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Induction sensor

Discussion in 'Electronic Basics' started by Joseph, Apr 19, 2007.

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  1. Joseph

    Joseph Guest

    I'd like to build a metal detecting induction proximity sensor. Something
    that can pick up a stainless disk (size of a dime) at .2 inch. Flipping
    through the Allied catalog I see page after page of cylindrical (M18)
    induction sensors.

    Is there a common technique "everyone" is using that I should look into?
    Can someone give me some circuit details to point me in the right direction?
  2. Pete D

    Pete D Guest

    ot used one, but i think they detect metal moving past the sensor end at
  3. Joseph

    Joseph Guest

    What I really meant to ask...
    Is there a typical circuit design they use and what is it? There are so
    many of nearly identical size and specs that I'm taking a guess there is a
    common circuit they all use.
  4. All the units I have seen are based on an internal
    oscillator (a few hundred kilohertz for he small ones) and a
    means to measure the eddy current load on that oscillator,
    with a decision point to switch the output logic signal,
    indicating metal in the field at some specific magnitude of
    eddy current load on the oscillator coil. The two main
    types are ones that spray field out all around the end of
    the coil, as well as in front of it, and those that have
    field shaping cores that keep almost all of the field in
    front of the coil (concentric poles), so that the unit can
    be flush mounted in a hole in metal without seeing this
    metal, or close bedside other units, without interacting
    with each other. The down side of the shielded form is that
    it cuts the sensing range for a given diameter unit, about
    in half.
  5. Jamie

    Jamie Guest

    You may want to get a large surface diameter type. Stainless is not as
    sensitive as other types of metals.
    Depending on your application, 2 wire types require a 120/240 AC in
    the loop of lets say a relay coil.., you can get normally
    closed/normally open types and the average handling current is around
    200ma, that is normally enough for a 120v 10amp type relay coil with a
    medium mechanical mass.

    The 3 wire types require DC voltage 12..24 volts DC on most of them
    going to the Brown Wire (+), Blue wire(-) and the black wire will be a
    transistor output.

    You can get those in PNP, NPN, N-on/N-off etc..
    the handling current on those are around 200 Ma DC>
    for the NPN types, it is used as a sink (pulls to common when on). We
    this type, you can switch a variety of devices that use a varying range
    of DC voltages, you simply pull it to ground/common..
    With the PNP type, it becomes a source device. What this does is
    generates output from the same (+) as the sensor is connected too.
    The PNP types are not so popular..

    So you have your pick now.
  6. I don't think you can possibly build one of these sensors
    for what you can but several.

    Here is a low priced source:
  7. Joseph

    Joseph Guest

    Despite the small costs of the available units I'd still like to build one

    I have some pot cores and magnet wire for the coil, and uPs and relays etc
    for the logic and output. But...the oscillator and detection circuit is new
    to me and where I need some direction. I've googled and found some good
    theory but very little on circuit specifics (except a 'hobby' metal
    detector). Any help is appreciated.
  8. I don't have a construction article for you, or even a
    schematic, but I think you might get started by building a
    Hartley Oscillator and sense the change in its DC supply
    consumption as the sensed metal quenches its oscillation.
  9. How about an oscillator of which an induction coil is a component. The
    frequency changes when metal approaches the coil. An appropriate filter
    lets through the changed frequency. The level of the signal is detected and
    if high enough a buzzer sounds. Would that work?

  10. Sorry, I meant to say that, but must have assumed it was
    obvious. The oscillator inductor is the coil that produces
    a field in front of the sensor.
    The ones I have seen have the oscillator actually lose
    enough gain to stop oscillating, and the supply current
    takes a step (down, I assume). The logic output just
    detects the changes in the supply current to the oscillator.
  11. Is this attribute unique to Hartley oscillators? Does it not apply to other
    types as well (Pierce, Colpitts, etc)?
  12. I think it applies to any LC oscillator. I picked the
    Hartley, because if you are going to wind your own inductor,
    it puts more of the complexity into the coil, and less intro
    the rest of the circuit. It is not hard to bring the tap
    out on a home made coil.
  13. Man, it has been a long time since I last built a 1
    transistor oscillator. I plinked around a little and
    quickly discovered that I have the supply current situation

    The oscillator is biased to draw a significant and
    continuous supply current, but once it gets going, the
    rectification effect of the base junction kills the forward
    bias for most of the cycle, greatly lowering the average
    collector current, concentrating it into narrow pulses at
    the positive peak of the cycle. Putting a piece of metal in
    the inductor field not only lowers the inductance, but also
    the Q, so the oscillation tends to start and die out ,
    rapidly, so that the transistor spends most of the time in
    an intermediate state between no oscillation (high current)
    and active positive feedback oscillation (class C low
    current operation). The hardest part for me to model is the
    eddy current losses in the target as the distance is varied.
  14. Justin West

    Justin West Guest

    This may or may not help,
    you can download a free .pdf which has "6 components and less"
    electronic projects. One of the said projects was a metal detector.
    Might be something to give you a few ideas.

    Regards, JW
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