This has been a pretty long-running hobby project of mine: A real world traffic light that shows the status of our CI server. I think I maintained and overengineered this for almost a decade and implemented it with various hardware and programming languages.

The newest iteration is done in Elixir, with the help of Nerves and Phoenix.

Client (on a Raspberry PI): https://github.com/fabrik42/traffic-light-client-elixir

Server: https://github.com/fabrik42/traffic-light-server-elixir