Fence-sitter warning system, how it works

Farmers are often soft targets for attacks and other forms of crime. The newly developed fence-sitter warning system alerts farmers to possible fence tampering.

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South African engineer Ernst Pretorius recently made it to the shortlist of the UK’s Royal Academy of Engineering’s African Engineering Innovation Award with his fence-sitter warning system that alerts a farmer to someone tampering with the farm’s fences called the fence-sitter.


READ: SAPS and AfriForum collaborate to deal with crimes on farms

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The fence-sitter warning system

A sensor, developed as part of his degree in Engineering at the University of Pretoria, stimulated his ideas on preventing stock theft. The fence-sitter warning system sensor was originally a component in a system to alert a farmer to abnormal flock behaviour, such as sheep running or moving away from the main flock. What concerned Pretorius was that a farmer would often be far away from the flock when the alarm sounded, so that sheep may already have been slaughtered by the time he arrived on the scene.

The solution came after Unis Pretorius, an ex-colleague and dairy farmer from Delmas, mentioned to him that stock thieves would typically cut the fence for a quick escape. “As a taut fence wire is an excellent conductor of sound, I decided to use a fence in a ‘tin can telephone’ system to detect tampering and alert the farmer in time to react,” recalls Pretorius.


The unit, named the Draadsitter (fence-sitter warning system), consists of two sensors that are attached to a fence post. One sensor measures the ambient environmental sounds as well as any sound caused by tampering, all transmitted as waves along on the wire. The other sensor measures only ambient environmental sounds.

An alarm is triggered when the difference between the sensor readings falls outside a pre-set range. The system thus filters out most environmental background sounds such as thunder, amplifying the sounds associated with actual fence tampering.

The first prototype was tested on an old fence. It detected tampering to about 50m away in both directions from the wiring post. Six months later, Pretorius had developed a more sensitive prototype that detected tampering up to 300m on either side of the wiring post on a good fence with taut wires and solid wooden straining posts.

But, when tested on an electric fence, the high voltage’s electric field triggered false alarms in the sensors and amplifier. Pretorius therefore encapsulated the unit in a metal box – as in a Faraday cage – to shield the sensors and amplifier from this electric field.

The lower electromagnetic noise level inside the new enclosure allowed greater amplification of the sound, enabling the sensor to detect sound up to a tested 429m in either direction from the post, or a tested total of 858m of good game fence with wooden posts, 22 taut wires and the post in the middle. It is possible for the unit to calculate quite precise distances, variable due to the tension of the wires and the steel in the fence. The initial units will only identify the nearest unit on a fence system that triggers on the sound.

Next, Pretorius further developed the sensor to differentiate between somebody cutting the wire and otherwise tampering with it. “Fencing wires exert forces on a wiring post,” he explains.

“These forces balance each other out. When one wire is cut, the other wires must compensate to bring everything back into balance. “This causes a slight vibration in the wiring post. If the level of this vibration falls outside an allowable range, it triggers the alarm.”

The sensor units has two default settings: sensitive (reacting to sound) or insensitive (only reacting to cutting of a wire). Pretorius recommends using ‘sensitive’ around the farmyard, and ‘insensitive’ in camps where an animal rubbing against the fence could trigger a false alarm.

Sensors can be switched remotely by radio signal to ‘sensitive’ via a base station when an attack is suspected, or to ‘insensitive’ or to their default state.

The prototype is about the size of a cellphone, but slightly thicker. It is fastened to the wiring posts by high-tension steel straps, making it difficult to steal. Tampering is especially difficult with the sensor placed on a high post such as on game fencing.

The prototype is equipped with a secure radio link, to prevent altering, overriding or jamming messages from being sent.

Pretorius has incorporated a temperature sensor in the unit to alert a farmer of fire. The sensor transmits a warning signal when it senses a temperature of above 65°C. He is also working on sensors to alert a farmer to when someone passes through a gate or other entrance.

Each sensor unit works on two AA batteries, which are expected to last in the system for longer than two years.
To save time in developing the fence-sitter system, the first prototype sets up manually. When setting the sensors, the farmer records the location of each sensor to locate the source of an incident should the alarm go off.

In future, the system will be improved to be set up electronically via a smart phone. The phone will be used to change system settings and save the GPS co-ordinates of each sensor unit. The base unit will transmit a message to the farmer’s phone, identifying the type of intrusion and GPS co-ordinate of the site in question.

The fence-sitter base unit would typically be set up in the farmhouse or office. An outdoor base unit, set up on a pole and run off independent solar panels, is being planned.

Available 2015
Pretorius hopes to complete the final fence-sitter prototype by December and aims to start marketing by June next year. The initial units will use several imported components and consequently will be somewhat expensive. Eventually it will be mass-produced using more locally manufactured components, so will be affordable to more farmers. “My hope for this system is that it will help to make a farm a safe place again,” he concludes.

Phone Ernst Pretorius on 012 361 3842 or email him at [email protected].