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detecting a magnet

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

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  1. Brian

    Brian Guest

    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 Pearce

    Don Pearce Guest

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

    d

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

    Brian Guest

    say it crosses the plane in 2 seconds.

    Are there ways of detecting a stationary magnet?
     
  4. Don Pearce

    Don Pearce Guest

    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. 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 Eldred

    Bob Eldred Guest

    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. 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 Heller

    Leon Heller Guest

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

    Leon
     
  9. Joerg

    Joerg Guest

    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. Brian

    Brian Guest

    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 Larkin

    John Larkin Guest

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

    John
     
  12. Mark

    Mark Guest


    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. Mark

    Mark Guest


    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. Brian

    Brian Guest

    Much smaller scale, much smaller target, proportionally much longer range.
     
  15. Guy Macon

    Guy Macon Guest

    Dang! Now I have to throw away my chunk of iron glued to a switch!
     
  16. System used by 3M was to basically use it to tune an oscillator. When
    metal entered the loop, the frequency changes. Traditionally, you had a
    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 Smith

    Ken Smith Guest


    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 Smith

    Ken Smith Guest

    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 Eldred

    Bob Eldred Guest

    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
     
  20. Put a solenoid on a crank and spin it around.
     
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