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From: moroney@dec-jon.UUCP
Newsgroups: net.analog,net.auto
Subject: Re: Car sensors
Message-ID: <2685@decwrl.DEC.COM>
Date: Thu, 1-May-86 00:30:14 EDT
Article-I.D.: decwrl.2685
Posted: Thu May  1 00:30:14 1986
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Organization: Digital Equipment Corporation
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>> 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.

To pick a minor nit, the "inductive" type actually senses the CONductivity of
metal over it, as a kind of transformer with a shorted turn when a car showed
up.  I found this out when they installed such a sensor in front of the house I
used to live at the corner of a busy city street and a very minor city street.
This sensor could be activated by a ~1 foot square of (non-ferrous) aluminum
foil. This light would turn red ONLY when the sensor was activated (or a
crosswalk button was pushed) so I knew I triggered it. 

One thing amusing about this particular setup, the sensor started about 2 feet
behind a stop line, which itself was behind a marked crosswalk.  Some fools in
small cars didn't seem to know what stop lines or crosswalks were, and stopped
right in the crosswalk, not over the sensor at all (which required a car to be
on it ~5 seconds to trigger) and they would wait for the light to turn green,
which it didn't... eventually they would run the red.  If they stopped behind
behind the stop line in the first place...

Some of the "ultrasonic" sensors are <=20kHz - there used to be one that I
could hear.  It got replaced by the buried wire kind when that intersection
got rebuilt.   Still LOTS of old ultrasonic sensors in the Buffalo, NY area.

-Mike