David Nebenzahl wrote:
On 2/21/2010 5:41 PM David Nebenzahl spake thus:
On 2/21/2010 4:12 PM David Nebenzahl spake thus:
I'd like to get responses, ideally, from folks who actually know how
these things work, and not the usual Usenet speculation.
I install a lot of motion-detector lights for clients. A lot of these
installations are problematic. I just adjusted one for the third or
fourth time because the light was staying on all the time, even though
the sensor unit (I replaced the entire unit with a Heath-Zenity sensor
recently) was working properly.
It might help for me to know how, exactly, these sensors work. By
"sensors" I mean the actual sensor, as well asw the entire unit with the
support electronics.
So I found this circuit out there in the wild:
http://www.freeinfosociety.com/elect...ew.php?id=2074
It uses something called a PIR. Does that stand for piezoelectric
infrared? I've seen other references to piezo sensors: how do those
work? Do they both send and receive a signal? (The device shown here is
a 3-terminal thingie.)
My guess was wrong; it's "pyroelectric", not piezo.
Hey, whaddya know; actually found useful stuff Googling, like this page
that has a pretty good explanation plus a schematic:
http://www.glolab.com/pirparts/infrared.html
Passive Infra Red, meaning that it doesn't have its own IR source.
It works from a pair of IR sensors, and looks for a difference in the IR
level on the pair. That is why it takes a few seconds to a minute for
them to settle when first powered up. Some early PIR detectors came
with several sets of lenses to adjust the beam width & height. You
looked at the manual and picked the one you needed for the desired
operation.
http://www.google.com/search?num=100&hl=en&newwindow=1&safe=off&client=f irefox-a&rls=org.mozilla:en-US
fficial&ei=JeeBS-muLtPX8Aap-PGMBA&sa=X&oi=spell&resnum=0&ct=result&cd=1&ved=0C AsQBSgA&q=Passive+Infrared&spell=1
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