Microwave vs motion detector

January 27, 2009 1:50 PM

Why would my mum's (inside) microwave set off her (outside) motion detector?

My mum had a light with motion detector installed in her back yard a while back. For practical reasons [easy access to electricity at security-light-level] this was installed within a foot or so of her microwave . Obviously with a wall in between (microwave inside, motion detector outside).

But now, whenever she uses the microwave, the light comes on [only during darkness, i.e. when the motion detector is on]. So basically it interprets this as "motion". [after the microwave is done, the light remains on for the customary 30secs]

What could be the reason for this?
(NB I'm not so much looking for "how can we fix this" - she's had a switch installed to disable the light while she uses the microwave. tho if there's sth really obvious please do share)

Thanks!

posted by ClarissaWAM to home & garden (9 comments total)

The Wikipedia article on motion detectors mentions that some of them work using microwaves. I offer no explanation - however, I can offer my suspicions that it is probably not a coincidence.
posted by Brettus at 2:05 PM on January 27

I wonder if the illumination source, IR or microwave, isn't dimming some when the microwave kicks on, enough to trigger the sensor, since all it really detects is a rapid change in received rays, whatever type they are. It's unlikely that microwaves are making it thru even the flimsiest of external walls.
posted by nomisxid at 2:40 PM on January 27

If the motion detector, as suggested in Brettus's link, works by sending out microwaves and listening for echoes, microwave leakage from the oven could quite sensibly be interpreted by the sensor as a received echo. Or microwaves could actually be echoing off of moving objects.

You might be able to shield the detector from the oven by putting a piece of aluminum foil or chicken wire between the oven and the wall, or between the motion sensor and the wall, or both.
posted by fantabulous timewaster at 2:49 PM on January 27

Some possible candidates that may or may not be plausible, but which I don't know enough to be able to rule out:
- electric noise on the same power supply as the sensor is using,
- microwave emissions reaching sensor
- EM interference with sensor circuitry,
- Vibration
- Any other candidates?

I suggest you apply some SCIENCE!

Hypothesis 1: it might be electric noise on the same power supply as the sensor is using
Test ideas:
- Operate the microwave after plugging it into a wall socket on the far side of the house via an extension cord. Observe nightlight.
- Operate an appliance of similar load (eg a kilowatt or more) on the wall socket the microwave is currently using. Observe nightlight.

And so on. Invent tests for each candidate hypothesis until you've narrowed things down to one and confirmed it with a predictive test.
posted by -harlequin- at 3:30 PM on January 27 [1 favorite]

I'm guessing nomisxid has it--microwaves draw a lot of power, especially when the microwave emitter clicks on. It seems most plausible to me that, if the motion sensor is on the same circuit as the microwave (do they both turn off when you click off the microwave's circuit breaker?), this drop in power is sort of reseting the motion detector.

If you want to apply SCIENCE!, try plugging a lamp into the plug next to the microwave's plug and turn it on before you start the microwave. If the lamp dims next time she makes a Hot Pocket, then this is probably your answer.
posted by wondercow at 4:43 PM on January 27

Is it near a window? It could actually be sensing motion inside the house
posted by Gungho at 5:51 PM on January 27

Thanks for your replies everyone.

The electric noise is an interesting suggestion as I actually think the microwave is a bit broken, I always used to get really bad electric shocks from it as well.

I'll be back to mark best answers after running these by my mum (not sure how far she can test as the micrwave is built in and I suspect she may not have easy access to the socket).
posted by ClarissaWAM at 8:23 AM on January 28

If you're getting electrical shocks from the microwave, it is too dangerous to use. Even when unplugged, those things have capacitors (say, 2400V). It's too easy to imagine a wet food/wet floor situation that could send a charge through a person's heart and kill them instantly.

Used microwaves are pretty cheap. Don't risk it.
posted by dhartung at 8:37 AM on January 29

(just back to mark some best answers as I clean up. for the record, this was never satisfactorily resolved)

Not marking wondercow as it was later determined that the power source for microwave and motion detector is not shared.
Not marking gungho cuz motion detector is NOT set off by movement in the kitchen, which is much more frequent than microwave use.
Not marking Brettus cuz IIRC we found out these motion detectors are not powered by microwave.
And not marking dhartung cuz (much as I appreciate the concern), that was not the question.
posted by ClarissaWAM at 4:21 PM on May 22