Electric Fence Detector

Discussion in 'The Projects Forum' started by Betadel, Sep 26, 2013.

  1. Betadel

    Thread Starter New Member

    May 11, 2011
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    Hello, I want to make a detector that can tell whether a fence is on and live or if it's off by whatever reason. Also, ideally I don't want to have to touch the fence in anyway to install it.

    After detecting it I would then pass it by a μC to do some other stuff.

    My problem is what kind of sensor to use. Something like this is what I want:

    http://www.zarebasystems.com/store/electric-fence-accessories/efa2

    Does anyone know what kind of circuit this thing uses?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Electric fences typically use an earth ground return and output high voltage pulses.
    This unit more than likely uses a neon detector to sense a nearby charge on a Fence, and un-sensed by you, it uses your body as a return to earth, it requires a very small charge for the sensor to detect.
    Max.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    It's an electric field detector, i.e. a radio. But I don't know the details. Is there a reason you don't like the commercial device? It might cost more and be more difficult to make your own.

    Gaaah, beaten again by Max!
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Not sure whether they still sell them, there used to be an electricians screwdriver with a neon light in the handle and a metalic pocket clip at the top, this was used as a quick check to detect if a the live side of a 120-240v AC socket was actually live, you had to make sure you held the pocket clip as the return was to earth the same way.
    Max.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    I have one of those! Always wondered how it worked with just one conductor. Didn't realize the missing conductor was me.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You'd probably soon realise it if you touched one of those neon screwdrivers to an electric fence :D. I doubt it's rated for fence voltage.
     
  7. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Used to check the fences with a metal bar. Rest one end on the ground, and bring the other end up near the fence (maybe 1/8 inch away) and if it's good, it will arc, and you'll hear the click_click_click.
     
    #12 likes this.
  8. Betadel

    Thread Starter New Member

    May 11, 2011
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    You mean something like this: http://en.wikipedia.org/wiki/Resistive_opto-isolator ?

    If so, why neon and not a photodiode? And also, can it resist the high voltage pulses (up to 10kV?) of a fence?
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    An opto Isolator is a bit different technology, Neon is used as a low pressure gas that conducts with very low current, to operate at higher voltages the electrodes are simply taken farther apart.
    Neon signs are an example of the higher voltage application up to 10kv etc.
    Max.
     
  10. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Avago makes an opto isolator with super low current on state hcpl-something I've used to detect 1kv with a 20 megohm resistor.

    Hcpl-4731 IIRC.
     
  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    When I was younger and playing on my uncles farm, I took my fence detector with me. It was called Tim, my little brother. :)
     
  12. #12

    Expert

    Nov 30, 2010
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    In 1980 I used a neon bulb and a CdS photocell. I glued them together with a bit of clear silicon and then coated the whole thing with black silicon. It's pretty slow, but it was fast enough to couple a phone ringing signal to a loud bell for the partially deaf guy.
     
  13. Art

    Distinguished Member

    Sep 10, 2007
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    19 x 10K resistors in series connected to a 10K Pot (also in series),
    so total 20 x 10K resistors in series connect directly between the fence HT
    and fence ground, which can be embedded into the electric fence tape I think.

    neon in series with 2 x 2.2K resistors connected between fence ground,
    and the wiper of the pot (divider).
    Now the circuit can measure between 1.8 and 10.8 Kv.
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Methinks those resistor values are way too low. 10kV/200k = 50mA. Each resistor would have 10kV/20 = 500V across it and would dissipate 500 x 50/1000 = 25W. The total loading on the fence would be 20 x 25 = 500W (effectively a dead short).
     
  15. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    And don't forget that most "fencers", the voltage generating circuits, have the ability to up their volts and current when needed. It's called 'weed burning', for when a plant grows up against the fence it gets killed. So any type of short will be met with higher than normal output until the short goes away.
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you got hold of a simple neon indicator lamp you could experiment as to the distance away it will register, as you should not need to actually contact the fence, after determining the distance, a insulated extension could be made?
    Max.
     
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