Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!pacbell!sactoh0!americ!erk
From: erk@americ.UUCP (Erick Parsons)
Newsgroups: sci.electronics
Subject: Re: Car Detectors for Traffic Lights
Message-ID: <0542.AA0542@americ>
Date: 3 Jun 89 00:00:47 GMT
Followup-To: sci.electronics
Distribution: na
Lines: 67


><Does anybody know how the car detectors for traffic lights work?  Are
><they tuned circuits?  If so, what frequency?

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

	Ill give this one a try though it's been a few years since I've
actually read any literature on vehicle detectors. I may be wrong
on some points but overall it is sound.

	Basically you will have two type of vehicle detectors in any 
*Modern* traffic actuated signal system. There are magnetometer
detectors and loop detectors. The magnetometer detectors are very
straightforward in operation and theory and are likened to proximity
sensors used in industrial control systems. It is a high current
device and therefore low impedance making High power a necessity
to induce into it. A costly, bulky and possibly futile project. 
These are being phased out in my county (Sacramento) and being
replaced with:

	Loop detectors, or Induction detectors are a reletively new type
of sensor (as compared to magnetometer det's) and work on the 
principle of changing the resonance of a tuned coil of wire in the
street so as to cause a small frequency shift at the sensing device
in the controller cabinet. The loops are buried in about 3-4 inches
of asphalt and connect directly to the detector (electronic device
that senses the inductance change). There is not a whole lot of wire
to induce into and coupled with the fact that asphalt makes a Da*n
good ground a radio transmission would appear futile for tripping
one of these beasties. (Our local Elec. Co. SMUD sometimes uses the
roads for ground conn's to their system by sawcutting the asphalt
and laying their bare ground wire in the sawcut covering it and 
running it to their ground point on the power pole). The detector
connected to the loops also has some built in features (uP cntrl)
that will try to avoid any bogus vehicle calls being sent to the
controller equipment. An average detector will have such features
as automatic cancellation of a gradual buildup in conductive material
placed in close proximity to the loop i.e. rain, snow, garbage 
muffler parts whatever, that appear to be smaller than a bicycle.
Larger pieces that could cause the detector to think that a vehicle
is parked on the sensor will be cancelled out after a fairly long 
wait, I would guess about 10 minutes. As to your question what
frequency do they run at, there is no set frequency, the loops
cannot possibly be made exactly the same any two times and coupled
with the fact that there will probably be 4 to 8 loops on one 
channel the error multiplies. On the other end the detectors
are constantly adjusting for frequency drifts, have 3 frequency 
settings and 10 sensetivity levels. I wouldn't bother trying
as you will in all likelyhood pull your hair out ITP,  In the
event that you do find a way to mount a 500KW transmitter on your
car (truck, big rig) and do manage to trip the sensors through 
some sort of wizardry you will in all likelyhood trip all of the
other sensors and cause the I/S to cycle through before serving
you, probably locking up the system completely for the ~10 minutes
it takes to cancel junk on the loops and pissing off the City,
County, State  personnel that maintain and repair these buggers. 

Relax:
Don't hurry be wappy :-)
And remember to give those boys (and girls) a brake.


--
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  Erick Parsons    //    Words for the wise:   *If it works don't fix it.*
  Sacramento Ca   //        mail to:   ...pacbell!sactoh0!americ!erk
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