Just wondering if anyone with a newborn who has those Angelcare baby monitors with sensor pad.
Has anyone opened up motion sensor pad? My wife wont let me and I am dying to know how it works. There are only 2 wires and I cant imagine its too hitech but it seems to detect baby breathing movements through a thick mattress???
Never seen one, is it firm to the touch or spongy?
If the former, my guess would be pressure/resistive, if the latter then possibly an air bladder with passive pressure sensor? Which is cheaper? Have to watch out for punctures if it's a bladder...
-- Chris.
Its springy. The slightest tap it measures about .4 to over 2v AC for a sharp tap. Not sensitive to sound, just heavy vibration. I thought piezo but in that case wouldnt need a moving spongy plate which what this is. Its designed to set off an alarm if baby doesnt move (breathe) in a 15 sec period.
piezo's not too good at low freuency. could be a magnetic pickup? is there any evidence of a permanent magnet - does ant part of it deflect a compass needle.
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So it *produces* a voltage without external supply/excitation?
yes correct, without anypower it produces up to 2v acpeaks . the pad is 2 splitplastic pads connected together on some sort of flexible gymbal with a few mm of rocking movement. I cant open the pad without damaging it.
I read this as 2 inflexible pads with, my guess, a flexible piezoelectric film/ribbon (such as
-- FWIW, Chris.
cth Inscribed thus:
MIL has a similar device that warns us if she gets out of bed, though I suspect that its some kind of pressure sensitive device. Its very similar to an electric under blanket in appearance.
How about a big dynamic mic, with the spongy pillow part being the diaphragm. This would generate a voltage proportional to movement.
Someone is going to use one of these as a kick drum microphone one day.
maybe its among this stuff:
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