Fence Charger Animal Contact Detector Circuit with Audible Buzzer

In one of my electric fence charger articles, one reader asked me something interesting. He wanted to know whether a buzzer could be made to sound whenever an animal touches the fence.

At first I was thinking about a current sensing method because that looks like the obvious way. But after looking at how electric fences actually work, I realized current is not always the best thing to watch here.

See, most electric fence energizers do not supply continuous power. They send very short high voltage pulses and the energy in each pulse is limited. So when an animal touches the fence, then current may still be only a few milliamps or maybe a few tens of milliamps, depending on the animal, the ground condition, moisture in the soil and things like that.

So now instead of looking at current, it makes more sense to look at the fence pulse voltage itself.

When nobody is touching the fence then pulse voltage stays close to its normal value. But when an animal comes and touches the wire, so part of that pulse energy gets absorbed through the animal and through the ground return path. Because of that, the pulse voltage drops.

That voltage drop is much easier to detect, and we can use it to switch ON a buzzer.

Basic Operating Principle

The idea is actually quite simple. We take a very tiny sample from the fence pulse through a high resistance network, then that sample is rectified and stored in a capacitor. That stored voltage is fed to an LM393 comparator.

As long as the fence is working normally, the stored voltage stays above a preset level. When an animal touches the fence, then the pulse amplitude falls and the stored voltage also falls.

The moment it goes below the preset setting, the LM393 changes state and switches ON a transistorized buzzer stage. So the buzzer starts sounding whenever the fence voltage drops noticeably because of an animal contact.

High Voltage Sampling Network

One thing is very important here. The fence voltage must never be connected directly to the electronic circuit. Instead we use a very high value resistor divider so only a tiny sample reaches the detector. The divider uses five 10M resistors connected in series.

Fence Wire, then 10M to 10M to 10M to 10M to 10M then to Detector Point

Total resistance then becomes:

Rt = 10M + 10M + 10M + 10M + 10M

Rt = 50MΩ

This very high resistance keeps the loading on the fence extremely small. At the same time it protects the low voltage electronics from the fence voltage.

Using several resistors instead of one also spreads the voltage stress across multiple parts which is generally safer.

Peak Detector Stage

The sampled pulse now goes into a peak detector stage. This stage uses one diode, one capacitor and one discharge resistor.

Whenever a fence pulse arrives, D1 conducts and charges C1. After the pulse disappears, then C1 does not discharge immediately because R1 allows only a slow discharge path. So now the short pulse gets converted into a smoother DC level which the comparator can easily monitor.

Comparator Stage

One section of the LM393 is used as the comparator.

The voltage stored on C1 is fed to pin 3, which is the non-inverting input. A 100k preset is connected across the supply and creates an adjustable reference voltage. The preset wiper goes to pin 2 which is the inverting input.

So now LM393 keeps comparing these two voltages continuously. One voltage comes from the fence pulse sample. The other voltage comes from the preset setting. That is basically the decision making section of the circuit.

Output Driver Stage

The LM393 output is connected through a 10k resistor to the base of a BC547 transistor. When the comparator output goes high, then BC547 turns ON. Once BC547 conducts, the buzzer gets power and starts sounding.

The transistor is simply working like a switch for the buzzer.

Circuit Operation

Now let us see how the whole thing behaves in practice. Under normal conditions, the fence pulse amplitude remains high.

The divider samples a small portion of that pulse and charges capacitor C1 to a level higher than the preset threshold. Since the voltage at pin 3 is higher than the voltage at pin 2, the comparator stays in its normal state and the buzzer remains OFF. Now suppose an animal touches the fence.

Then part of the pulse energy gets absorbed through the animal and ground path. Because of that, the pulse amplitude becomes lower.

The sampled voltage also becomes lower and C1 charges to a lower level. If that voltage falls below the preset threshold, then the comparator output changes state immediately. BC547 turns ON.

The buzzer starts sounding. So now the user gets an instant indication that a significant load has appeared on the fence.

When the animal moves away, the fence voltage returns to normal again. C1 charges back to its previous level. The comparator returns to its normal state and the buzzer switches OFF automatically.

Setting Up the Circuit

After assembling everything, power the circuit from a regulated 12V supply. Allow the fence charger to run normally without any load connected. Now adjust the 100k preset until the buzzer remains OFF during normal operation. Next apply a test load between the fence and ground.You can use a suitable resistance for this purpose.

