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AC Proximity Detector

Discussion in 'Electronic Design' started by [email protected], Sep 3, 2007.

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  1. Guest


    Out of curiosity - how do those hand held AC proximity sensors work ?
    (e.g. the pen sized devices that you hold near to a live mains wire
    which light up when AC is present). Is there an off the shelf IC that
    you can purchase to make one yourself ? Anyone have a circuit diagram
    for one ?

    Reason for asking, I want to build an electronic circuit to detect the
    presence of 240v in a wire - without disturbing the wire.

    Many thanks,
  2. Jamie

    Jamie Guest

    Electrostatic charges are on the surface of the wire.
    Those simple probes you're thinking about are just hi-z
    input sensing circuits via a couple of diodes to rectify
    charge on the surface so it can drive a CMOS or FET type
    sensing circuit. Normally input load are tailored to work in the
    desired range and maybe a clamping protection diode to save the
    Electricity produces charges on the surface of the conducting
    If you would like a PDF file on the subject of how electricity flows?
    I think I may have something designed for students. You can email me if
    you wish.
  3. MooseFET

    MooseFET Guest

    The simplest form of that uses an NE-2 neon glow bulb. They don't
    work very well, but they don't need any batteries. It just uses the
    AC electrostatic field and the fact that you are providing the
    connection on the other side of the bulb.

    The modern ones use a little bit of electronics. You don't need
    much. Just a high input impedance and enough output current to drive
    the LED. There is no need to rectify and level detect or anything
    like that. The amplified 60Hz is basically applied to the LED

    If it wasn't for the ESD problem, this circuit would work:

    D1 470R
    C1 !!-----!<-----/\/\----+9V
    PROBE <---!!--+---!! TN0601N3
    ! !!----
    20V /-/ !
    ZENER ^ !
    ! !
    ---------+----------------9V Return and your fingers

    C1 is about 1000pF and rated at 3KV.

    D1 is a green super-bright LED

    The Zener attempts to protect the MOSFET etc but it gets destroyed
    after a few minutes of fooling around with it. This circuit will also
    detect people combing their hair and turning pages in books etc. It
    works over quite a wide band width.

    If I was building one for real, I'd use some sort of low powered op-
    amp. They make quite a few that can run for a long time on a 9V
    battery. With an op-amp you can make a narrow band filter to reject
    everything but the 60Hz.
  4. , of course.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  5. Joel Kolstad

    Joel Kolstad Guest

    Calling that circuit "basically a 60Hz e-field radio receiver" is kind of an
    insult to all the Real Radios out there, Don. :)
  6. Robert Baer

    Robert Baer Guest

    Very simple, they use CMOS logic where the first (inverting) gate is
    biased in the linear region with a high value feedback resistor.
    That makes the input sensitive to small signals that get amplified by
    that gate; the output is then passed to the second stage which amplifies
    it more and saturates (signal swings from ground to supply voltage).
    That signal swingcan be detected by another gate, which can drive an LED.
    I have seen three variations of this, and one of them had an
    adjustable sensitivity.
  7. Yeah, I've always thought that it should be the opposite, since the
    can on carbon-zinc batteries is the (-), but that's the standard..

    Best regards,
    Spehro Pefhany
  8. Rich Grise

    Rich Grise Guest

    So, then, Don's symbol is "right" in that the thick black one, the
    carbon, is plus, and the thin line, the zinc envelope, is minus.

    That's the way I've _always_ done it, and I don't care if everybody
    else in the world does it wrong. ;-)

  9. ehsjr

    ehsjr Guest


    As long as you label the side _you_ designate as +
    it won't be misunderstood, even if you do it *right*.

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