Low cost electric fence indicator light

I have an electric fence which is powered by an energizer that sends up to 9KV pulses every 1.5s.

I can buy brand name indicator lights that attach to the fence with a clip and have a ground probe, and which flash every time the fence pulses provided the voltage is above 3kv. They are very useful as they indicate from a distance, which sections of the fence are working and which have a short.
However they are quite expensive and despite being designed to be outside, not that robust. So am wondering whether it would be possible to build a cheaper alternative. Ideally with an LED that is bright enough to be seen from a distance- 500m+

Thanks in advance.

Hi Cath

That is going to require a few watts of power which an electric fence will not provide. While the fence will provide kVs of volte there is very little current available. If there were it would be pretty lethal. It is a supply with a very high source impedance.

If you had a short to ground none of the fence would be working. OK to indicate a break (open circuit) though.

The fence would not provide enough current to power any sort of LED so your indicator (I presume passive, unpowered) would be some sort of Neon indicator which need next to no current.

A LED of any sort would have to be separately powered and the only solution I can see is have a sensing circuit looking at a neon device which will activate a suitably powered LED.
Cheers Bob

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Hi Cath

I don’t think so.

Read on and perhaps you might decide they are really not that expensive after all.

I said previously that the only device that I know of which provide visible indications at the level of current probably available from your electric fence would be neon devices. There are some around which switch at 90V and require about 0.5mA which might be OK.

The series resistance to drop 9kV at 0.5mA is 18MΩ. Now this sounds fairly easy but with voltages up at 9kV there are other factors.

This cannot be done with 1 resistor of 18MΩ. There would be more current leaking across the resistor body that going through it. The commonly available resistors would have an allowable maximum of 500V or so across them. 18 resistors of 1MΩ would each have 500V across it so the number of resistors in series would have to be 18 at minimum. A safer alternative would be 24 resistors of 750kΩ or 36 of 500kΩ which would give 333V and 250V respectively. The next problem is the mounting material which has to withstand the same voltage gradient as the resistor string.

The next problem is using these flashes to trigger and drive a higher powered LED so it can be seen at a distance.

As you may see, the electronic problems are not so great but at 9kV the mechanical difficulties multiply somewhat. You might be starting to think the commercial device might not be so expensive after all.

You may have seen a high voltage probe for a multimeter. There is a reason it is so long and has fins moulded into the plastic body.
Cheers Bob

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Hey Cath,

It looks like Bob has done an in depth dive on this one and supplied a bunch of information that can help get you on your way here. Obviously a big problem here is that such a high voltage and low current can make it difficult to power something that relies on a decent amount of current to function, especially when we are talking 500+ metre distances.

While I don’t have any experience in dealing with this type of thing, it seems bobs suggestion may be the right way to go here, plus it would be more cost effective than purchasing what are likely proprietary parts.


Hi Cath,

Is this the sort of indicator light you would normally use on your fence: Pulsar Electric Fence Light | JVA Technologies ?

Cheers, Steve

Hi Steve

I had not seen that brand before- the ones I normally purchase are made by Gallagher and are about $45 a throw. That one looks a lot simpler than the Gallagher one. Thanks for sharing!


Thanks Bob for going into such depth on something I thought would be a lot simpler to recreate with consumer electronics that it looks like it is going to be! I have about six sections of fence that are gated with an on-off switch and resolving shorts involves driving between the switches with a fence tester and trying to locate which bit of fence is shorting. It’s often a very tedious job! The lights are great in that they do show at a glance, especially at night, which sections of fence are working or not.

The device I currently use is made by Gallagher and doesn’t seem to have it’s own power supply, but maybe it does have a small battery inside the casing. I should take apart one of the dead ones I have and see if it does.

Thanks for taking the time to explain the details -I have a much greater understanding of why it isn’t as easy as I thought!

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Hi Cath

Like I said before if the fence is shorted to ground none of it will be working. That is all the sections powered by a particular energiser.

Might be why it is dead. But if it is replaceable surely it should be accessible.

Re Stephen’s link. This device requires 1 Joule (1 Watt per second) to operate. So 2 devices would require 2 Joules etc. The 2 wires on this device seem pretty close so might be border line. If the insulation is adequate and the weather not too humid must be OK or they would not sell it. There must be more than the 7 LEDs inside surely.

