I built my first electric fence circuit back in 2011, to protect the perimeter of a tent from bears. The circuit still works today and gives a very good performance. A few changes have been made like adding the neon bulb protection for induction coil spikes sent back in the primary circuit.

The circuit runs on 12V (batteries), making this a very portable fence energizer solution. It uses 2x 555 timers, one runs in astable mode and is used to generate the frequency interval, while the second one controls the pulse length and so the output voltage. A calculator for the 555 timers can be found here.

The updated circuit diagram for this version of the circuit is available:
electric_fence_circuit_induction_coil_driver_555
Here are the Eagle files, with both the SCH and the PCB layout: fence-4.3-orig

Miniaturized version

Thanks to SMD components and various timer alternatives, such as the double timer 556, we can shrink this circuit even further, without making any compromises. Here is an improved version that I've just designed and tested with excellent results:
electric_fence_circuit_1 electric_fence_circuit_2 electric_fence_circuit_3 electric_fence_circuit_4
The circuit diagram:
miniaturized_electric_fence_circuit_pcb miniaturized_electric_fence_circuit_sch
The eagle files: fence-4.2

Both the designs can operate on battery and have low power consumption.

Does it work against bears?

This circuit was not tested against bears nor other animals, but given it's ratings it would surely create huge amounts of pain. Here is a demo video showing a commercial unit that does work:

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