A reed switch on a door. When the door opens, light an LED for five seconds and report it to the server you built in Module 01. No new ideas here, just every decision from this module, made once, in order. Get one wrong and it tells you which fault you just built.
A reed on a clean GPIO read with the pull-up, an LED driven through a resistor, a non-blocking five-second hold, and the event reported to your own server. Every wrong turn you avoided was one of the module's faults.
It runs on a breadboard. It is not deployable yet: the joints vibrate loose and it has no enclosure. Soldering makes it permanent, CAD prints its case, Salvage sources its parts. That is the next skill.