[woodcote]

I use the LapTracker-Auto Timer app on iPhone and iPad for single car testing, rally stages etc. It can be used with multiple cars (up to 4 lanes), although setting up the phone to ensure no false lap counting will take time. Ideally, you want a rigid frame to place the phone over the track, so it doesn’t move and there are no moving shadows to trigger a lap. Something like Lego would be a good prototyping material.

I’d also recommend having mains power nearby so you can plug in a charger - or use an auxiliary power pack. Also use the phone in airplane mode…

An app is certainly a cheap alternative. The raw data must be recorded in a spreadsheet or to a piece of paper. You can do a screen grab of the results for reference. Here’s a single car screen…