I'm currently working on https://beontimer.com a free timing app for running and cycling races.

It started off because I wanted to see if I could print QR codes on a piece of paper and use these to detect people crossing a lap if finish line.

That proved more difficult than I thought, the QR codes were not easy to scan from far away and while moving. It is still in alpha stage.

What does work is a simple manual mode for people to use at their races.