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Discussion Starter #1
Re: [EVDL] EV Digest, Vol 3, Issue 8

For anyone needing to be sensed by an inductive loop sensor in the
road bed you can use any metal loop three to six feet in diameter
placed over the sensor loop. The circuit is just looking for a change
in frequency of a tuned circuit made from the loop and a capacitor in
the loop detector box. The loop may be iron, copper, or aluminum. All
will cause a change in the inductance of the sensor loop.


[email protected] wrote:

>
> Message: 11
> Date: Tue, 2 Oct 2007 17:24:17 -0500
> From: "FRED JEANETTE MERTENS" <[email protected]>
> Subject: Re: [EVDL] Sensors at drive throughs
> To: "Electric Vehicle Discussion List" <[email protected]>
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset="iso-8859-1"
>
> well they could be using a reflective light emitting relay like
> is used on
> production lines . however the most likely device is a proximity
> sensor .
> ----- Original Message -----
> From: Lee Hart<mailto:[email protected]>
> To: Electric Vehicle Discussion List<mailto:[email protected]>
> Sent: Tuesday, October 02, 2007 12:29 PM
> Subject: Re: [EVDL] Sensors at drive throughs
>
>
> Dewey, Jody R ATC COMNAVAIRLANT, N422G5G wrote:
>> People have said that for years. They sell the magnet for
>> motorcycles
>> to do just that. Most don't work. I thought they were actually a
>> fiber
>> of light that gets pinched when weight is on the slab.
>
> The usual traffic sensor is a loop of wire buried in the
> pavement. It
> forms an inductor. The circuit measures the inductance of this loop.
> When a vehicle drives over it, the iron and steel in its chassis
> increases the loop's inductance. This is sensed to trigger the
> "vehicle
> present" indicator.
>
> A permanent magnet won't have any effect, except that due to the
> small
> amount of iron it adds. Water, snow, wet leaves etc. only have a
> trivial
> effect. Interestingly, conductive non-ferrous metals like copper or
> aluminum *decrease* the inductance of the loop. My old Comutavan had
> plenty of iron, but had even more aluminum and so did not trigger
> these
> vehicle detectors.
>
> If you need something really small and light to trigger it, you
> could
> use a loop of wire with a variable capacitor across the ends.
> You'd have
> to tune it to a lower frequency than the traffic sensor was
> using. When
> your coil passed over theirs, it would pull their frequency down to
> trigger it. Unfortunately, this method only works for that certain
> make/model of traffic detector. There is no "standard" frequency for
> these things, so they vary from place to place.
>
> [When I was in college, they had a traffic sensor to control the
> gate to
> enter the parking lot. As a prank, we made a loop out of coat hanger
> wire with a capacitor across the end, and tuned it to be
> detected. The
> loop was duct taped to the gate itself. Every time the gate went
> down,
> it detected a car, and opened it. Now it didn't see a car, and
> closed
> the gate, which went back up again, etc. The gate went up-down-up-
> down
> until the car counter thought the lot was full, at which time it
> stopped
> letting more people in. People were confronted with an empty lot
> and a
> sign that said "Full"!]



Mike Swift
Two things only the people anxiously desire, bread and circuses.
Decimus Junius Juvenalls




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Discussion Starter #2
Re: [EVDL] EV Digest, Vol 3, Issue 8

Do you think a ferrite wound inductor and capacitor would achieve the
same result in a small package?
--
Martin K

Mike Swift wrote:
> For anyone needing to be sensed by an inductive loop sensor in the
> road bed you can use any metal loop three to six feet in diameter
> placed over the sensor loop. The circuit is just looking for a change
> in frequency of a tuned circuit made from the loop and a capacitor in
> the loop detector box. The loop may be iron, copper, or aluminum. All
> will cause a change in the inductance of the sensor loop.
>
>
>

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