I replaced the motion detector by the front door and I took the old one apart just to see what was inside. Cracking it open was a destructive process - the case ended up in a pile of little plastic bits. I am puzzled as to how it operates. I thought motion detectors mostly relied on ultrasonics, but there is a plastic shield that goes in front of this thing. Plastic case, plastic horn, plastic shield.
 |
| The emitter / sensor is the small round can with the black rectangle on top in the center of the board. |
|
 |
| Plastic feed horn, from the back. |
|
 |
| Plastic feed horn in place on top of emitter / sensor. |
My guess : it's a horn antenna for the ultrasound (think ear-flap). Echoed sound gets picked up by the horn, fed to the transducer. Sound frequency is heterodyned with the transmitted signal and the (Doppler) difference fed through a low-pass filter for amplification, rectified and used to trigger the alarm if above threshold.
ReplyDelete