Then adjust the preset again until the buzzer activates whenever a noticeable voltage drop occurs. This adjustment decides how sensitive the detector will be. So you can make it more sensitive or less sensitive depending on the application...

Concept using Current Transformer

So now this circuit watches that current increase and whenever a strong fence pulse current appears, then it switches ON a buzzer to tell us that something has touched the fence.

Circuit Operation

This circuit is built around LM358 and one small current transformer called T1.

The fence high tension wire is simply passed once through the center hole of the toroidal core. That itself becomes the primary winding, so there is no separate primary winding to make. Then on the same T1 core we wind a secondary coil having several hundred of wire turns and connect it with the detector circuit.

Whenever a pulse current goes through the fence wire, then a matching voltage gets induced into the secondary winding of T1.

Current Transformer Stage

T1 is actually doing the sensing job here. When fence is sitting normally, then pulse current is very small so only a weak signal appears across the secondary winding.

But when an animal touches the fence then fence current jumps up suddenly and therefore a much bigger voltage appears at the transformer secondary.

D1 and D2 are there for protection. Transformer pulses can sometimes become quite sharp so these diodes stop excessive positive and negative spikes from reaching the LM358 input.

C1 passes only the pulse signal into the non-inverting input of IC1A and blocks any unwanted DC from entering.

Signal Amplification

The first op amp section, IC1A, is used as a high gain amplifier.

R1 and R2 create a reference voltage point. R3, R4 and R5 decide the gain and switching level. R4 is the sensitivity control. If we increase it one way then circuit becomes more sensitive, if moved the other way then sensitivity reduces.

The amplified signal finally comes out from pin 1 of IC1A.

Pulse Detection and Storage

The signal from pin 1 next goes through D3. D3 rectifies the pulse and charges C2 whenever a sufficiently strong fence pulse is detected.

Fence chargers normally give only one pulse every second or around that timing, so without storage the signal would disappear between pulses. That is why we use C2. It stores the pulse energy for some time and keeps the voltage available between successive fence pulses.

R6 slowly discharges C2.

Here we have, C2 = 47 µF, R6 = 1 MΩ. So now the discharge time becomes:

T = R × C

T = 1,000,000 × 47 × 10⁻⁶

T ≈ 47 seconds

Which is quite long, so even short fence pulses can be detected properly without missing them.

Comparator Stage

The second op amp section IC1B works as a comparator. The stored voltage from C2 goes to pin 5 which is the non-inverting input. R7 and R8 create a fixed threshold voltage for the inverting input.

Whenever voltage on C2 rises above this threshold, then output of IC1B immediately goes high.

Buzzer Driver

The output from IC1B drives transistor Q1 through R9. When Q1 switches ON, then current starts flowing through the buzzer and the buzzer sounds. As long as fence current keeps producing strong enough pulses, C2 remains charged and buzzer stays active. When the animal leaves the fence, then no strong pulse current remains, so C2 slowly discharges through R6.

After some time voltage drops below the threshold and then buzzer switches OFF.

Current Transformer Construction

T1 can be made using a small ferrite toroidal core.

Typical winding details are:

• Primary: 1 turn (fence wire passing through the core)
• Secondary: 800 turns of 34 SWG enamelled copper wire
• The exact turn count is not very critical.
• Sometimes 500 turns works nicely, sometimes 1000 turns may give better sensitivity depending on the transformer core and fence conditions.

Adjustment Procedure

• Power the circuit from a regulated 12 V supply.
• Keep preset R4 roughly at the center position first.
• Run the fence charger normally.
• Now adjust R4 slowly until the buzzer remains OFF during normal fence leakage conditions.
• Next apply a test load or suitable resistor across the fence output.
• Then adjust R4 little by little until the buzzer responds only when a significant fence current is present.

Features

1. Non-contact current sensing
2. Complete electrical isolation from the high voltage fence wire
3. Adjustable sensitivity
4. Simple LM358 based design
5. Suitable for battery operated fence chargers
6. Audible indication whenever fence contact occurs

So now whenever an animal or any other load causes a noticeable increase in fence pulse current, then circuit detects it and gives an audible warning through the buzzer, which makes fence monitoring much easier.