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Weak Magnetic Field Sensing

Discussion in 'Electronic Basics' started by Christopher, Mar 3, 2006.

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  1. Christopher

    Christopher Guest

    Hello Everyone,

    Weak Magnetic Field Sensing

    Before I begin to chase the madness into the evening, I would like to
    know if using a magnetic sensor like the Hall Effect would allow me to
    convert "a small magnetic rod's distance into resistance. " The few
    circuits sensing magnetism I see seem to be more of a window
    comparator behavior or 0/1 or a tripped on state.

    Ideal: is a strong "hand held" magnet moving 8" to 1" from the sensor,
    this would create a resistance change of 10k down to 1k or something
    to that effect not necessarily that resistance change.

    Question: Is this conversion possible or unreasonable?

    Thank you for any input or humor,


    * * *

    Temecula CA.USA
  2. Bob Masta

    Bob Masta Guest

    What exactly are you trying to accomplish?
    Do you really need resistance as the output?
    Most important, do you need to detect a
    stationary magnet, or only a moving one?

    If you want to detect a moving magnet, all you
    need is a big coil of wire and a simple circuit
    to detect the voltage induced in it by the
    magnet's motion. However, when the magnet
    stops, the output stops as well.

    Best regards,

    Bob Masta

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

    Anna Banana Guest


    I can see what you are getting at.. maybe I am a different species?.
    Yes this conversion is possible, but might require some cumbersome

    The true hall effect sensor you speak of outputs a voltage
    proportional to both the applied field
    and transverse bias current. In the 'easy' sense what you'd like to do is
    unreasonable, but it can be acheived in effect,
    approximately, with additional circuitry... with some likely requirement of
    isolating the hall effect bias current voltage source from the rest of your
    circuits. I think you can forget about using the hall device directly to
    emulate some kind of resistor. However it's output voltage can be amplified
    and sent through a transconductor of some kind which approximates a
    resistor, or more readily an analog to digital converter driving a digital
    potentiometer.. with some added resistors, or offset scheme to get your
    range expressed in 64, 100, 256 ohmic range steps.

    LakeShore has hall effect sensors of the type you are interested in. I
    suspect Digikey or Allied also carries more than one proportional hall
    effect device, and one that is easy to use... though most are as you say.

    There are lots of way to skin a cat (no cats were harmed during testing)
    .. Since hall devices are linear with respect to applied field and current,
    the output will not vary linearly with distance, as the field strength of
    the magnetic rod will not correspond linearly over that range (but will
    appear close to linear over very very small incremental changes in distance
    between rod and sensor).

    There are magneto-resistive sensing elements out there, maybe there is
    some way you could use a Honeywell device.. still I would wonder why you
    couldnt use the changing voltage to effect whatever desired action somehow.

    You could set up 10, 0/1 devices over the 8-inch range, each switching in a
    new resistor with some clever way of latching the present state to hold
    R-value in the dead-zones between sensors. It's crude, but sort of linear.

    You can do this to some effect with a few components.. as you said, not
    necessarily linear, but over some range.

    I'm not sure how to set it up, but I've heard that if you shred the
    constitution, torture people for corporate profits, and
    (mile-long list truncated), resistance changes proportional to that... but I
    think it usually means boycotting beer for an hour at best, which isn't
    really resistance, or ohmically useful either.

  4. Christopher

    Christopher Guest

    Hello Bob and Hello Anna,

    You ask:
    Find the best method using any type of sensor or effect for a
    non-tactile, hand controlled volume control.



    I am attempting to turn my hand into a human potentiometer with at
    least an 8" sweep range. I thought holding a magnet could give me a
    usable effect or force to do this.

    I want to explore a new volume control method of a special musical
    instrument called a "Theremin". The normal theremin volume method uses
    a antenna and detects the shift in frequency of two perfectly balanced
    oscillators. It has very wide physical sensing area of more than one a
    foot diameter. Placing my hand out 12" then moving it next to the
    control antenna quiets the volume without touching it.

    Yes, I can do the above technique, along with an optical method and a
    mechanical balance of two magnets opposing one another method which
    works but is not responsive enough and is sloppy.

    I want to use a different approach that will have a working width in
    free space of about a 4" to 6" diameter and have detectable downward
    sweep range starting at least 8" to the outside, then have a
    controlled measurable output as my hand magnet moves down to the
    control sensor pickup.

    This will allow me to have more than one (maybe four) sensor/detector
    spread out in the original one foot diameter or musical volume control
    panel area of which my hand approaches.

    Non of the above values or measurements are critical and could be
    modified to adapt to the approach.

    I have a webpage I show hobbyists/musicians how to build my theremin
    design and this volume control method would be a great enhancement.

    "The theremin is the only musical instrument played without touching
    anything." It has can sound like a musical saw or an angels voice.

    This is Clara Rockmore Circa 1940 .mp3 777kb

    Thank you, your ideas are greatly appreciated.

    * * *

    Temecula CA.USA
  5. Bob Masta

    Bob Masta Guest


    Voltage induced in a coil by a moving magnet
    would have a velocity response, instead of
    the position response you seek. It might
    be worth investigating, but it would have
    a bit of a learning curve. However, since
    the Theremin (or any instrument, for that
    matter) has a learning curve, this doesn't
    need to be a show-stopper. It might turn
    out that rate is easier to control than position.
    You might want some sort of gestural
    interpretation to activate peak hold, so you
    don't need to keep moving at a constant
    velocity to keep a constant volume.

    I'd think that some sort of photo-optical
    arrangement would work pretty well for
    position sensing, but you'd probably need
    something like rows of LEDs and photodiodes
    to replace the Theremin antennae (or arrayed
    along the antennae). There could be separate
    zones for different controls, but I don't know
    how you could control more than a couple at
    once. (Assuming you have no more than the
    normal allotment of appendages!)

    How about a photo method where instead
    of a magnet, you hold a disk of polarizing
    film, or a transparent color wheel. I'm thinking
    there might be some way to control multiple
    channels by using distance as well as orientation,
    but I confess this is right off the top of my head
    and not thought out at all.

    Best regards,

    Bob Masta

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