin counter units triggered simply by magnetic reed switches glued under the track. Unfortunately, I can't see the counter modules that I used listed on their site anymore - just this more expensive "Multi-functional Up/Down Counter" by Velleman
Still, it might be worth investigating if you fancy turning your hand to a bit of electronics.
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