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From: crandell@ut-sally.UUCP (Jim Crandell)
Newsgroups: net.analog
Subject: Re: PIR (passive infrared)detectors
Message-ID: <2201@ut-sally.UUCP>
Date: Wed, 26-Jun-85 23:28:06 EDT
Article-I.D.: ut-sally.2201
Posted: Wed Jun 26 23:28:06 1985
Date-Received: Fri, 28-Jun-85 03:23:47 EDT
References: <81@biomed.UUCP>
Organization: U. Texas CS Dept., Austin, Texas
Lines: 33

> 
>            I was interested in buying a PIR detector to mount on the side of
> my house to detect anyone around the cars in the yard and activate a relay
> to turn on some floodlights for a minute or so.

     At least some of the newer ones work pretty well indoors, although
I haven't heard of any for outdoor installation, and I suspect that most
of the common ones would be problematic there.  To mention a couple of
items, they may not perform well in environments where temperature
varies widely, and most of them don't have too much range.  They easily
ignore the hot asphalt, because it doesn't move (they're motion detectors,
you know).  However, a massive, hot object in the background can obscure the  
IR image of the thing you actually want to sense.  A kitty walking by may
or may not pose a problem, depending on the size and temperature of the
kitty and the sensitivity of the sensor, which can generally be adjusted
over some range (the sensor, that is; adjusting the kitty is not usually
a primary recommendation, especially if it's not yours).  The beam shape
is determined by the optics (lenses and/or mirrors) used in conjunction
with the detector, and it can be just about any shape the designer can
imagine.  One popular shape is the ``fingers'' configuration, which can
be likened to the ribs of a fan.  PIR detectors sometimes appear able
to ``see'' through house windows; many glasses transmit quite a bit of
IR radiation.  The heat-absorbing glass used in most car windows is a
different matter, however, as you might expect.  In indoor installations
it is becoming popular to combine PIR detectors with detectors of some
other type (e.g., ultrasonic or microwave) to improve the reliability
of the indication.  It is not greater sensitivity that is generally
being sought, though, but greater freedom from false alarms (i.e., the
alarm signals are ANDed, not ORed).
-- 

    Jim Crandell, C. S. Dept., The University of Texas at Austin
               {ihnp4,seismo,ctvax}!ut-sally!crandell