Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site ut-sally.UUCP Path: utzoo!watmath!clyde!bonnie!akgua!gatech!ut-sally!crandell 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