Does anyone have a circuit for a simple electric fence indicator?

Discussion in 'Homebrew and Kit Projects' started by N1AUP, Sep 7, 2008.

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  1. N1AUP

    N1AUP XML Subscriber QRZ Page

    I'd like to build a neon indicator that I could hang on an electric fence to confirm that the fence is actually energized. Fences put out anywhere from 1000 to 7000 volts.

    Does anyone have a simple, rugged design that would work here? I've heard suggestions of using a fluorescent light bulb between the fence and ground, but I can't help but believe that this would get broken by the fenced animals in short order.

  2. KC9NMC

    KC9NMC Ham Member QRZ Page

    I think you'd have to use a powered, active monitor, as neon bulbs are just going to load down the fence itself. That is, most electric fences can't put out much current (on purpose! You want to shock the animal/person, not kill it), and loading the fence down with more than one neon bulb/resistor will likely drop the voltage below that needed for effective operation.

    I'd build a simple LED circuit that is controlled by an enhancement-mode MOSFET. So, you have an LED, a crrent control resistor, and a MOSFET. The gate of the mosfet can be tied to a probe wire that comes close but does not actually touch the fence- it will sense the E-field. Yes, many mosfets will do this! The LED can be powered by a little AgO/Zn coin cell battery or a pair of AA alkalines (you'll need at least 2 volts to forward bias an LED).

    If you need more specifics let me know.
  3. K8ERV

    K8ERV QRZ Member QRZ Page

    I bet you could hear the pulses as noise bursts on a nearby am radiddio.

    OR, just back a cow up to the fence and check the color of it's eyes.

    TOM K8ERV Montrose Colo
    Last edited: Sep 7, 2008
  4. KE5MC

    KE5MC Premium Subscriber QRZ Page

    I agree that the fence is high impedance and low current for non-lethal effect. Again if only using the NE2, once the gas ionizes it has a very low impedance and loads the fence. Place a resistor of meg ohm value (1 to 5? s.w.a.g.) would limit the loading and possibly allow the NE2 to light dimly. Might be difficult to see in bright daylight.

    Another possibly is the non-contact voltage detector for 120/220/440 AC power circuit. Not sure they work for electric fences, but they might.

    Re-reading the OP I am not sure if the intent is long term fixed attachment to the fence or a carry in the pocket and use when near the fence.
    Last edited: Sep 7, 2008
  5. W4HAY

    W4HAY XML Subscriber QRZ Page

    Tractor Supply and Southern States have them at prices less than what you could build one for.

    There's also a battery powered unit that simply hangs on the fence and if it doesn't sense a pulse every few seconds, begins flashing a light that can be seen for considerable distance at night. Very handy for that back forty, especially when the weather's bad!

    If you do want to build one, an NE-2 fires at about 75 volts, so you can make a voltage divider that presents a high resistance load to the fence. Electric fence chargers are current limited, but not THAT much. A low impedance unit can knock you on your <bleep> if your boots are damp. Don't ask how I know this.
    Last edited: Sep 7, 2008
  6. K8ERV

    K8ERV QRZ Member QRZ Page

  7. W0LPQ

    W0LPQ Ham Member QRZ Page

    An NE-2 type won't load down a fence. They are low current devices. Ionize around 90 to 110Volts or so.

    An NE-2 and series resistor would be ideal. Just figure out the voltage on the fence and calculate. Ground one side of the resistor (or the NE-2) and the other side to the fence.
  8. KR2D

    KR2D Ham Member QRZ Page

    Just convince your little brother to grab the fence wire. You'll know in short order if the fence is energized. Hey, it worked for me :D

    Only problem was, he wouldn't do it again the next time we visited Uncle Johnny's horses. So I showed him that the fence was safe by touching the wire myself - carefully timed between clicks of the controller. He didn't understand how the fence charger worked, but he found out real soon :D
  9. W4HAY

    W4HAY XML Subscriber QRZ Page

    Ionization devices are constant voltage components, hence the use as voltage regulators. It's the resistor(s) that limit the current.

    I installed a pilot lamp assembly containing a NE-2 as a line voltage indicator for a power supply, thinking it had an internal resistor. It didn't, and tripped a branch breaker in the power panel. The bulb would glow afterward, but the black coating on the inside of the bulb made it difficult to see.

    My 1966 GE Glow-Lamp Manual shows several versions of the "NE-2" bulb. Minimum breakdown voltage is specified as 65, maximum as 80, extinguishing voltage as greater than 50, and design current as 0.3 mA.

    Ambient light levels affect the ionization voltage, and some units incorporate radioactive materials to minimize this "dark effect".
  10. KB7UXE

    KB7UXE Ham Member QRZ Page

    true story:
    2 old guys, one kid hunting...
    all stop to take a leak.. 2 elders fail to warn kid about peeing on wire...
    true story..
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