False microwave sensor activations in a confined space

I purchased a few RCWL-0516 microwave presence sensors to switch on an extraction fan. The sensor was intended to be placed in the false ceiling space above the Gyproc in the shower. The sensor tested perfectly thru a piece pf Gyproc in my living room after adjusting the sensitivity resistor, however when placed in the confined space above the shower ceiling it false activates continuously, even at lowest sensitivity. I have tried screening the back and sides of the sensor, but this has not helped. I am unaware of any interference as nearby fans and downlights were switched off.
Has anybody had experience of this issue, and how it may be resolved?
Thanks, Richard

Hi Richard,

Does the behaviour still happen when the sensor is used in the same space but not behind the Gyproc?

It does use a frequency outside of normal Bluetooth and Wifi so they shouldn’t be an issue.
Different materials have different election and absorption capabilities so I’m wondering if the shape/materials of the room are causing a stange reflection that is being detected as a person.

Hi Richard
The gyproc in the ceiling wouldn’t be of the insulating kind with an aluminium sheet backing by any chance would it.

Anyway although the product splurge is delightfully empty of any technical details it is likely to be operating in the several GHz range and you got lucky with your original piece of gyproc. Being Radar anything solid should reflect the signal. If the text is correct and this is Doppler Radar the target will have to be moving toward or away from the sensor.
Cheers Bob

Thanks for your comments Aaron.
Yes, the same behaviour occurs if the sensor is below Gyroc.Yes, the small bathroom is fully tiled except the ceiling, so it seems that excessive reflections may be triggering the sensor even though there is no movement.
I have tested 3 of these sensors and they yield the same results.
I have ordered another microwave sensor with a more directional patch antenna in the hope it may perform better in my bathroom environment.
If that doesn’t work then I intend to try an IR sensor through the Gyproc ceiling, which is not ideal.

Bob, both my test piece of Gyproc and the bathroom ceiling do not have an aluminium layer.
I believe the sensor frequency is 3.2GHz, which should penetrate Gyproc, according to several websites.

Hi Richard
3.2G, that is somewhere the “10cm” radar used on large ships. I was looking at the physical size of the antenna on the units PC board. Looks more like about 10GHz. Where did you get the 3.2GHz from?

Marine radar (which is Pulse Echo as against doppler) has one shortcoming called sea clutter which manifests itself as speckles on the display. Caused by white caps on the sea in windy weather so the target can be really made of anything. This is more prevalent in the higher frequency 3cm (10GHz) units. That is why I find it strange that you say it worked perfectly through a sheet of Gyproc. I don’t think it should have.
Cheers Bob

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Hi Bob, Raspberry Pi Pico RCWL-0516 Microwave Radar Proximity Sensor Arduino IDE | Random Nerd Tutorials and All About RCWL-0516 Microwave Radar Motion Sensor : 15 Steps - Instructables both say the frequency is 3.8GHZ and can penetrate non metallic materials.

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Hi Richard
3.18GHz NOT 3.8GHz
Yes the incident wave might but the reflections could be anything. If you were using this for point to point communications yes it might be OK but if you are expecting reflections as in radar I believe you need free air line of sight so you can have some control over what is happening.

Some of the text in that link is a bit strange. Early in the article it says it detects “any objects” (as long as the object is moving) which means you will get a reflection off “any object”. Fair enough, that is OK.
Later it says it works through walls etc. The author should make up his mind here. Even though the wall is not moving WRT the radar unit it will still reflect a signal which would probably be strong enough to override any reflected signal from a moving object on the other side of the wall. The article says this unit works through walls without any degradation in performance. Leave me out of it, Any reflected signal off an intervening wall must surely confuse the issue somewhat. A bit hard to predict with any certainty exactly what would happen here.

My humble thoughts. Save yourself some trouble and don’t operate through a wall.
Cheers Bob

It would be interesting to see some performance numbers on that antenna at 3.2GHz

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Have you tried any type of shileding to solve this problem?