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
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