As a matter of curiosity, I had to test out an assumption, that these LJ12A3-4-ZBX (NPN NO 4mm) proximity switches are pretty much bullet proof (unless you run end-on into them).

I connected 3 of them up to a 15v power supply and used an LED and a 4k7 resistor as an indicator. If you common the outputs and trigger one sensor all 3 indicator LEDs on the sensors light and the output goes to GND to indicate a Home run. No smoke, no drama.

Connecting the output to GND lights the indicator goes on, even though the sensor is not triggered, but the unit is not damaged.

Connecting the output to +15v and triggering the sensor - OUCH, I thought I had caused it to fail, but a few minutes later it was back in service.

So maybe these units are idiot proof by design, but it seems difficult to turn them into smoke generators.

Cheers,

Rob