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a little RF help, please?

Discussion in 'Electronic Design' started by almo, Aug 16, 2006.

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  1. almo

    almo Guest

    I want to find a way to detect when a very small solenoid is energized
    for about 10 msec. By "small" I'm talking about 1.5 watts, and turned
    on with 6 volts, and physically very small, like 3/16 inch diameter. I
    want to detect it from about 2 feet away. So, it's going to be
    emitting an electro-magnetic signal, but to save me a lot of trial and
    error, if someone can get me pointed in the right direction, I'd
    appreciate it. As soon as I post this I'm just going to see if I can
    detect it using a piece of wire as an antenna, then try a coil of wire,
    etc., and just watch the oscilloscope.

  2. The magnetic field is going to be very difficult to detect, as magnetic
    loops go down as the cube of the distance. It's likely to be weaker
    than the Earth's magnetic field. To evaluate this, hold a compass two
    feet from the solenoid as you energize it. Oops, 10msec isnt going to
    be enough time for the compass needle to react.

    The rf radiation is going to be really really miniscule and very low
    frequency, as the current through a solenoid is going to change quite
    slowly. A wire hooked up to an audio amplifier or scope just might
    pick up something. But a Harley driving by is likely to swamp out the
  3. Luhan

    Luhan Guest

    Given your stated conditions, I believe no solution is possible.

    You need to re-pose the problem from a more elemental level. Why are
    you activating the solenoid? Why do you need to detect it?

  4. Barry Lennox

    Barry Lennox Guest

    I suspect you cannot do it by detecting the magnetic field. 2 feet
    from a solenoid will see the field close to the noise. What about
    putting an IR LED/resistor across the solenoid? It would then be easy
    to detect the IR pulse a few feet away.

    Barry Lennox
  5. almo

    almo Guest

    Thanks everyone. I think there is no practical solution to the problem
    as it was presented. The situation was to detect the solenid being
    energized but not having access to the system containing the solenoid,
    so "non-invasive" would have been the key word. Of course, in theory
    (where everything works) I would use an "ideal" amplifier, and with
    some fourier analysis I could pick out "my" little signal from all the
    larger ambient signals. Also, I would normally just say what it is,
    and why I'm trying to do this, but this is something I'll patent and
    it'll help pay the rent, although not using the RF from 2 feet away.
    I'm debating on starting my own company right now, versus working for
    some big company again and just getting "assignments," and then yearly
    performance reviews by people who couldn't design their way out of a
    wet paper bag.

    But I've found that it never hurts to ask. These google groups, and
    usenet, put me in touch with everybody in the world who knows something
    about anything. Strangely, where I worked, (I quit recently) being
    caught browsing the user groups might be considered as just screwing
    off. But, whenever I need to do something without re-inventing the
    wheel, these groups are the first place to go. Maybe somebody doesn't
    know, but they know a guy who has a friend, etc.

    In fact...I do have another RF question, a real world type question,
    about my Sirius Radio antenna, which is not working. So, I'll post
    that one now.
  6. Well, let's not give up just yet. As I understand it, your solenoid and
    wiring and power supply are completely contained within a noncoductive two
    foot sphere. Is that about it?

    Now you are going to hit that solenoid with a pulse 10 milliseconds wide and
    the solenoid is going to move, but not so as to mechanically disturb the
    sphere. The problem with detecting RF, is that RF by definition is a
    constantly varying magnetic field, and your magnetic field, after the
    initial burst of magnetization, is a relatively constant field, not
    generating any RF.

    But before we go away and say we can't do it, is my scenario exactly

  7. J.A. Legris

    J.A. Legris Guest

    Here's a long shot. Do it the way shoplifting detectors do it. When the
    solenoid is energized it's core is probably magnetically saturated or
    close to it. You may be able to detect this by applying a powerful
    alternating magnetic field that drives it in and out of saturation,
    producing a signal at a harmonic of the driving frequency that could be
    detected with a sensitive receiver. Shouldn't require more than a
    thousand watts or so with coils the size of barrel hoops :-(
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