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In article <18811@cup.portal.com>, mmm@cup.portal.com (Mark Robert Thorson) writes:
> Does anybody know how the car detectors for traffic lights work?  Are
> they tuned circuits?  If so, what frequency?

	The very first vehicle detectors were treadle switches that were
recessed into the pavement.  Needless to say, the constant mechanical
stress and effects of weather did not contribute to the reliability of
such detectors.

	During the 1950's when electronic car detectors first saw widespread
use, the detection methods were primarily ultrasonic and microwave.  Various
reliability problems with such detector methodology have resulted in the
the magnetic disturbance detector being the favored mode of detection.

	Magnetic disturbance detectors operate through a buried loop of
wire.  There are various schemes in use, some of which use two coils, and
others use a single coil.  The simplest technique involves a single coil
inserted as one arm of an inductance bridge, with the vehicle metal mass
cauing an increase in inductance.  A more reliable technique uses two
coils, with the vehicle metal mass causing a change in permiability of
the "air" core.  Since both of these magnetic techniques require AC
excitation and since the coils are large, the excitation frequencies in
use range between 50 and 150 kHz.  I have an older (mid 1970's vintage)
Eagle Signal unit in my driveway as an announciator, and it runs around
75 kHz.

	I have also seen a Hall effect sensor for vehicle indication
which requires only a single point for burial in the roadway, as
opposed to cutting a 360 degree loop in the pavement.

> More importantly, can they be activated from a distance?

	On the assumption that a magnetic disturbance detector is involved,
the answer is most likely no.  If memory serves me correctly, the Eagle
Signal unit that I have uses a coincidence circuit which requires detected
signals to be in-phase with the excitation oscillator, thereby minimizing
the probability of external interference.

	I won't even begin to ponder why you pose this question. :-)

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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