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From: fred@umcp-cs.UUCP (Fred Blonder)
Newsgroups: net.analog
Subject: Re: Car Detectors
Message-ID: <2662@umcp-cs.UUCP>
Date: Wed, 30-Jul-86 01:49:10 EDT
Article-I.D.: umcp-cs.2662
Posted: Wed Jul 30 01:49:10 1986
Date-Received: Thu, 31-Jul-86 21:40:17 EDT
References: <2974@watvlsi.UUCP> <4839@topaz.RUTGERS.EDU> <992@kitty.UUCP> <4347@srcsip.UUCP>
Organization: Computer Sci. Dept, U of Maryland, College Park, MD
Lines: 33
Summary: Question: How can I intentionally trip a car detector?


    In article <992@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:

	In article <4839@topaz.RUTGERS.EDU>, josh@topaz.RUTGERS.EDU writes:

	What's the circuitry behind the buried-wire car detectors at
	stoplights?  Technical details please.

    The buried wire vehicle detectors used with traffic lights work
    on the principle of an inductance bridge.  The buried loop is
    one arm of a low frequency inductance bridge (generally 100
    KHz to 200 KHz excitation).  When a vehicle is present within
    the loop, the inductance increases from its free air value by
    virtue of the ferrous metal in the vehicle acting as a "core".
    This change in inductance is detected by a bridge circuit in
    some signal devices, and by detuning the frequency of an LC
    tank in an oscillator in other devices. . . .

Question: Would it be feasible to build a transponder-like device
(preferably out of parts available at the local Radio Shack) to
carry on my bicycle so I can trip traffic lights when I'm riding
home late at night and there are no cars to trip the sensors? I've
found that the inductive-loop car detectors around here don't detect
my bike frame unless I dismount and lay the bike down in the street
on top of the loop, which is not only a bother, but dangerous as well
if a car DOES actually come along then (Murphy's law). Some lights
do not have a safety timeout, or maybe it's just absurdly long.

Does anyone have any idea of the legal implications of such a device?
-- 
					Fred Blonder (301) 454-7690
					harpo!seismo!umcp-cs!fred
					Fred@Maryland.ARPA