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The light in my bathroom is controlled by a radar motion sensor. The idea was that radar based sensors can be hidden inside the lamp itself and thus be invisible (so no sensor, no switch, only the lamp is visible). Unfortunately, since it's a small bathroom and some of its walls are made of plaster, the radar sensor detects movement in the corridor as well, turning the light on every time somebody walks by.

Could the radar motion sensor's detection zone be shaped somehow? As the sensor is located on the ceiling, narrowing the detection cone would do the trick. Unfortunately, the device itself only offers control over the light duration, maximum darkness level when it still activates and the sensor's sensitivity (setting this to a low value helps a bit, but is inconvenient because you have to do an elaborate hand dance to turn the light back on again on longer toilet sessions... and it still sometimes detects people in the corridor).

I tried making a cone out of aluminum kitchen foil, but it didn't seem to work at all. Any other ideas?

  • Apparently radar motion sensors are also called microwave motion sensors; I managed to find what appears to be my exact sensor model here: sharkward.com/enproductshow.asp?ID=38 – Gregor Petrin Dec 28 '12 at 7:10
  • Is moving the detector feasible? Does it detect movement in the hall with the door shut? – The Evil Greebo Dec 28 '12 at 12:52
  • There's one possible location to move the sensor but I'll probably need the electrician to move it there. Will try after new year's when he can come around. Otherwise the idea is for the sensor to be hidden somewhere so I can't move it around freely. – Gregor Petrin Dec 29 '12 at 22:25
  • The link you provided says that the reach can be modified from 1m to 8m, is this feasable on yours? – Lima Oct 16 '15 at 3:26
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    The company probably changed their webpage because the sensor listed at that link currently is PIR and doesn't even look that similar to mine. Otherwise my sensor stopped working over a year ago - I was tweaking it so often that the plastic knobs for setting the sensitivities got disconnected from the electronics. After fixing that and assembling the sensor I must have touched some wires together and the sensor was destroyed in a loud bang as I turned the fuse back on :) – Gregor Petrin Oct 16 '15 at 4:06
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I think your strategy of making a cone out of aluminum foil was on the right track, but your choice of material was not. Think about what happens when you put various materials in the microwave. Paper, glass, ceramic: no effect, these will be ineffective. Aluminum foil: unintended consequences, do not use. Then consider what the microwave itself is made out of. You are essentially making the window (which is a person window, the negative image of a sensor window) This suggests the use of a grounded metal mesh or foam. Locally in NYC, the SpyStore sells copper-nickel mesh fabric for unstated purpose. You should also be able to find self-adhesive EMI/RF shielding foam at various industrial suppliers such as McMaster-Carr. You will want to wire either material to ground. I would start by trying to disable the sensor completely by this method (of covering it with grounded metal mesh or foam.) Then open a small aperture in the opposite direction of the corridor. Increase the size of the aperture until the functionality is as you desire. Use white self-adhesive paper label stock cut slightly larger than your masking material to return the sensor to an all white appearance if that is important.

  • I did not look at the size of the sensor hemisphere but if it corresponds in size to similar appearing things I have seen then you might try lining one-half of a ping-pong ball or an "acrylic hemisphere" with the grounded metal mesh or foam and the process of incremental removal. The finished appearance is tidier and it gives a way to keep that material in the correct shape without having to go to the effort involved in cutting a flat material to cover a spherical object. – Aaron K Sep 29 '18 at 6:04
  • Hello, and welcome to Stack Exchange. The reason aluminum foil is bad in the microwave is that the foil blocks/absorbs the hundreds of watts of microwave power, which heats the foil destructively. Assuming this person's motion sensor isn't putting out hundreds of watts of power (I doubt it's even putting out one), that shouldn't be the issue here. – Daniel Griscom Sep 29 '18 at 15:52
  • Thank you, I have since switched to an IR sensor that is quite visible (and ugly :) ), but I might try your suggestion when I feel like tinkering around the house. In fact at work I'm just tackling a project where we'll need to simulate bad connectivity to a small device and your advice will be similarly handy! I'm from Europe, not North America, but I'm sure I'll be able to find the materials you mentioned. Thanks again! – Gregor Petrin Oct 1 '18 at 6:59
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It's a bit difficult. In theory you could potentially change the antenna and change the radiation pattern but you would have to be an engineer to know what you're changing and how.

Some like this from RS actually have some adjustment for orientation and sensitivity:

http://docs-asia.electrocomponents.com/webdocs/12ad/0900766b812ad06a.pdf

Then one like this has no adjustments, however seems to have coverage in almost all one direction.

http://docs-asia.electrocomponents.com/webdocs/12ad/0900766b812ad053.pdf

So depending on floorplan may be able to have coverage say towards outside or similar that may work ?

http://cableplan.ie/downloads/pdfs/microwave_presence_detector.pdf

Is another adjustable one.

The issue is that microwave can penetrate and if you look at the specs of a lot of these they can and do cover a large area. There may be a way of applying something to the walls whether conductive paint, a mesh, metal sheeting or so on. Lot of effort though to work around a sensor.

It is a bit of an issue, some PIRs can change orientation and/or have ways to mask part of mirror or lens to change the coverage pattern in addition to sensitivity.

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The problem is that even if your foil cone changed the radiation pattern of the microwave antenna in the sensor so that it no longer transmits energy in the direction of the wall, some of the other microwave energy in the room is going to be reflected by the surfaces that it hits (and some absorbed or passed through, those surfaces). It's a bathroom, so you probably have mirrors, and they reflect microwaves just as well as they do visible light, adding to the problem.

You may be better off replacing the radar detector with a PIR-based occupancy sensor, either a separate switch or part of the light fixture. If you don't want to change the wiring, there are adapter kits available, and I've also started to see PIRs built into the light bulbs themselves.

  • My problem is that a regular PIR doesn't detect presence through glass and I would like the sensor to remain hidden inside the lamp for aesthetic reasons. – Gregor Petrin Dec 29 '12 at 22:27
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I use the inexpensive microwave occupancy sensor

https://www.amazon.com/Solu-Microwave-10-525GHz-microwave-Detectors/dp/B00ZWPKO5E/

There are similar, but costlier offerings. This has only 180 degree detection angle. I mounted it on a partition wall made of ply wood, and it does not respond to people moving on the other side of the wall.

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