This is a Spring Boot application accessible via standard web browser that sits on a Raspberry Pi and is connected to a photocell. When the model car passes the start line, the photocell is interrupted and the timekeeper displays lap times and total race time. See it in action on youtube.

1. Get a photocell with open collector transistor output, like this one.
2. Connect photocell power and ground as needed, and its output to pin 13.
3. Build fun-timekeeper.jar using maven or download the latest release.
4. Launch the timekeeper on your raspberry using java:

   java -jar -DlapsPerRace=3 fun-timekeeper-1.0.jar

5. Open http://address-of-your-pi:8080/ in the browser.
6. Click Start Race and put pedal to the metal.

To clean the records remove races.json and restart the app.