a.k.a. WSITEM-201521 (or WSITEM-201591).
An over-engineered 12V plant light timer.
When initially turned on, will connect the power (negative terminal; positive is always connected) to the 'out' terminal for 12 hours, then disconnect for 12 hours, and repeat.
Short-pressing the button (less than half a second) resets to 12 hours of on. Long-pressing (more than half a second) resets to 12 hours of off.
May be expanded upon later to also measure and report temperature and humidity. At which point it will no longer be v2.1, but v2.2 or something.
May also serve as a template for other 'stupid little microcontroller projects', including documenting it with a web page like this one and a QR code, etc.
Arduino code: x-git-object:19de97654cd60dbe1101e28e6da79bb397b031b1#RelayTimer/RelayTimer.ino.
Fritzing file: PlantLightTimer2.1-wiring.fzz
3D-printed parts: WSITEM-201438 / p1800 (panel), WSITEM-201500 / p1808 (case).
"WSITEM-201591" on the label was a typo, and should have been "WSITEM-201521". To avoid having to make a new label or update the above photo, I have defined 'WSITEM-201591' as being an alias for 'WSITEM-201521'.