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From: aalonzo@eagle.wesleyan.edu
Newsgroups: sci.electronics
Subject: Re: IR receiver on traffic lights ?
Message-ID: <1991Apr15.090530.41753@eagle.wesleyan.edu>
Date: 15 Apr 91 14:05:30 GMT
References: <1991Apr12.042157.6685@engage.enet.dec.com> <1991Apr12.063837.7120@bradley.bradley.edu>
Distribution: na
Organization: Wesleyan University
Lines: 36

>>Some systems have a photoelectric sensor aimed at the traffic lanes and a phase
>>locked loop circuit that is synchronized to the flashing emergency lights on
>>the emergency vehicles. When the PLL detects the flashing lights in a certain
>>direction, it turns the light green in that direction.
> 
> Wouldn't this also detect the flashing lights on construction vehicles
> or tow trucks?
> 
> Does the sensor simply pick up the presence of any flashing light or
> does it confirm that it is actually a police or fire vehicle by
> measuring frequency of flashes or something similar?  If it does
> measure the frequency, wouldn't it be susceptible to error due to
> slight variances in the flash frequency caused by low or high battery
> voltage in the vehicle?


I worked for a while in the engineering department at Whelen Engineering, a
large manufacturer of warning & strobe lights for emergency vehicles.  They
made a special white, directional, rapid flashing strobe light that would
trigger recievers at intersections.  The
system  I believe was developed by Xerox?.  What it consisted of was
a rapid flashing strobe light (200 - 300 flashes per minute) aimed at the
intersection.  A reciever at the intersection decoded the high flash rate and
turned the proper signal green to allow passage of the emergency vehicle.  The
flash rate of the strobe on the firetruck could be maintained pretty stable
(+/- 3 FPM), and was directional and aimed at the stop lights.    
 
It would be pretty unlikely that the random flashes of a tow truck, etc. would
match the trigger frequency for the sample period required to turm the light
green.  I hope this sheds some light on the subject....)

______________________________________________________________________________
Allen Alonzo
Computer Service Supervisor
Wesleyan University
AALONZO@EAGLE.WESLEYAN.EDU