Electric fence energiser

might be almost the same as a camera flash circuit. maybe you can hack a small disposable jobbie?

What are you trying to keep out?

Get a 12V coil from an old car, and something to interrupt the primary.

No circuits, but plenty of electric fence energizers on ebay.

-- Sir Benjamin Middlethwaite

Yes that's the easy bit

This is the bit I need the help with ;-(

Sounds kewl. You put a push button by the fence and ask the animal or whatever to push it before touching the fence.

Old gas cooker igniter module,...bugger that hurt :-)

-- Sir Benjamin Middlethwaite

A DPDT relay. Use one pole to get the relay to click on/off (by connecting the coil in series with the n/c), and the other pole to switch your primary. You'll need to put a large-ish capacitor across the relay coil to slow down the clicking, once every couple of seconds is about right.

pmsl

-- Sir Benjamin Middlethwaite

Movement detector of a security lamp. :-)

-- Sir Benjamin Middlethwaite

Yes but at around £50 I should be able to make one for nought

Something to prevent pitting and destruction of the relay contacts?

I wonder about the RFI aspects of all of this as well. Do commercial units have relays or semiconductor switching, I wonder?

Although this isn't quite in the league of Tesla machines able to compete with North Hessary Tor, a relay banging away into a coil is likely to generate enough RF to be a nuisance in populated places.

Condenser from a points based ignition system should be fine. You could probably also find the relay in the scrap yard along with the coil and battery.

Old ones were relays..new ones semiconductor.

In theory it won't cause any more interference than a car ignition system as it uses the same parts.

sponix

Yes, that wouldn't be a bad idea - another cap across the relay contacts switching the coil - in fact, a car ignition cap (condensor) would be perfect.

We've had both types. Strangely, the relay ones last quite a bit longer. Poor heatsinking on the switching semiconductor I suspect.

The capacitor should bring the level down to car-emissions levels I'd have thought.

Thanks But I think spark gap transmitter's have been banned for some time ;-(

Something like this but with a DPDT relay instead of the switch:

Take the module of one of these...

-- Sir Benjamin Middlethwaite

All you'd be doing is re-creating a non-electronic car ignition system. With a capacitor across the relay contacts, RF emissions should be very reasonable.

Yes you are quit correct that was a case of me opening mouth before engaging brain. I was really looking for a semiconductor type circuit but if all else fails I will try the relay option Thanks

That's easy - use a 2N3055 transistor to drive your coil. Use a 555 to switch the 3055. Need more details, or does that make sense?