Simple Electronic Fencer


Introduction: Simple Electronic Fencer

I have two jack russle dogs who love to rip up flowers, much to my partners annoyance, so I desided to have a root around in my project parts box and see what I could come up with. Low and behold I found that I had an old ignition coil from an old retired Mercedes car of mine, all I needed to do was to pulse its primary coil with 12 volts to produce a very high voltage in the secondary.
I now needed to design and build a circuit that would pulse a relay to feeb power to the ingnition coil. Using my bread board I set up a simple NE555 with a couple of resistors and a capacitor to give me 50% duty cycle. I then fed the output into a simple RC network that feeds a power transistor. Because the ingnition coil consumes a lot of current I needed to pulse the relay so that the transistor is only on when the RC is charging, I selected the values to give me about .25 of a second on time for the transistor, this is just enough to pulse the relay and cause the coil to deliver a powerful high voltage shock.
I then had to limit the current passing through the ignition coil so I used a power resistor of 82 ohms to allow approx 1amp to pass through the primary winding, this limits the power of the output to a save level that won't hurt my dogs. I tested this on my self, at first it didn't work very well until I hammered a 2 foot copper pipe into the ground and connected this to the negative of my circuit, now I was getting a nasty but harmless shock from the unit approx once every 2 seconds. It works very well, I used agricultural fencer wire with insulators around the flower beds and after a couple of shocks the dogs are happy to leave the flowers well alone.




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    9 Discussions

    i am looking for something like this, I have to keep neighbors cats away from hunting my quails.. would you please give some more details about this project, the electronics parts and the circuit diagram...

    3 replies

    You would want to limit the current even more because cats are small and more fragile than dogs. Also this seems a bit extreme cause there just birds.

    Is your concern about birds that they'll land on the fence and get shocked? To complete the circuit, you need to be in contact with a ground and the hot - so being perched on the wire doesn't harm them. Consider HV transmission lines. big birds get fried only when they have a wingspan large enough to manage to contact two of the wires.

    Electric fences are used around the world to corral livestock. You don't see dead birds around them.

    Hi Wudaal,
    Yes of course, I am happy to provide whatever details you may need. The circuit diagram has all of the component values, these don't need to be strictly adhered to, you can change the R/C values to suit your needs. The important values are the resistor bias values.

    I used a power resistor in series with the ignition coil to reduce the current going to this as it can draw 6-10 amps quite happily which would flatten a battery pretty quickly and provide a shock powerful enough to injure and animal. I used a 22 Ohm metal encased power resistor mounted on a heatsink as it tends to get quite hot.

    The relay I used is a low voltage (12v) high impedance with contacts capable of handling mains at 5 amps, mine is switching just over an amp into the primary coil of the ignition coil.

    Please let me know what detils you require and I can elaborate on those. Mine has been working away very well since I built it a number of months ago.

    One very important thing is to earth the ignition coil near to where you will be using the electric fense. I used a 3 foot long copper bar, hammered it into the ground near my fence and used a jubilee clip to attach the wire that goes to the ground of my coil, without this it will not work. Initially I used 6 inch piece of copper but it didn't work very well.

    Looking forward to hearing from you.
    Kind regards.

    Hello Uglymudda,

    I wondered if you could give me an update on this project. I also had a couple of questions.

    You say to mount the coil near where you earth the fence, how did you choose to weather proof the device? Also did you lay the Copper pipe horizontally or vertically?

    I'm looking use a setup similar to this to go across a metal gate between the house and garage. One side has tarmac and the other is flag stoned. I would need to insulate the gate which should be easy enough and I would be interested to know how you would choose to earth the device in such a situation.

    Many thanks for your time

    1 reply

    Hi Petebob,
    My coil is mounted to the electronics in a small project box, the box its self is water proof, I bought it from B&Q, its an electrical outdoor box, set me back about 30 Euro's, its big enough to house my battery and the ignition coil. I found that a lot of coils give very different outputs and some are dual output, I found that I got best results from an old Mercedes ignition coil.
    The copper pipe is used to ground the coil, so hammer this into the ground as close to the gate as possible (vertically) then place a jubilee clip on the the top of the pipe so that you can secure the ground wire to it. Unless you have a very good earth its not very effective. You don't have to use a copper pipe, I used one of these as I had one handy, you can buy earth rods in any good electrical store. So long as about 2 feet of it is buried well into the ground. Laying it horizontally will not work. Once you have connected the earth rod (pipe) be sure to tape up the connection good and tight to prevent the wet getting at it or it will corrode very quickly and the ground will become ineffective very quickly. Insulating your gate will be difficult, so what I suggest is to place some wire mesh on your gate and insure that it is electrically isolated from the gate.A few simple plastic washers should do the trick. Remember that the voltage coming from the coil is around 8,000-20,000 volts, so be careful, I have got quite a few shocks from mine. It is good practice to put 1 mega Ohm resistor in series with the output from the coil to seriously reduce the current that can flow from this thing.
    Feel free to ask more questions and let me know how you get on with yours.
    Kind Regards.

    HI Uglymudda,
    Thank you very much for your so much quick reply.
    I can not see the diagram clearly... please check and you will find me right. can you share a high resolution image so that i can see the values very clearly..

    what I am planning is that I will get a motion detecting switch, and will use it to activate the circuit you have designed, as the neighbor's cat comes rarely, so using it in this way will save some power. do you have any better idea about it in this way.

    Regarding grounding the coil, what do you think if I connect it with the water supply tap?

    Our Eid vacations are getting near, so I will get enough time to work on this project.

    Thanks for you time Mr.Uglymudda. hope will read from you soon.

    Best Regards

    1 reply

    Hello Wudaal,
    Yes I see what you mean about the circuit diagram, Ill upload a better quality one this evening. I designed this unit to be left on for extended periods of time, I use a car battery to power it and that lasts for about two-three weeks of continuous use.

    I wouldn't really recommend connecting the ground to the water pipes as most pipes under ground are hydra and not copper, i would opt for a proper earth bar, apologies, my earth bar is 3 feet long, not two as stated in the original post.

    Ill upload a better circuit diagram later today.
    Kind Regards,