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How does this motion sensor work?
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TheBigT:
Hi Everyone,

I was having trouble with a car alarm that was installed in my car by the previous owner. It turned out that the root cause was an oversensitive motion sensor that was installed under the roof liner (see attached pics). The maker claims that it can detect movement inside the car (https://www.defa.com/product/motion-sensor-25m/), but I've never seen a design like this, and I'm puzzled as to how it works. Can anyone shed some light on its principle of operation?

Thanks,
  TheBigT
SiliconWizard:
Looks like some kind of simplified doppler radar to me.

DaJMasta:
My first guess was using that bar as some kind of deflection measurement, but it's not that flexible.  I know you can use piezo elements for this, maybe the plastic piece under the bar is the key?

I agree the thing on the end of it sort of looks like an RF stub.
glarsson:
Probably a radar implemented using one transistor. See this video: https://youtu.be/9WiJJgIi3W0
Benta:
The brass strip is a vibration sensor (sensor itself must be under the strip) for when the car is being moved/towed.
For the RF part I don't know.
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