While I have no experience with electric fences I do have quite a bit of experience in high voltage (7.5kV +) measurement in high power transmitters where the meter multiplier resistor is made up with lots of resistors as described above. In one case mounted on a quality insulating board about 300mm X 300mm with a lot of 250kΩ 2W resistors mounted on it. The application is similar to my above description, to get the voltage gradient across individual resistors down to a manageable level using “off the shelf” components. I daresay there might be very high insulation type resistors about somewhere but not exactly common or I suppose very cheap ( cheaper was a word you used).

You have not provided any details re your fence to date so the power capability is unknown. I doubt it would be much as like I stated it would be lethal.
Cheers Bob

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Hi Blayden

No “in depth” research needed. Been there, done that as the saying goes. A parallel situation is the multiplier resistors for high voltage (kvolts) metering in high power transmitters. These are usually “off the shelf” components used with maintenance in mind. The end user does not want to have to go chasing “special” components to get a unit back on line. In my experience if done properly in the first place (as with most things) very little trouble is experienced in this area. I think I have seen 1 resistor in such a string fail.

A thing to watch here is the soldering technique. There can be no sharp or pointy bits. In this particular application there is a meter movement switched to various functions. When the movement is connected there is a gradual voltage gradient across each resistor getting down to near zero at the meter. However, when the meter is disconnected the full high voltage will appear at each resistor junction. With sharp bits this could cause some corona discharge and air ionisation at these points. Highly undesirable. Too far back to remember now but there might have been a “bleed” resistor arranged at the bottom of this string to prevent this.
Cheers Bob

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Hey Cath, Bob, Blayden and Steve,

Just throwing in my two cents here, I would also be surprised if you can get a fence to kick out that much current continuously. I wonder whether the brand name solution is trickle charging up a cap/battery then just discharging in bursts, or has its own power supply and just uses a sensor to detect the pulses.

If you are trying to put together a custom solution, rather than trying to use the power off of the energizer. I’d suggest using some large LiPos or another form of active power supply for the unit instead, possibly with some solar cells to keep it charged up (for example, the Farmo Electric Fence LoraWAN Sensors use 3500mAh Lithium Thionyl Chloride batteries, that apparently allow the sensors to operate for around a year before they need replacement)

I have a tester from Gallagher that indicates faults in electrtic fences.

You hang it on the hotwire with its metal clip and connect the ground wire to earth.

It will indicate which side the device the fault is on (left or right) and the approximate distance to the fault.

I’ve always assumed it operated like the TDR I used in troubleshooting back in my Cat3 (and earlier) network days!


Hi Bryce
Like I said. I have no experience with electric fences or how the HV is generated. Could be like a CD ignition, charge up a cap then discharge through a device like an auto ignition coil.

Irrespective of all this the object here is I believe to monitor and display the HV pulses and the techniques I described still apply if you wish to use easily obtainable bits. Does not matter much just how the HV is generated.

Your observation

I definitely agree with. Otherwise it would be very dangerous indeed. The idea here being that the supply has a very high source impedance. When accidentally touched your body resistance will form a voltage divider with said impedance and the voltage at your body will fall to a low and safe value. Gives one a bite sure, but relatively harmless. Pretty much like the zap off an old TV picture tube. All the damage is secondary when you bash your head on the TV cabinet.

When all is digested the main object of this post was to see if it was feasible to build a device more economically that a commercial product. In a nutshell, personally I don’t think so. (my personal thought)
Cheers Bob


Hi Alan

Finding out how that works would be a good start but maybe not so easy.

But Cath’s original query was something that is fitted permanently and gives a visual of status more or less on a full time basis. I don’t know about the 500M though. That Gallagher device would be great to trouble shoot a fence known to be faulty but maybe not a permanent fixture.
Cheers Bob
Just re read your reply. It could indeed be a TDR device. I have often used the technique to locate co-ax cable faults. It is actually also used to monitor the performance of TV TX antennas. If this is the case the fence would need to be un powered, ie; OFF.


Hi Bob

I did a bit more web searching and found these diagram: https://electricfencelight.com/wp-content/uploads/2016/05/Knocker-Spark-Gap-Adjustment-Instructions.pdf

Could it be that the spark gap means the light won’t work when something touches the fence as that provides an easier path to ground ?

I believe this is very different to the first light I found, by the way.

Cheers, Steve


Hi Stephen

Yes, but that would be the case with any of these indicators anyway. With this sort of thing it is only the initial moment that bites. Like a car ignition. If you can grab a spark plug lead and hang on to it the initial jolt makes one jumpy but if you hold on to it the rest are nowhere as savage. I once knew a bloke that could stop a 4 cyl engine by spreading his fingers across all the plug leads.

