Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!sdd.hp.com!hp-pcd!hplsla!tomb
From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: IR receiver on traffic lights ?
Message-ID: <5170128@hplsla.HP.COM>
Date: 12 Apr 91 16:36:40 GMT
References: <1991Apr11.211957.7309@lynx.CS.ORST.EDU>
Organization: HP Lake Stevens, WA
Lines: 18

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 ?

Systems I've seen used pulsed light; it's easy to look for the AC
component at a particular frequency.  It's also pretty easy to filter
the receiver so it's looking at the wavelength of peak emission of the
transmitter.  The receivers tend to be pretty directional, too:  aren't
they in pipes perhaps 1" diameter and 6" long?

I was always annoyed that empty, "out of service" busses in Santa Clara
county would trip the light and hold up long lines of left-turning
cars for another cycle of the lights.  Busses there used them more
than emergency vehicles!