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When I turn my microwave oven over the stove on, it will cause a motion sensor light in the hallway next to the kitchen to got off and on. This affect can be reproduced anytime. I did notice that we stand in front of the microwave instead of out of the room, the motion sensor light goes on then off then on repeatedly.

How is the microwave affecting the motion sensing light that is about 4 ft. away behind a plaster wall? Is it dangerous?

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More on microwave ovens and other electronic devices: – Qmechanic Mar 7 '12 at 15:49

Most likely is electrical noise being coupled back into the mains wiring from the microwaves power supply. The microwaves are generated by a magnetron that uses high frequency and high voltage supplies, these are probably noisy in a cheap consumer appliance.

Fridges were always notorious for spoiling hifi listening when the compressor motor kicked in and sent spikes down the mains wiring.

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Your explanation is probably correct as well, but this may be interesting: As a kid, I once built a radio transmitter. I didn't have a good antenna, so I experimented with a long wire. I once wired it directly to the receiving antenna of a commercial radio. I remember that the microwave caused static in the radio in this case. – Manishearth Mar 7 '12 at 16:51
Now I don't know what the microwave did exactly, and I'm not sure if it would have happened when the wire wasn't hardwired but just held in the air, but it seems that a microwave can tamper with EM stuff as well. Motion sensors use IR. Then again, it could just be some microwaves(dont think that they can escape;but may be wrong) hijacking the long wire. – Manishearth Mar 7 '12 at 16:55
@Manishearth - probably RF noise form the magnetron. You build consumer devices to 'only just' pass FCC/CE regualtions – Martin Beckett Mar 7 '12 at 16:55
Aah yes; a device that passes regulations shouldn't get confused by magnetron noise &c. Hopefully ;) – Manishearth Mar 7 '12 at 17:06
@Manishearth - I meant your home made radio was probably pickign up a wide band of RF noise from the magnetron - especially at close range. The light motion sensor is probably responding to electrical noise coupled onto the AC power line. Especially since its circuitry is built to be even cheaper than the microwave! – Martin Beckett Mar 7 '12 at 17:16

CW (continuous-wave) motion detectors do use microwave frequencies.

Some of them (like this one on Amazon) transmit at 2.45 GHz (the microwave oven band). It's possible you have one of those installed and that the normal leakage from your oven is being detected as motion.

Normal microwave oven leakage is about $2 mW/cm^2$ which is probably within the detection range of a radar detector.

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