Re: Indoor tabletop motion detector???

Apr 01, 2005 1 Replies
J

I read in sci.electronics.design that snipped-for-privacy@hotmail.com wrote (in ) about 'Indoor tabletop motion detector???', on Fri, 1 Apr 2005:



Well, it's not that simple. The IR from the LED is short wavelength
>IR, about 800 nanometers. The IR that the PIR motion sensor senses is
>long wave IR, maybe 7000 to 15000 nanometers. So it's like pointing a
>VHF transmitter at an AM radio and expecting it to receive something.
>Ain't gonna happen.
>
>However I could put a spark coil in place of the LED and light a gas
>jet or something. That should do it. Probably burn the place down
>too! :-o
>

Replace the LED with a resistor that gets warm?



(s.e.marketplace removed as irrelevant)


Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
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W

a

Yeah, I thought about that, too. One problem here is that the PIR circuits that I've seen have two things. One is a coupling capacitor that's between stages. Another is the PIR sensor itself which has two sensors and the output is the difference of the two. I guess that's to prevent a parked car's warm hood from swamping the sensor and preventing it from turning off. A resistor would be much like that, with no apparent motion. So if you have a _moving_ warm body, it'll trigger the sensor.

I'm not sure if lighting a small lamp in front of the sensor would do the trick. If it was sensitive to that, the lamp that it turns on, once on, would swamp the sensor and it would never turn off.

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