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Critter Zapper

Discussion in 'General Electronics Discussion' started by Blank Stare, Jul 29, 2017.

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  1. Blank Stare

    Blank Stare

    48
    0
    Oct 10, 2012
    I need help designing a circuit that will take power from a 6v DC battery, and change it to an extremely high voltage, but extremely low amperage ZAP!, to discourage raccoons from destroying a piece of equipment I own, without doing any permanent damage to the raccoons. I think the proper circuit will use electronics to make an artificial sine wave, so that the shock has the desired effect.

    Think electronic dog collar, as far as the kind of current specs I want for output. Or you could think micro-horse fence. I have seen some devices on the market designed to do similar duty, that state they have an output of 10,000 to 20,000 volts, but they do not state the amps.

    I would buy something ready made, but I need to make it fit inside the compartment that all the rest of the equipment's parts are in, in order to protect it from weather, and the critters.

    Ideally, the completed device will be about the size of a deck of cards. It MUST be powered by a 6 volt rechargeable battery, that is recharged by a solar cell elsewhere on the equipment.

    The ground output lead from the device will attach to a cage I will be putting around the equipment, and the positive lead will be wrapped around plastic parts that are several inches, to a foot from the cage. A critter reaching through the cage will reach for the object they desire (debris on the automatic corn spreader), and brush against the positive lead. They will then receive a surprising shock, as their arm completes the circuit between the cage and the positive lead wire.

    I can not stress enough that this must be non-lethal. As annoying as these little banditos can be, they are not doing anything unexpected... they are just foraging. I even think they are cool creatures - but they are ruining my equipment, and preventing me from my purpose. After they get jolted few times, they will learn to forage elsewhere, and everyone is happy.

    I have built, and even designed a few simple circuits. Nothing fancy, and I had help each time. Once I had a schematic for the circuit, I was able to order parts online, and assemble them on my kitchen table.

    At the moment, the biggest unknown is how many volts/amps output to design the circuit for. Once that is known, I can start trying to figure out how to size the electronics for the job.

    Anyone care to help out?

    Thanks in advance.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
    2,780
    Jan 21, 2010
    Design a circuit that will generate about 300V and use this to charge a capacitor (say a couple of uF). The boost circuit need only run when the capacitor voltage is (say) < 250V (so it doesn't run all the time).

    Now, 300V across a 10uF is pretty nasty. I doubt it will kill anything of reasonable size, but if you want to be sure, use this to charge a smaller capacitor (say 0.22uF) through a 100k resistor.

    Now, for a schematic...

    upload_2017-7-29_16-15-16.png

    IC1a is an oscillator. This is buffered by IC1c through f to drive the gate of mosfet Q1.

    Q1, L1, D1, and C2 boost the input voltage and store energy in C2.

    D2, R2, and R3 form a voltage divider which puts about 4V on the input of IC1b when the voltage on C2 rises to about 300V. This causes the output of IC1b to go low, stopping the oscillator and forcing Q1 off.

    As the voltage on C2 drops, the voltage on the input of IC1b drops. When the voltage on C2 drops to about 250V, the voltage on the input of IC1b drops to 2V. This causes the output of IC1b to go high, the oscillator restarts, and C2 is charged up again.

    Because the charge on C2 is quite high, and may be more than is required to startle an animal, R4 is used to charge a smaller capacitor C3. C3 is connected to whatever will startle the animal. R4 also limits the long term current, protecting the circuit if the output gets shorted.

    R2 and R3 affect the power usage dramatically. The values here will result in approx 6mA average current drain on top of what is required to charge the capacitor and overcome other losses. The current drawn while charging is likely to be significantly more than 6mA :) To a large extent, the inductance of L1 will determine the max current drain because the frequency of operation is constant.

    The main part of this has been built and tested, the tricks for getting this working will be:
    1. Using a CMOS hex Schmitt trigger that allows a 6V or more input voltage. 74HC has an absolute max of 7V, but that's sailing a bit close to the wind, I'd use a 40106.
    2. A 200V zener diode. You probably won't find one. A series of high voltage zeners in series should be fine.
    3. D1 something with a 1000V PIV rating
    4. C2 and C3, 400V or higher rated. The exact value of C2 is not too important.
    5. Q1 is the most tricky part to find. You want a 400V or higher voltage mosfet that can be turned on sufficiently with Vgs of 6V (or lower as the battery goes flat)
     
    twister likes this.
  3. kellys_eye

    kellys_eye

    4,275
    1,146
    Jun 25, 2010
    You want an 'electric fence energiser'. Making a 'permanent' high voltage supply is wasteful of energy especially if you want to power it from a battery so a 'pulsed' output saves considerably.

    Here's the schematic of a typical unit that was published in an electronics magazine. It can be adapted to 6V and the original ignition coil replaced with a reverse-wired miniature mains transformer (use a 0-6V a.c. output, 240V a.c primary device) to deliver the output although with such modifications you will get a reduced output.

    Bear in mind that you need sufficient energy to be effective and too low a voltage/current won't necessarily deter to the extent you imagine so I'd recommend you use the circuit as-is for effectiveness. Note that the energy output can be adjusted (switched) but you can also hard-wire the specific resistance to make it a fixed output.

    To make an electrical path for the shock to be effective there has to be a 'return' - it is usually the ground but this is not particularly electrically efficient which is why the output voltage tends to be quite high - alternatively you can make you own 'return' path by using electric fence tape. This is a nylon woven strip about 1cm wide that has rows of stainless (thin) bare wire stitched into it. You can wire the thread alternately and connect to the output of the energiser so that touching the TAPE - rather than requiring a path to/thru ground - provides the shock.

    The tape isn't expensive - I got a reel of it (couple of hundred metres) for £10 ($15).

    Energiser.png

    Dang! I hit 'post' ages ago but didn't see that Steve had beat me too it!
     
    twister likes this.
  4. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    I have managed to beat Steve on one occasion only, he is fast on the draw.:)

    I have repaired several fence energisers, the normal circuit is to charge a capacitor to 300V or so and then to discharge this through a 1:20 step up transformer. The high voltage is necessary to punch through furr and dry skin. The energy can be up to 20J but 0.1J is probably all you need.
    The pulse is normally 1/sec.
    It would be difficult to put this into a small box since there needs to be considerable insulation distances.
    The pulse can get into other electronic equipment and damage it. You need a very good earth separated from other equipment earth.

    There are high voltage (300V) boost convertors on ebay for very little money which will run on a range of voltages, they use an oscillator, transformer and Cockroft/Walton multiplier.I have a couple here but other things have taken priority.

    A good high voltage transformer is needed, I would not use a reversed mains transformer because the insulation will not be good enough, insulation is needed between layers. The ignition coil seems to be frowned upon, I do not see why, perhaps it is the high secondary resistance so will not run a long fence wire.

    I had my garage broken into, my car window smashed and my coat stolen. I connected an energiser to the metal door but was told this was illegal so I removed it. A requirement is that a warning label must be attached to an electric fence. Can your raccoons read?:)
     
  5. kellys_eye

    kellys_eye

    4,275
    1,146
    Jun 25, 2010
    A mains transformer is good for 1.5kV pulses - sufficient to deter over a small area and definitely for the 'box' the OP wants to protect.

    Raccoons can probably read better than the low life's that broke into your garage....
     
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