Re: detecting a magnet
From: Brian (brian_at_w3gate.com)
Date: 02/03/05
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Date: Thu, 3 Feb 2005 17:35:56 -0600
Much smaller scale, much smaller target, proportionally much longer range.
"Mark" <makolber@yahoo.com> wrote in message
news:1107471453.151065.22920@f14g2000cwb.googlegroups.com...
>
> Bob Eldred wrote:
>> "Brian" <brian@w3gate.com> wrote in message
>> news:VJadnZKl8qofnp_fRVn-tA@centurytel.net...
>> > If I had a flat, 2 foot by two foot coil lying flat, how far above
> it
>> could
>> > I reasonably expect the detect a rare earth magnet, perhaps the
> size of an
>> > M&M?
>>
>> An ordinary inductive coil requires the magnet to be in motion and
> the
>> voltage induced is a function of the number of turns and the rate of
> change
>> of the flux, dphi)/dt. Therefore the speed of a moving magnet is very
>> important. Practically you might be able to sense the magnet moving a
> foot
>> away or so depending on the number of turns in the coil and the speed
> of the
>> magnet. It is a signal to noise issue and power line interference
> (hum)
>> would limit what you can detect.
>>
>> However it is possible to detect small stationary magnetic fields
> (DC) by
>> arranging and energizing coils in a device called a flux gate. These
> things
>> can be designed to measure fields as low as 1/1000 of the earths
> magnetic
>> field or even lower, less than a milligauss and could easily measure
> your
>> magnet many feet from the fluxgate. Being DC, it's easy to get rid of
> the
>> power line interference. There is also hall effect devices which has
> been
>> mentioned but they are not particularly sensitive. Beyond that, there
> is a
>> quantum mechanical device called a "squid" that can measure
> extremely small
>> fields like the field from the iron in a single blood cell. These are
>> probably way beyond what you need or could pay for but it gives you
> an idea
>> of the technology available. What are you trying to do?
>> Bob
>
>
> I'm guessing, maybe wrongly that the OP is interested in those vehicle
> detection loops burried in the ground at red lights. These sometimes
> fail to respond to motorcycles and some people sell magnets that you
> can attach to your motorcycle that alegedly allow the coil to see your
> motorcycle.
>
> I believe the principal of these coils is a tuned resonant tank that is
> detuned by a large hunk of metal. I don't believe that the magnet
> helps in this case but there are people out there that will swear that
> they do work.
>
> Mark
>
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