detecting a magnet

[Guy Macon]

There is a
musical instrument called a Theramin where the frequency of an oscillator is
varied by waving ones hand over an antenna. This is beat with another
oscillator producing an audio tone played through an amp and speaker. You
can hear these things in space movies and other places where weird etherial
music is used.

_Good Vibrations_ by the Beach Boys.

 

[Rich Webb]

[snip...snip...]

BTW: A good flux gate will detect a cow magnet up to about 100 feet. A
SQUID only improves on this distance by about a factor of 20.

A cow magnet? This would be a magnetized cow, possibly one that has a
large uptake of iron from feeding on ferrous soils? Or a magnet used to
stick a (small) cow to a (large) refrigerator?

....clickety... google ...clickety...

Well, by the Wrinkled Sack of Zeus! I have lead a sheltered life.

 

[Brian]

[snip...snip...]

BTW: A good flux gate will detect a cow magnet up to about 100 feet. A
SQUID only improves on this distance by about a factor of 20.

A cow magnet? This would be a magnetized cow, possibly one that has a
large uptake of iron from feeding on ferrous soils? Or a magnet used to
stick a (small) cow to a (large) refrigerator?

...clickety... google ...clickety...

Well, by the Wrinkled Sack of Zeus! I have lead a sheltered life.

For those of you without "hick" experience....
Lotta metal aroud for cows to pick up and eat. Ya stuff a big magent down
their gullet, catches the debris before it perforates something

 

[Brian]

Somebody below mentioned a car detector that used the metal locator
principle of changing an oscillators frequency when a metal object (car)
alters the field of a coil. This wouldn't work very well on a dog or a
nonmetalic box but changing frequency by capacitance would. There is a
musical instrument called a Theramin where the frequency of an oscillator
is
varied by waving ones hand over an antenna. This is beat with another
oscillator producing an audio tone played through an amp and speaker. You
can hear these things in space movies and other places where weird
etherial
music is used. In any case you could make a dog detector that worked on
this principle; i.e., varying the frequency of an RF oscillator by
changing
the capacitance in a tank circuit. An oscillator running at several
hundred
kilohertz having an insulated metal plate connected to its tank could
easily
detect a dog or other nearby object by change in frequency. No magnets or
metalic objects required. Do a search on Theramin and see if that idea
might
meet your needs.
Bob

I have played with the capacitance detection in the past. Range stinks.

 

[Active8]

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.

They're called metal detectors when you put them on the end of a
stick. Beat it against a ref and listen to it click.

I don't believe that the magnet
helps in this case but there are people out there that will swear that
they do work.

Maybe as the cycle pulls up to the light, the coil senses the flux
that wasn't there a split second ago (dphi/dt) and it gives the
detector enough bump to make some noise. I've seen dumb detectors
switch the lights after a right turn on red. Annoying little
bastrds, they are.

 

[Mike Fields]

I am tryning to accomplish "mental exercise". Sometimes I see things and it
just makes me ponder how to do it.

This "application" could be many, but is similar to RFID I suppose. Tagging
something, but without identification and a completely passive tag. Could be
to see if a box is on a pallet, or a dog is in his doghouse.

Easy - just run a cat past the dog house to see if he is home ... (also
counts
as a "cat scan" )

 

[Mike Harrison]

A Hall Effect sensor is the only option for a stationary magnet. It is
also the best choice for such a slow moving magnet.

Or a GMR sensor - more sensitive. NVE and Philips make them.
For ultimate sensitivity, try a fluxgate magnetometer.

 

[John Fields]

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.

 

[John Woodgate]

I read in sci.electronics.design that John Fields <jfields@austininstrum

---
If you believe that you're right and that the people who disagree with
you are wrong, prove that you're right. Get a magnet, demagnetize it,
run some frequency VS proximity tests and then get it magnetized and
run the same tests over again.

At audio frequencies, you might find a difference, due to the differing
incremental permeabilities of the magnet when demagnetized and
magnetized. But the difference could well be quite small.

 

[Andrey]

If you don't want to mess with coils, think about solid state sensors.

They go, in order of increase of sensitivity:
Hall Effect sensor - GMR sensor - Magnetoresistive sensor - Magnetometer.

Solid State Magnetometer (like the one made by Honeywell) is made around
Magnetoresistive sensor(s) but with well thought signal conditioning which
increases it's range. It will give you 2 - 3 feet range for the magnet you
describe.

You should think in advance what you gonna do about stray targets and false
triggering.

Andrey


Magnetometer.

 

[Ken Smith]

Or a GMR sensor - more sensitive. NVE and Philips make them.
For ultimate sensitivity, try a fluxgate magnetometer.

No, for the ultimate try a low temperature SQUID. The have a noise of
something like 10fT / sqrt(Hz).

 

[Ken Smith]

Easy - just run a cat past the dog house to see if he is home ... (also
counts
as a "cat scan" )

The reaction depends on the dog. I think feeding the dog radioactive food
and using automatic radiation detector would work better: That would be
Radio-Acitve Dog Automatic Ranging (RADAR)

 

[Guy Macon]

I think feeding the dog radioactive food and using automatic
radiation detector would work better: That would be
Radio-Active Dog Automatic Ranging (RADAR)

Those who have tried this have posted glowing reports.