# detecting a magnet

Discussion in 'Electronic Design' started by Brian, Feb 3, 2005.

1. ### BrianGuest

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?

2. ### Don PearceGuest

Roughly proportional to how fast the magnet is moving - give us a
clue.

d

Pearce Consulting
http://www.pearce.uk.com

3. ### BrianGuest

say it crosses the plane in 2 seconds.

Are there ways of detecting a stationary magnet?

4. ### Don PearceGuest

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

The magnitude of signal you get depends on stuff like how strong the
magnet is.

d

Pearce Consulting
http://www.pearce.uk.com

5. ### Spehro PefhanyGuest

Ah, flat statements are (almost ;-)) always wrong. What about a
fluxgate senor? Or a reed? Or magnetoresistive (eg. GMR) sensor?
There are probably others I've forgotton

But the first would require core material, and the others don't use
the coil that the OP posited.

Best regards,
Spehro Pefhany

6. ### Bob EldredGuest

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

7. ### Gregory L. HansenGuest

A fluxgate magnetometer (Google search terms) can probably be built easily
enough by the hobbyist. You need a high permeability material like
mumetal, a few solenoids, and electronics.

E.g. http://beale.best.vwh.net/measure/fluxgate/

8. ### Leon HellerGuest

A SQUID will work, as well. They work best when they are very cold.

Leon

9. ### JoergGuest

Hi Spehro,
Yep. Reed is really easy, just take a reed relay and use only the inner
glass body with the contacts. That's how I usually did it. It cost next
to nothing but that might have changed. Of course, it'll age after
umpteen thousand cycles and once I had a reed contact break off and
float about in the little glass tube.

Such contacts can also be purchased separately with proper mounting
holes, matching screws and all. They are used in alarm systems to detect
when an intruder starts to move a sliding window (the magnet slides
away). Possibly a store like Radio Shack has these.

Regards, Joerg

10. ### BrianGuest

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. Basically
thinking of ways to have a flat "mat" and detect the presence of something
within 2-4 feet directly above it.

In the end, I do it cuz I am a geek

11. ### John LarkinGuest

Semi-seriously, you could use a compass and an led-phototransistor
sensor. You might resolve milligauss that way.

John

12. ### MarkGuest

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

13. ### MarkGuest

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

14. ### BrianGuest

Much smaller scale, much smaller target, proportionally much longer range.

15. ### Guy MaconGuest

Dang! Now I have to throw away my chunk of iron glued to a switch!

16. ### Charles EdmondsonGuest

System used by 3M was to basically use it to tune an oscillator. When
filter for the frequency, and detected when it changed enough to no
longer pass signal. 3M just used a digital counter to measure the
frequency. When count changed by about 5, a car was there!

17. ### Ken SmithGuest

A SQUID would work. PNI makes a magneto-inductive sensor that uses a
nonlinear core to detect the field at a lower cost than a fluxgate.

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.

18. ### Ken SmithGuest

I think 2 toroids inside two solenoids will work better.

You place a toroid inside a solenoid with its axis at right angle to the
bore of the solenoid. You drive the toroid with an AC current that just
saturates it in each direction. The solenoid coil will develop a voltage
proportional to the external field at twice the drive frequency.

Such a design will have trouble with picking up stray AC fields. If you
place 2 side by side, drive the toriods at 90 degrees to each other and
wire the coils to subtract, you get more than twice the signal and a lot
less pick up.

19. ### Bob EldredGuest

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