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Sunday, November 13, 2011

Zombie Detector

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.

1 comment:

Ole Phat Stu said...

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.