I'm thinking of hacking up something simple to check toilet occupancy in an office. Ideally, the system should be unobtrusive and out of the way.
I know you can use a PIR sensor to track moving objects (What sensor to choose to track human presence indoors?) – however, if somebody is sitting still on a cubicle, that will probably not register since they're stationary.
Apparently parking spot sensors use both IR and magnetic fields to detect cars (http://www.nedap.com/business-units/identification-systems/parking-detection-makes-sense/). Obviously the second part of that won't work on people.
I know you can get those IR-based sensors that bounce a beam off a reflector, however, I'm hoping to be able to mount just one sensor on the ceiling, if possible.
Is there another type of IR or ultra-sound sensors that might work here?
Has anybody got any particular parts or approaches they could suggest? Any caveats I should be aware of for this use case?