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From: moroney@ramblr.enet.dec.com
Newsgroups: sci.electronics
Subject: Re: IR receiver on traffic lights ?
Message-ID: <1991Apr12.042157.6685@engage.enet.dec.com>
Date: 12 Apr 91 05:21:18 GMT
Sender: news@engage.enet.dec.com (USENET News System)
Distribution: na
Organization: Digital Equipment Corporation
Lines: 16

In article <1991Apr11.211957.7309@lynx.CS.ORST.EDU>, youngqd@jacobs.cs.orst.edu (Dean Youngquist) writes...
>The fire trucks in my town seem to trip traffic lights to green in
>their direction as they are approaching the intersection.  Does anyone
>know how they do this?  I thought they might have a system using
>infra-red light ?  If it is infra-red how can they work in the day-time?
>Doesn't the sun put out infra-red that would interfere ?

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.

Other lights may use radio triggers, or (especially those right near fire/police
stations) controlled right from the station.

-Mike