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How do you Build a Magnet Detector

Discussion in 'Electronic Basics' started by Cindy, Jan 18, 2006.

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  1. Cindy

    Cindy Guest

    I'm looking for a circuit that can detect a magnet passing through a
    field about 1 foot away from the detector. The magnets are the small
    powerful type as found in harddrives. We need to detect if a person
    is carrying a magnet in his or her pocket or purse.

    Is this possible?

  2. Xtrchessreal

    Xtrchessreal Guest

    I wish you luck on that.

    FYI: You need to design around credit cards, discs, mp3 players, etc as
    to not ruin a persons life.
  3. Ron Hubbard

    Ron Hubbard Guest

    Try a battery connected to a resistor and a LED. Place a reed switch on
    one of the supply lines. If a magnet comes by, the reed switch will
    close and the LED will light. This is simple and effective if you can
    get the circuit reasonably close to the magnets. If you build this,
    make sure it's in a plastic case-- not metal.

  4. Jordan

    Jordan Guest

    A magnetic compass might work?

    Other extreme - a gauss meter.

  5. Jasen Betts

    Jasen Betts Guest

    you could try a compass with wires either side of the needle, the magnet
    deflects the needle and it contacts the wire... it worked for submerged
    sea mines, it's not suitable for a handheld detector (and less than ideal
    for portable) but may work for your aplication.

  6. Bob Masta

    Bob Masta Guest

    Not sure what you mean by a magnet "passing through a field".
    Do you already have some sort of magnetic field present?
    To simply detect a moving magnet, you just need it to pass
    by a coil of wire. The moving magnetic field of the magnet
    will induce a curent in the coil, which you can detect. The
    trick is making the setup sensitive enough. Having a soft
    iron core in the coil will help. You may need to have an
    unusual core shape, such that the face of the coil is
    much taller than wide, in order to allow for people having
    the magnet at different heights on their bodies. Or you
    may want to use a column of some readily available
    relay coils, appropriately wired in series.

    Note that the magnet should pass perpendicular to
    the axis of the coil.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  7. spudnuty

    spudnuty Guest

    Scientific American had an article in their Amateur Scientist some time
    ago. Get it at the library or on their CD. The device detected very
    minute variations in the earths magnetic field. As I remember it was
    like an ultra-sensitive galvanometer. It used a tiny mirror on a
    torsion thread in a magnetic field. The mirror had a laser pointer
    bounced off it. I'm sure this thing could detect a magnet within 10
    feet. A wire loop around a door frame and a sensitive enough detector
    would also work.
    This also gets into the area of violating someone's rights to privacy
    and freedom from illegal search. Anyone scanned would need to be
  8. Cindy

    Cindy Guest

    Thanks for all your replies. I guess I should have told you why... A
    friend of mine works at a video store and they are losing a lot of
    DVD's to someone who figured out the security lock device on the DVD
    case. The security device that sets off the sensor at the door is
    attatched to the DVD case, not the DVD. So, the thief unlocks the box
    using 2 strong magnets, removes the DVD and makes a clean undetected
    getaway. Knowing someone enters with a powerful magnet can give them
    a heads up so they can keep an eye on the person.
  9. Rich Webb

    Rich Webb Guest

    Very possible.

    Look for info on "fluxgate magnetometers." There's a brief intro at, lots of links at, and some
    relatively inexpensive (and easy to interface) gadgets at Also possible
    to roll (wind) yer own.

    It's probably even possible to mount the sensors in such a way as to
    derive real-time X-Y positioning of the magnet.
  10. Ron Hubbard

    Ron Hubbard Guest

    Add to the coil an op-amp with a gain of say ten, and it's output
    connected to a transistor to act as a driver for a LED or a small
    relay. While the magnet(s) moving through the coil will only produce a
    short burst of current, the relay can be used to turn on a simple 555
    oscillator to produce a tone of your choice.

  11. Jasen Betts

    Jasen Betts Guest

    I got sold one with the device still attached, it took me a while to figure
    out how to unlatch it. he hard drive magnet was at-hand fimly attached to
    the refrigerator and did the trick easily, I was wondering if your application
    was related.
    magnets are easy to shield from sensors.

