Outdoor Security System Sensors

Outdoor Security System Sensors

These types of sensors would be found most of the time mounted on fences or installed on the perimeter of the protected area.

Vibration (Shaker) or Inertia Sensors

These simple devices are mounted on barriers and are used primarily to detect an attack on the structure itself. The technology relies on an unstable mechanical configuration that forms part of the electrical circuit. When movement or vibration occurs, the unstable portion of the circuit moves and breaks the current flow, which produces an alarm. The technology of the devices varies and can be sensitive to different levels of vibration. The medium transmitting the vibration must be correctly selected for the specific sensor as they are best suited to different types of structures and configurations. More sophisticated sensors use piezo-electric components rather than mechanical circuits, which can be tuned to be extremely sensitive to vibration. These sensors are more durable and more resistant to tampering.

pros: Very reliable sensors, low false alarm rate and middle place in the price range. cons: Must be fence mounted would be the main con. Its rather high price deters many customers, but its effectiveness offsets its high price. conclusion:The best money for value fence mounted configuration.

Passive Magnetic Field Detection

This buried security system is based on the Magnetic Anomaly Detection principle of operation. The system uses an electromagnetic field generator powering with two wires running in parallel. Both wires run along the perimeter and are usually installed about 5 inches apart on top of a wall or about foot buried in the ground. The wires are connected to a signal processor which analyze any change in the magnetic field. This kind of buried security system sensor cable could be buried on the top of almost any kind of wall to provide a regular wall detection ability or be buried in the ground.

pros: Very low false alarm rate, can be put on top of any wall, very high change to detect real burglars. cons: Can't be installed in near high voltage line or radars and airports. conclusion: The best solution in the buried sensors range.


This proximity system can be installed on building perimeters, fences, and walls, and also has the ability to be installed free standing on dedicated poles. The system uses an electromagnetic field generator powering one wire, with another sensing wire running parallel to it. Both wires run along the perimeter and are usually installed about 800 millimetres apart. The sensing wire is connected to a signal processor that analyses:

Amplitude change (mass of intruder), Rate change (movement of intruder), Preset disturbance time (time the intruder is in the pattern). These items define the characteristics of an intruder and when all three are detected simultaneously, an alarm signal is generated. The barrier can provide protection from the ground to about 4 metres of altitude. It is usually configured in zones of about 200 metre lengths depending on the number of sensor wires installed.

pros: concealed as a buried form. cons: expensive, short zones which means more electronic (more money), high rate of false alarms as it might sound as it can define a cat from a human in reality it doesn't work that well as well as extreme weather causes false alarms. conclusion: As in the buried security systems field the Passive Magnetic Field Detection will do a better job most of the time.

Microwave Barriers

The operation of a microwave barrier is very simple. This type of device develop an electromagnetic beam using high frequency waves that passes from a transmitter to a receiver, creating an invisible but sensitive protection wall. When the receiver detects a difference of condition within its beam (and hence a possible intrusion), begins a detailed analysis of the situation that, if considered a real intrusion, it provide an alarm signal that can be treated in analogue or digital form.

Microphonic Systems

Microphonic based systems vary in design but each is generally based on the detection of an intruder attempting to cut or climb over a chainwire fence. Usually the microphonic detection systems are installed as sensor cables attached to rigid chainwire fences, however some specialised versions of these systems can also be installed as buried systems underground. Depending on the version selected, it can be sensitive to different levels of noise or vibration.

The system is based on coaxial sensor cable with the controller having the ability to differentiate between signals from the cable or chainwire being cut, an intruder climbing the fence, or bad weather conditions. The systems are designed to detect and analyse incoming electronic signals received from the sensor cable, and then to generate alarms from signals which exceed preset conditions.

The systems have adjustable electronics to permit installers to change the sensitivity of the alarm detectors to the suit specific environmental conditions. The tuning of the system is usually accomplished during commissioning of the detection devices.

pros: very cheap, very simple configuration, easy to install. cons: some systems has a very high rate of false alarms because some of these sensors has sensitivity problems as they might be too sensitive. conclusion: If you need a fence mounted sensor and you willing to add some more money for a reliable system go with the Vibration system.

Taut Wire Fence Systems

A taut wire perimeter security system is basically an independent screen of tensioned tripwires usually mounted on a fence or wall. Alternatively, the screen can be made so thick that there is no need for a supporting chainwire fence. These systems are designed to detect any physical attempt to penetrate the barrier. Taut wire systems can operate with a variety of switches or detectors that sense movement at each end of the tensioned wires. These switches or detectors can be a simple mechanical contact, static force transducer or an electronic strain gauge.

Unwanted alarms caused by animals and birds can be avoided by adjusting the sensors to ignore objects that exert small amounts of pressure on the wires. It should be noted that this type of system is vulnerable to intruders digging under the fence. A concrete footing directly below the fence is installed to prevent this type of attack.

pros: low rate of false alarms, very reliable sensors and high rate of detection. cons: Very expensive, complicated to install and old technology. conclusion: Very good but very expensive system that uses 20-year-old technology; these days there is no reason to choose Taut wire over the other fence-mounted sensors.

Fibre Optic Cable

A fibre-optic cable can be used to detect intruders by measuring the difference in the amount of light sent through the fibre core. If the cable is disturbed, light will ‘leak’ out and the receiver unit will detect a difference in the amount of light received. The cable can be attached directly to a chainwire fence or bonded into a barbed steel tape that is used to protect the tops of walls and fences. This type of barbed tape provides a good physical deterrent as well as giving an immediate alarm if the tape is cut or severely distorted.

pros: very similar to the Microphonic system, very simple configuration, easy to install. cons: high rate of false alarm or no alarms at all, some sell it as buried system which function VERY bad as buried or on top of a wall. conclusion: Some people choose fiber optic systems only because of the price which is a very wrong decision for a security system, as a fence mounted the microphonic and the Vibration sensors would do a better job for the same price range.


This system employs an electro-magnetic field disturbance principle based on two unshielded (or ‘leaky’) coaxial cables buried about 10-15cm deep and located at about 2.1 metres apart. The transmitter emits continuous Radio Frequency (RF) energy along one cable and the energy is received by the other cable. When the change in field strength weakens due to the presence of an object and reaches a pre-set lower threshold, an alarm condition is generated. The system is unobtrusive when it has been installed correctly, however care must be taken to ensure the surrounding soil offers good drainage in order to reduce nuisance alarms.

pros: concealed as a buried form. cons: affected by RF noise, high rate of false alarms, hard to install. conclusion: Choose one of the other 2 buried security systems sensors.

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