Quick question

[Whitling2k]

Now then! I have a few supersonic cat scarers in the garden to stop them from laying landlines on my lawn. Currently each scarer eats through 2x 9v batteries every month or so.. Which is getting expensive! They do have an option for powering then with a 9v DC, 500ma mains adapter. The problem being that their official adapter only supports one device per adapter, and I don't want a bank of low draw devices taking up all my spare sockets!!

Apparently they draw way less than the 500ma.

Will it be possible to buy a 3rd party adapter, and wire several of them up from it in parallel? If I did, would I need more than 9v or more milliamps?

I have a relatively large garden too, so we're talking 40m of wire taking the 9v to them. I was thinking bell or alarm wire. Will I also need to account for these long runs in my calcs.

I'm pretty sure it's a simple calculation, but can't remember enough of my A-level physics!!

Cheers

Whitling2k

 

[JohnW2]

Now then! I have a few supersonic cat scarers in the garden to stop them from laying landlines on my lawn. Currently each scarer eats through 2x 9v batteries every month or so.. Which is getting expensive!

I presume you have considered, and for some reason rejected, the use of rechargeable batteries?

They do have an option for powering then with a 9v DC, 500ma mains adapter. The problem being that their official adapter only supports one device per adapter, and I don't want a bank of low draw devices taking up all my spare sockets!! .... Apparently they draw way less than the 500ma. ... Will it be possible to buy a 3rd party adapter, and wire several of them up from it in parallel? If I did, would I need more than 9v or more milliamps?

In principle it's possible. What you primarily need would be "more milliamps" - in fact N x X mA if you have N devices which each draw X mA. However, you might need more volts as well ...

I have a relatively large garden too, so we're talking 40m of wire taking the 9v to them. I was thinking bell or alarm wire. Will I also need to account for these long runs in my calcs.

Yes, you certainly need to take voltage drop into account, and the types of cable you are contemplating might not be adequate. If it were simply a long run to a single device, you could simply use a supply of a sufficiently high voltage that, with the voltage drop due to the operating current, about 9V was delivered to the device. However, if there are several devices at substantially different distances along a cable run, it's not quite so simple. If you chose the voltage so that the first one got 9V, the last one might get a lot less than 9V (hence might not work). If you chose the voltage so that the last one got 9V, the early ones might get a lot more than 9V (hence might blow up!). That problem could only be avoided by having sufficiently hefty cable that the voltage drops were not very great, hence differences between voltages supplied to the various devices not too great.

If you could find out (or measure) roughly how much current one of these devices draws at 9V, and also tell us the approximate distances involved (distance from supply to first device, from first to second device etc.) then we could suggest what sort of cable (and voltage) you would need.

Kind Regards, John

 

[Whitling2k]

yes... cat poo... not the nicest thing to be scraping off my 1 year old's feet because she selfishly wants to play out the garden. Worst thing is, round here they don't even bother trying to hide it - middle of the lawn, paving slabs, even the drive.

Anyway...

Rechargables: Discounted these because I want something I can plug in and forget really.

Cables: (Outlet) - 10m - (device 1) - 20m - (device 2) - 10 m - (device - 3) - 20m - (outlet) So I was thinking of making a ring circuit. No idea why, just seemed sensible.

Draw: Not on specification, on box or website. Can anything be deduced from 2x 9v batteries? I do have a multimeter, but I have no idea how to use it. I can post a pic if you can suggest which settings to use!

I can see what you are saying about over-voltageing to cancel out the length of the cable, but then risking burning them out I guess I could just run 3x 20m cables from a single output - which would make each device have a similar drop-off (is resistance proportional to length, and consistently so?)

Do they work? Actually (and to my suprise) they do on the whole. The cats don't like them, and try to outsmart them at first. It took a few goes to get their placement right, i.e. sound not reflected too much, PIR in the right place, right height etc. But now, 9 times out of 10, if we find some poop in the garden it's either the hedgehog, or the batteries have died. They are not fool-proof, but do help.

There are just sooo many cats now a days though. I think an old couple up the road take in nearly dead cats - some are deaf, and they just ignore the ultra-sonic squeeks. Unfortunately it's these nearly dead ones that (literally) drop their cat-diarrhoeay bowels anywhere. yuch!

Whitling2k

 

[JohnW2]

Rechargables: Discounted these because I want something I can plug in and forget really.

Fair enough.

