Electrified fencing is usually used to control animals with smaller and cheaper fence constructions than would otherwise be necessary with non-electrified wires. A typical live fence installation has a long wire or wires starting from an electric fence energizer and extending to surrounding fields in various complex configurations. In principle, the voltage of an electric fence depends on the desired power of the shock and the distance up to which shocking occurs. This varies from about 2,000 V (2 kV) to about 10,000 V (10 kV). A 10,000-V output is the maximum voltage allowed by international regulations.
Faults in electric fences are very difficult to locate because the average live fence holds kilometers of wires. The cordless live fence tester circuit, presented here, is a quick and easy solution for farmers and electric fence owners who want to know at a glance if there is power on their electric fence. By simply pointing the “antenna” of the tester toward the fence, you can find out whether or not it has power. In the case of a live (electrified) fence, a red light will blink in the tester. One advantage of this simple design is that there is no direct connection to the fence. The circuit can detect radiated signals/high-voltage pulses coming through the air from inches away. The 3-V (1.5 V x 2) battery-powered design is centered around the 74HC14 (IC1), a hex inverter with Schmitt-trigger inputs. A Schmitt-trigger input can transform a slowly changing input signal into a sharply defined, jitter-free output signal. In the circuit, the “antenna” (probe) is the free end of the 1M resistor (R1) connected to a small rectangular copper strip, which should be isolated for safety with a suitable heat-shrink tube. Switch S1 is the power on/off switch and LED2 (green) is the power status indicator. LED1 (red) indicates the presence of electricity in the close proximity of the antenna.
My prototype, built on a small piece of perfboard, was tested with a desktop HV generator (1 kV). Since I do not have an electric fence, the current status of this design is “working, but not sure whether it is working well or not with different fences.” I have been tinkering with this for weeks, trying many things (updates will be added in due time) and probably making mistakes (which I would like to learn from). Thank you in advance for telling me when I am wrong!
Although this is a very simple design that gives a visible indication on an LED, there is often a need to measure the actual voltage level on the fence. After all, electric fencing relies on high voltage to form an effective barrier against intruders. Therefore, voltage should be used as the primary means to determine the presence or absence of faults. Nowadays, the simplest method is to feed that voltage directly into the ADC of a microcontroller. Shown below is an example circuit of the hardware interface for this task, made with some inexpensive passive components. Follow this trail to build your own microcontroller-based live fence voltage tester. First, try out this Arduino LCD Voltmeter project: startingelectronics.org/projects/.
Live Fence generates voltage in kV range, which could be lethal. Experimentation should only be attempted by those experienced with high-level voltages and safety procedures.