Low Power
Electric Pet Deterrent


"Scat, Cat!"

We have been plagued by neighbours' cats using a flower bed, which we use for a herb garden, as their local ablution block. Not that I am against cats, just don't relish the thought of cat doo complimenting the salad dressing!

Having tried everything "natural" i.e. citrus peel, coffee grinds, and even pepper corns (stopping short of chemical alternatives they sell in hardware stores especially as one doesn't really know what contaminants they use in these concoctions!), it appeared nothing would deter the more resilient moggies. More on this at the end of the article.

The following is an ecologically friendly, chemical free means to keep these furry visitors from using the areas one does not want used by them. It does not require "topping up" or replenishing, and costs about 10p per year to run.

They use it on farms to contain horses, cattle, and even sheep. It's quickly moved from place to place, and once removed there are no "trace elements" that need to leach out before one can use the ground.

I'm referring here to an electric fence. This is just a lower power version for smaller bodies. The really good part is one only needs to put up one that works, and any "dummy" fence is also regarded, through association, as hostile.

The same thing can comfortably be used for keeping foxes off dustbins, or any other furry friend (or enemy, depending on the angle of view!) from areas you don't want them to be in.


Scat Cat

The circuit uses current limited direct rectified mains of 230VAC with respect to ground. Please do be careful (although everything has been done to make this as safe as possible - see later).

Only the Live is used, and R1 and R2 limit the maximum current to about 1mA. The neon lamp, L1, indicates a good return connection via Ground (i.e. terra firma). Do not be tempted to use Neutral as this will cause RCDs to operate.

C1, D1, and D2 form a voltage doubler circuit. This forces both halves of the cycle to be used as half-wave rectification would cause 'plating' of the ground electrode. C3 is the storage capacitor - more on this later.

R3 and R4 form a voltage divider and combined with the neon lamp, L2, sets the trigger voltage of the circuit (about 250VDC). Gate trigger current is provided by C2 with R5 limiting the gate current. R6 ensures no false triggering.

The SCR, when triggered, forces the energy stored in C3 into the step-up transformer. R7 and D3 protect the SCR from negative going commutating voltages.

The step-up transformer is a non-critical car ignition coil. It has two major advantages - it is cheap for starters, and is pretty close to fully weatherproof when used with the correct lead cap on the high-voltage pin.


There are two ways to build the circuit. But, first things first..

Always place R1 as close to the mains Live as possible so as to reduce the likelihood of high fault currents should something short out. If using a plug and lead (see the two construction methods below), replace the fuse in the plug with R1 (maybe going so far as to mount the neon, L1, in the plug too).

The choice most would make would be to build the circuit in a weatherproof box at the coil. It is then strongly suggested that the circuit be powered via the current limiting resistors which remain with the plug so that the cable between the plug and circuit is current limited (the neon remains in view in the comfort of indoors to ensure the circuit is operating).

I chose to build the whole circuit in a PSU plug-style box - this remaining indoors - with a 2-core cable between circuit and coil, the latter sited at a convenient spot under the sting wire. The available strike current is reduced slightly, but such reduction is acceptable when it is remembered all one wants to do is make visiting the area uncomfortable.


Do not be tempted to replace R1 and R2 with one resistor of 200k (or thereabouts). Two resistors are deliberately used as most standard resistors are only rated for 250VDC. Using one resistor significantly increases the likelihood of flash-over.

Do not be tempted to become cruel and up the value of C3 (the storage capacitor). The idea is to sting the feline visitor, not kill it! In this instance, C3 holds under 100mJ of energy which is about the same as a hefty static discharge - which is more than safe should a human (especially a tiny one) touch the wire!

As an aside, the cats appear to have stayed away completely since installing the stinger which proves animals are sensitive to such electrical fields (except for the birds which appear to have increased, probably because of the extra safety of having no cats about!).

For the really nervous amongst you who are wary about directly connecting to the mains, use two identical transformers (example 230V:15V) back-to-back - and if you don't know what I mean, you should not be playing with circuits like this! Another method is to use an AC output 'mains adapter' feeding a transformer of similar voltage and using the 230V side to feed the circuit. Either of these two methods provides double isolation from the mains (just remember you now need to bring the return to the circuit).


It is very important to site the step-up transformer at the start or, better still, the centre of the sting wire. Do not be tempted to site the step-up transformer indoors and route the high-voltage lead to the sting wire; It can create large ground voltages which can have all sorts of effects.

The secret for good operation is a good Ground connection and to keep the Ground wire as short as possible. For the ground electrode I used an old aluminium baking tray - the connection to it via a wire soldered to a lug then screwed to the tray. The tray was dug in as far as possible in the soil below the step-up transformer. The old fashioned copper-clad ground rods will work just as well.

The sting wire is supported by any decent insulated material - strangely enough, lacquered ornamental bamboo is very good, as well as Perspex and plastic electrical conduit. Short lengths of polyethylene rope can be used where the wire needs to be terminated against conductive structures.

The sting wire can be extended across paths etc. using the inner of solid dielectric TV coax ('foam' based coax is no good), but here a little work needs to be done. Strip the inner out of the coax, remove the shield, and then pull the inner back in to the outer sheath so as to protect the inner as it is hygroscopic (not forgetting to seal the ends when done!).


Some time in to the summer night
and out of view from the owner's sight
a cat came to visit with just one plight
to make a deposit of ghastly might!

Even water from the garden hose
has not stopped moggy's familiar pose
and again it the herb garden chose
to leave behind what gets up one's nose

Again scraped away earth till it saw fit
to plant its body down to sit
but soon it was heard to hiss and spit
as the wire beside it took hold and bit!

In the morn was found the point it tried
to ablute before it was stung on the hide
but I can safely say and also confide...

... our herbs are safe for your inside.


I keep stumbling across forums and blogs where people both praise and utter frustration at sound based pet repellers. Most seem to agree one should not even consider the battery versions, but even using mains powered versions seems to bring failed attempts at chasing the more resilient pest-pets. They forget, animals can get used to sounds too! My aversion to sound based... what about the birds?

I've noticed the next door cat hop over the fence, stride towards its old 'loo', and then suddenly stop in its tracks, prick its ears up, then scarper at a rate of knots. It heard the 'click' of the fence and remembered a rather unpleasant sting its hide once before felt! And, it didn't stop to look for another spot in the garden. It was gone. Ah! Mission accomplished!

As for chemical; Well, I've stumbled across a pellet that uses lion urine as the base to propel the smaller feline visitor into next year! The scent gives them the shivers; Being territorial they feel their nine lives are in danger of being used up all at once should the owner of the new smell suddenly reappear!

But... was the whole idea not to get rid of the unpleasant odour? And not to replace one with another?

And, as for the stinger being 'barbaric'... please! The forums and websites perpetuating these claims will often place such comments right next to the section on 'sterilization"! Enough said.

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© 09.07.05 / 07.04.07