Cables: (Outlet) - 10m - (device 1) - 20m - (device 2) - 10 m - (device - 3) - 20m - (outlet) So I was thinking of making a ring circuit. No idea why, just seemed sensible.

A ring would help a bit, but it will make the sums a little more complicated!

Draw: Not on specification, on box or website. Can anything be deduced from 2x 9v batteries? I do have a multimeter, but I have no idea how to use it. I can post a pic if you can suggest which settings to use!

We do need to have an idea of how much current they draw. Could you post pics of your multimeter and also one showing the batteries 'in situ# in one of the devices?

I can see what you are saying about over-voltageing to cancel out the length of the cable, but then risking burning them out I guess I could just run 3x 20m cables from a single output - which would make each device have a similar drop-off (is resistance proportional to length, and consistently so?)

Yes, separate (similar length) cables for each device would solve that problem, but it might not be necessary to go to such lengths.

KInd Regards, John

 

[JBR]

May I suggest a carefully positioned sprinkler (with IR sensor). I have two protecting my wife's herb garden and other areas near to the house (where the smell would be particularly noticeable) and find that they work very well.
Again, though, they use batteries but seem to last longer than you are experiencing.

 

[Whitling2k]

Morning!

Multimeter and device images.

I did note the bottom of the device says 9V, 1.8VA... is this a clue?

Whitling2k

 

[EFLImpudence]

If you had a dog, and you allowed it to roam uncontrolled, jumping fences into other people's gardens, defecating everywhere, killing wildlife, how long do you think you'd get away with that?

Not very long because the law calls for you to control what is a dangerous animal.

So why do you think cats should be different?

Because, in law and in fact, they are.

Maybe you can be the first person in history to come up with an answer which doesn't immediately destroy your case and confirm that I'm right.

The law allows for the nature of cats.
It also 'actually says' that you may not harm them.

It is no different than other things which the law allows or forbids despite your views.


http://www.thecatgroup.org.uk/pdfs/Cats-law-web.pdf

 

[JohnW2]

... I was thinking bell or alarm wire.

Cables: (Outlet) - 10m - (device 1) - 20m - (device 2) - 10 m - (device - 3) - 20m - (outlet) So I was thinking of making a ring circuit. No idea why, just seemed sensible.

Morning! ... Multimeter and device images. ... I did note the bottom of the device says 9V, 1.8VA... is this a clue?

OK, lets forget about the multimeter for a moment and assume that the 9V 1.8VA is correct - that equates to 200mA (0.2A) current draw per device (at 9V). Let's also forget about your 'ring' idea for a moment (although that would improve things). Assuming I've got my sums (done in haste!) right ....

AIUI, 4-core alarm cable typically has a resistance of around 0.1Ω per meter for each core. If you used pairs of those cores in parallel, each pair would therefore have a resistance of about 0.05Ω, a total resistance for the two pairs of about 0.1Ω. On that basis, if you fed all three devices 'in a line', the voltage drop (VD) between source and 1st device (10m) would be about 0.6V, VD between 1st and 2nd devices about 0.8V and between the 2nd and 3rd devices about 0.2V. That means that if you fed the system with 9V, the first device would get about 8.4V, the second about 7.6V and the third about 7.4V. Those are all a bit low, If one could find a 10V supply (which might be difficult), the devices would get about 9.4V, 8.6V and 8.4V respectively, which would very probably work OK.

If you used 6-core alarm cable (same size cores) with two 'triplets' of cores in parallel, with a 9V source the devices ought to end up with about 8.6V, 8.06V and 7.92V respectively - perhaps OK, but again a bit low. Again, if you could find a, say, 9.6V source, those voltages would become about 9.2V, 8.66V and 8.52V respectively.

If you used 1.0mm² flexible cable (each core about 0.023Ω per meter), with a 9V source the devices would get around 8.72V, 8.36V and 8.27V respectively - which would probably be OK. If you could find a source ~8.5V, it would be fine.

If you used 1.5mm² cable, with a 9V source the figures would become about 8.81V, 8.55V and 8.49V respectively, which ought to be OK - obviousness even better if the source provided a little over 9V (say about 9.3-9.4V).

If it were me, I think I'd probably go for the latter of those options - although a certain amount of 'bodging' would be necessary to connect pairs of 1.5mm² flex to suitable plugs to plug into the devices.

Kind Regards, John


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