[ogig]

I can see some uses, but calling this system batteries free seems a stretch. A sensor is worth nothing if it can't be read, and to read this you need a powered microphone and computing. Some already common magnetic door systems do the same; door plate and magnet movement is enough to create a detectable current, (using no external power), then that signal is read and computed by an electronic/digital system (using power).

[SOLAR_FIELDS]

Even the layout you describe has massive advantages over the status quo from a placement perspective. Having a reduced footprint device that goes in the actual measurement location that can phone home to a more robust central location like this is not only already very common but also the existing solutions that do it still suffer from the design constraint of requiring a battery which this innovation goes a long way towards

[phh]

I'm on the side of "clever, fun, but feels useless". But to defend the project, all sensors require a powered central system. It's pretty common for Zigbee to have one repeater per room [1], which is just what is needed for this system.

[1] Because any AC-powered Zigbee device is a repeater, so just a bulb or a plug is enough