Introduction: Simple Electronic Fencer
I have two jack russle dogs who love to rip up flowers, much to my partners annoyance, so I desided to have a root around in my project parts box and see what I could come up with. Low and behold I found that I had an old ignition coil from an old retired Mercedes car of mine, all I needed to do was to pulse its primary coil with 12 volts to produce a very high voltage in the secondary.
I now needed to design and build a circuit that would pulse a relay to feeb power to the ingnition coil. Using my bread board I set up a simple NE555 with a couple of resistors and a capacitor to give me 50% duty cycle. I then fed the output into a simple RC network that feeds a power transistor. Because the ingnition coil consumes a lot of current I needed to pulse the relay so that the transistor is only on when the RC is charging, I selected the values to give me about .25 of a second on time for the transistor, this is just enough to pulse the relay and cause the coil to deliver a powerful high voltage shock.
I then had to limit the current passing through the ignition coil so I used a power resistor of 82 ohms to allow approx 1amp to pass through the primary winding, this limits the power of the output to a save level that won't hurt my dogs. I tested this on my self, at first it didn't work very well until I hammered a 2 foot copper pipe into the ground and connected this to the negative of my circuit, now I was getting a nasty but harmless shock from the unit approx once every 2 seconds. It works very well, I used agricultural fencer wire with insulators around the flower beds and after a couple of shocks the dogs are happy to leave the flowers well alone.