Stephen wrote

There seems to have been an increase in the local cat population.
At first I tried to adopt a live and let live attitude but now cats are
sitting on my car, scratching at the fence panels, and going to the
toilet. Things have got so bad that I have dug up a couple of
flowerbeds and replaced them with gravel.

I have a toddler and I know dog mess contains germs
that can cause blindness, I don't know whether the
same is true of what the cats leave behind,

Worse with cats, it can kill you quite literally.

Tho the risk is quite low.

but regardless, I don't really want him playing with it!

Difficult to avoid with cats tho.

I have searched the internet and it claims some smells act as
deterrent: citronella is one suggestion. How volatile is that?

Quite.

Would it evaporate too quickly?

Yep, and it isnt really much of a deterrent either.

Many of he powders available from B&Q and garden centres
seem to contain pepper. The problem with these seems to be
that the smell washes away after the rain and has to be topped up.

Yep, and thats true of citronella too.

Is there a catering supplier I could get very
large (kg) quantities of pepper from cheaply?

LIkely, but I dont believe that will keep all cats away.

Ammonia seems to be another suggestion and Jeyes
fluid. Again Idon't know how volatile these are

Ammponia, very.

and the ingredients in Jeyes sound almost as unfriendly as the cat litter!

Worse actually.

There was a product that used to be highly recommended
which was a PIR activated water spray. I don't imagine it
was intelligent enough to aim and target the intruder, so
I presume it just operated a wide fan spray? Is that right?

I think these are £40-£50 and I would need more than one,
which makes things expensive. I could make one. I have seen
12v dc solenoid valves but do such things as 12v PIR exist?

You dont need one, just a tranformer for the solenoid.

The ones I have seen are 240v for lights. I'd prefer not to mix
and match voltages and SELV would be better for obvious reasons.

Sure, but it wouldnt be hard to keep the 240V well away from the
solenoid and water just by having the power supply well separated.

The RSPB web site seems to favour ultrasonic deterrents but these cost as
much. Has anyone made their own? Does anyone know the ideal frequency?

Thats got to be available on the net.

I remember using 40kHz piezo transducers once upon a time
but I wonder whether they would be loud enough or the right pitch.
Again PIR controlled but I think either of these could be made DIY
for a fraction of the retail price.

Sure, but I have real doubts about how effective it would be.

What do you think?

That a dog kept in the yard is much better for keeping cats away.

Corse there is the dog **** problem.

You can put out plates of milk with paracetamol dissolved in it.

Kills cats quick smart.

The neighbours might not be too thrilled tho.