Path: utzoo!attcan!uunet!wuarchive!sdd.hp.com!hplabs!hpfcso!hpldola!paul
From: paul@hpldola.HP.COM (Paul Bame)
Newsgroups: sci.electronics
Subject: Re: Sources of Cheap, but good Motion Detectors
Message-ID: <10960037@hpldola.HP.COM>
Date: 20 Oct 90 00:39:50 GMT
References: <10572@hubcap.clemson.edu>
Organization: HP Elec. Design Div. -ColoSpgs
Lines: 33


> Alsoditionally, I'd like to know if anyone comes
> up with a RELIABLE way of sneaking through the field. Everything I've tried
> failed, but if there's a way, I WANNA KNOW ABOUT IT. After all, it's MY sec-
> urity system, too. I want to know about the holes....

Most of the units seem to trigger due to changing light levels on the
IR sensor.  They use this funny plastic sheet to focus the IR in
such a way that the visual field is essentially broken up into zones -
each optical "zone" is separately focused upon the detector.  When something
moves across "zones", the detector sees a pulse.  Some units may require
only one pulse, some count 2 or more pulses (within some time limit)
before triggering to prevent false alarms (like a lamp coming on within
their field of view).

So, either never move between optical zones or do so very slowly.

I've had some success approaching a sensor slowly from afar without moving
laterally across the field of view.  This is risky since most of them seem
to also divide the vertical view into fields as well - though not as many.
I've also had luck moving VERY slowly laterally.  In units which require
more than one pulse to trigger, you should be able to move slowly enough
that they think they're just seeing lots of false triggers.

I've made up some of this but it's probably close - try it.

Blinding the sensor with an intense IR source might work - or at least
the de-sensitization might help the other schemes.



			-Paul "Spice is the Variety of Life" Bame
			paul@hpldola.hp.com	N0KCL