Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84 exptools; site ihlpa.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!ihnp4!ihlpa!dob
From: dob@ihlpa.UUCP (Daniel M. O'Brien)
Newsgroups: net.cycle
Subject: Re: Smart Traffic Lights?
Message-ID: <734@ihlpa.UUCP>
Date: Mon, 22-Jul-85 09:10:29 EDT
Article-I.D.: ihlpa.734
Posted: Mon Jul 22 09:10:29 1985
Date-Received: Thu, 25-Jul-85 00:22:22 EDT
References: <658@ihu1g.UUCP> <2637@ihuxf.UUCP>
Distribution: net
Organization: AT&T Bell Laboratories
Lines: 28

> > All kinds of stuff about stupid smart lights.
> > 
> > 				Rick Schieve
> > 				...ihnp4!ihu1g!rls
> 
> Does anyone know how these sensors work?  I had all ways assumed it was
> tripped by weight until a friend said he always thought they were 
> magneticly tripped.
> 
> 				Mike Nelson
> 				ihuxf!cuda

The sensors are just a giant inductive coil (3 ~ 4 turns) buried in the
ground. The coil is part of a tuned R-F network. When your car 
(and occasionally your motorcycle) drives over the coil it detunes the 
network causing a current draw which is detected by other parts of the
circuit causing a relay to close.

I have the best luck tripping the lights if I stop, not centered in the 
middle of the giant coil, but over an edge (with my front and rear tires
on top of the edge of the coil (most coil loops are square)).

---
			Daniel M. O'Brien (ihnp4!ihlpa!dob)
			AT&T Bell Laboratories
			IH 4A-258, x 4782
			Naperville-Wheaton Road
			Naperville, IL 60566