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From: moroney@ramblr.enet.dec.com
Newsgroups: sci.electronics
Subject: Re: IR receiver on traffic lights ?
Message-ID: <1991Apr12.211315.17786@engage.enet.dec.com>
Date: 12 Apr 91 21:09:10 GMT
Sender: news@engage.enet.dec.com (USENET News System)
Distribution: na
Organization: Digital Equipment Corporation
Lines: 33

In article <1991Apr12.063837.7120@bradley.bradley.edu>, sony@buhub.bradley.edu (David Lennie) writes...
>In <1991Apr12.042157.6685@engage.enet.dec.com> moroney@ramblr.enet.dec.com writes:
> 
>>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 am intrigued (obviously).  Explain furthur.

Most police and firetrucks nowadays have electronic flashers instead of the
old style light-with-rotating-mirror they had several years ago.  The tow
trucks, etc. still use the old style (cheaper).  The newer flashers are
timed electronically, often with a somewhat complex pattern, and the timing
isn't dependent on the battery voltage (as long as it's strong enough to run
the apparatus)  The timing on the sensor must be coordinated with the timing
of the emergency lights, so random tow trucks don't set them off while the
police cars/firetrucks do.

Even if the tow trucks occasionally trip it, no bug deal (as long as the driver
doesn't learn this and start abusing this knowledge)

-Mike