  12. Very possible. Look at magnetometers. A fluxgate magnetometer would
    certainly detect the field from such a magnet but you might be able to do it
    much simpler technology. Look at metal detector circuits and other circuits
    which detect currents induced in a loop or coil.

    Someone might care to comment on the possibility of feeding the two ends of
    a search coil into the + and- inputs of an op amp and detecting the output.

  13. A coil constructed in or on the door frame will generate a current when a
    magnet passes through it. Try an op-amp to detect the current in the coil.

  14. cbm5

    cbm5 Guest

    Have all entering customers roll in a pile of iron filings before they
    are allowed to browse the selection.

    Or, why not focus on the actual problem: one or more people who have
    figured out your system, and are morally OK with stealing. It's probably
    no more than a couple people. Buy or rent security cameras, taking note
    of the criminal's video preferences. Once you have a tape of the
    criminal in the act, alert the authorities to be on standby when you
    spot that person again. Have them wait outside so the criminal can be
    caught with the stolen items...problem is now solved.
  15. Guest

    Have you been building your own electronics stuff in the past? If not,

    then this is probably too complicated for a total newbie. But a newbie
    with lots of ambition might pull it off.

    If I was doing this, I'd put a tiny chip of mirror on a compass, then
    bounce the light from an LED off the mirror chip to a photodiode. Use
    cheap lens to focus the LED to cast a small spot on the photocell, then

    move things so the spot of light is only halfway on the photocell. If
    the compass moves, the photocell output changes. Put it all in a
    light-tight box. To make this more sensitive, just move the photocell
    farther away from the compass so that a small compass deflection moves
    the spot a larger amount (a larger box might be needed.)

    For the mirror chip, I'd smash a cheap front-surface mirror bought from
    surplus optics dealer. That, or try to put a silver or mercury plating
    on a
    microscope slide's cover-slip. Maybe put the whole compass in a cup
    of kerosene or light mineral oil to keep it from wobbling from floor

    Then rig up some kind of alarm circuit with a photodiode amplifier and
    a "window comparator" that fires a piezo beeper whenever the light gets

    brighter or dimmer.

    A totally different method would be to amplify the output of a linear
    Hall chip, then filter out the noise. Hall chips aren't too sensitive,

    but they can just about detect a signal as strong as the Earth's field
    (like a few Gauss.) This might detect a supermagnet from a couple
    of feet away. Or it might not be sensitive enough. The main problem
    is that the output voltage of a linear Hall chip will drift a bit, and
    drift could mimic the magnet signal.

    Linear Hall Chip $2.99

    Another method:
    If you have a cheap oscilloscope, shut off the H deflection and turn
    down the brightness. Use a tiny dot of opaque tape or foil to block
    the little green glowing spot. Temporarily glue a photodiode or
    over the location of the dot. Now if a magnetic field should cause
    the electron beam to deflect, the green spot will come out from behind
    the opaque dot and shine upon the photocell.

    (((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
    William J. Beaty SCIENCE HOBBYIST website
    EE/programmer/sci-exhibits amateur science, hobby projects, sci fair
    Seattle, WA 206-789-0775 unusual phenomena, tesla coils, weird sci
  16. Jasen Betts

    Jasen Betts Guest

    or use a sequin, or a fragment of a blank part of a surplus cd.

  17. Ron Hubbard

    Ron Hubbard Guest

    That's what I just said.

  18. Do you suppose that a multi turn coil, about to metres in diameter,
    shallowly buried under a driveway or path, if connected thus to an op-amp
    would be sufficient to detect a firearm or crowbar carried over it? I
    presume that such a coil would always detect a car?

  19. As sold in the US most DVDs have a security device inside the retail case,
    installed by the maker. Detector loops at the doors will alarm if the strip
    is not cancelled by being magnetised.
  20. Ron Hubbard

    Ron Hubbard Guest

    Yes, if you have enough gain. You can't get that kind of gain from a
    single op-amp stage but two (or more) chained together will provide
    sufficient gain without too much noise. You would also have to pick the
    op-amps carefully to get good gain without excessive noise that could
    drown out your incoming signal. Of course, you could probably do the
    same thing with several transistor amplifier stages which needn't be
    too complicated. If you look through some of the free circuitry
    websites that have all kinds of circuits for alarms, games, etc., you
    might find a circuit that you could easily adapt to your needs.

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