Phil Hobbs wrote in
:

On 2/21/2010 8:41 PM, David Nebenzahl wrote:
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.)


Close--pyroelectric, and all pyroelectrics are also piezoelectric,
afaik.

NOPE. PIR=Passive Infra-Red.

--
Jim Yanik
jyanik
at
localnet
dot com


Porch light sensors use a pyroelectric split cell, with the two
halves wired in series, and a MOSFET connected to the point in the
middle. The MOSFET usually has a 10M gate leak resistor.

The Fresnel lens in the front is made of high density polyethylene
(HDPE), which has about 40% transmittance in the 8-14 um band. It is
actually arranged as segments of several lenses (about a dozen or so),
so it casts a dozen images of everything in the field. Warm things that
move horizontally cause warm images to move across the split detector,
making an AC waveform. The FET's output is AC-coupled into another
amplifier that drives the triac.

Cheers

Phil Hobbs