Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!oakhill!craig
From: craig@oakhill.UUCP (Craig Shaw)
Newsgroups: sci.electronics
Subject: Re: Seeing in the dark (sort of)
Summary: Pulse count is better
Keywords: Show me
Message-ID: <2669@sol.oakhill.UUCP>
Date: 28 Nov 89 15:00:20 GMT
References: <1018@ariel.unm.edu> <2663@sol.oakhill.UUCP> <10190@ucsd.Edu>
Reply-To: craig@sol.UUCP (Craig Shaw)
Organization: Motorola Inc., Austin, Texas
Lines: 40

In article <10190@ucsd.Edu> brian@ucsd.edu (Brian Kantor) writes:
>IR sensors for alarm systems are often available in a version which does
>not operate its signalling contacts until it has been tripped multiple
>times.  That is the kind we have around here; they do trip when people
>walk past them but do NOT trip from glints of sunlight off windshields,
>flashbulbs, etc.

  The original post referred to security lighting sensors, at $15 - $17.
True, alarm system sensors are more reliable since most incorporate
pulse count techniques. They are also more expensive (if you find one
for less than $45 let me know!). None that I have seen seem sufficiently
weatherproof either.

  The false alarms will probably still be a problem with partly cloudy
conditions. The sun may be gradually uncovered (over a period of several
seconds) and could provide several pulses to trigger the sensor.

  Failure to trigger is also a potential problem if furry animals are
to be detected when the climate/setting provides a background radiation
level similar to the animal's. This is less likely to be a problem for
larger animals, but could be very significant for animals the size of
cats and smaller. My cat has never managed to set off my alarm system,
even though the IR detector beams extend to the floor. The PIR detector
I have is not of the pulse count variety, hence it should be more easily
triggered.

  Furthermore, the PIR manufacturers warn installers to be sure the
detection area doesn't cover windows, because they are prone to rapid
solar heating. Also, motion outside might be able to set off the alarm.

  Finally, if anyone can describe a commercial product (at any price)
that uses passive infared detection to detect people or animals during
the DAYTIME, I will be interested to hear about it. The only reason
this technology works on lighting controllers is that they only need
function at night, and failure to operate is not a big deal.

Sign me - Still Looking.
-- 

Craig D. Shaw - MCU Systems Design (512)891-2245  Motorola Inc., Austin, TX.


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