I was just thinking a while ago that it would be nice to know if the HV is present with one of these visual indicators connected to the fence. I could see how they could work. The 9kV leaps across the spark gap to the neon with enough energy to flash the lamp once then dies until the next pulse. I think under these conditions the rest of the fence would not be very effective until the device was removed. But the specs on that one you linked says it requires a minimum of 1 joule to operate which would be fair enough to operate LEDs but a neon may not require that much.

Will see if Cathy comes good with some fence info. Until that I have run out of ideas. I did look at the Gallagher site and they have models from 0.2 joules to tens of joules so there is quite a range possible.
Cheers Bob

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Here are some ideas: LED indicator light for electric shock fence | Electronics Forum (Circuits, Projects and Microcontrollers)

Hi Rooppoor
Good luck with most of that. Firstly find a 240kΩ resistor that will withstand 6000V then find the 25mA for the LEDs. That is the original idea. AC does make it easier for the transformer idea using spark plug wire but you still have to find the POWER to provide the visual result. Neon is the only obvious solution but usually not particularly bright at these power levels.
Cheers Bob

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Hi Cath.
I have been having a bit of a look at some of the fence monitor devices. I THINK (I stress “THINK”) the unpowered ones, ie; the ones that require power from the fence for a visual presentation, would be good at telling you the fence is operating and has no shorts to ground. But because they need power to illuminate something I don’t think they could be a permanent installation as there would not be enough bite left in the fence wire to be effective with animals while any indication device was illuminated. I THINK any device permanently installed would have to use next to nil or actually nil power from the fence, particularly as I read in your instance you have more than one on any fence run. In other words this add on needs to be separately powered.

Thinking about this an inductive pick up as in an auto ignition timing light will not work as to operate this type of pick up current needs to flow. This happens with ignition when the plug sparks or with your fence when shorted to ground by an animal or accidentally by other means.

But CAPACITIVE pick up might just work as the HV is pulsed. This could be used to trigger something like a 555 timer to provide a pulse to switch a LED for as long as you want. This only needs to be a fraction of a second to conserve battery power.

I have sketched up a block diagram of something that MIGHT work.

Unfortunately there would have to be some experimental work to be done preferably with an oscilloscope to determine the nature of the cap pick up etc. This might just need a couple of cm of high voltage wire (auto ignition wire) taped along the fence wire. The exposed end of the copper ignition wire MUST be kept well clear of the fence wire or an arc over could be disastrous. These experiments need to be carried out before any detail is put into the boxes on that sketch but unfortunately I don’t have access to an electric fence of any description to play with. (but I do have an oscilloscope).

With regard to building something up yourself I think this the best I can do with the info available. Something like that block diagram should work in principle but that could be a bit different in practice.

I am assuming throughout this that this fence is DC. Even if it is AC current has to flow (that is power has to be used) before an inductive pick up system will work. This is not going to happen unless a cow or something is making a circuit to ground…
Cheers Bob


Late to the party. An intriguing problem. The first specification that stood out to me is “bright enough to be seen from a distance- 500m+” - it doesn’t say if in daylight or at night. I don’t think this can be achieved easily without optics - either the person looking has binoculars or similar, or the flashing light is focused like in a torch.

So my design would be a capacitor divider as suggested by Bob - say 100pF and 100nF is a divide by 1000 and a 15kV 100pF capacitor is less than $5 from Element14. Butcher a solar light to get the solar cells, charging circuit and battery. Have the battery wired to a cheap torch triggered by the pulse from the divider. If the torch is oriented to point at the farmhouse back veranda then it should be easily visible at night, and can be checked using binoculars in daylight.

Not costed, so may be too much trouble and no cheaper than a commercial unit. So more a thought exercise.

Hi Alan
I don’t mean a physical capacitor, just a few cm of say auto ignition wire alongside the fence wire.

It is anticipated that there would be a number of these positioned on I don’t know how many fences. That info is not forthcoming. Also the envisaged number of units on any one fence module. Most of the devices I have seen seem to be diagnostic, I mean walk up to the fence, clip it on and see if the fence is working. Not to be left in situ. I get the feeling that a fence would not stand too many indicator devices hanging off it at once before it dies and ceases to be effective.
Cheers Bob