SmartAlarm Technology has a complete range of security products

Detecting Movement around Properties.

Photo Electric Beam sets

These consist of a minimum of 3 components

2 beam units to send and receive infrared between them
1 unit to receive a signal from the beams when the beams are broken.

The Receiver

The receiver unit is usually inside a building, typically a home and activates a buzzer when the beams are broken. This unit can also be connected to a SmartAlarm which uses the GSM network to send you a text message and call you on your mobile phone. The GSM dialer in the SmartAlarm works the same as a mobile phone and is completely wireless. This makes it ideal for use in cow sheds, and barns that have no telephone system. See SmartAlarm

The Beams

Photo electric beam sensors transmit a beam of infrared light to a remote receiver creating an "electronic fence". Sensors such as these are often used to detect movement in openings such as doorways, gateways or driveways. They are also used along fence or boundary perimeters: essentially they act as a trip wire. When the beam is broken or interrupted, an alarm signal is sent to a receiver unit either by wire or wirelessly.

These beam sensors consist of two components: a transmitter and a receiver which are placed opposite each other. The transmitter sends a consistent infrared beam of light to a receiver unit. The receiver opposite detects when the beam is present. If the receiver fails to receive at least 90% of the transmitted signal for as brief as 75 milliseconds (time of an intruder or vehicle crossing the beam), an alarm signal is generated.

Any object that breaks the beam such as birds, animals, blowing leaves or paper will interrupt the signal, therefore generating an alarm. Improper alignment of the transmitter, receiver or mirrors may generate an alarm. Mirrors can also collect dust, causing refraction or diffusion of the reflected beam. In order to help prevent false alarms 2 infrared beams are used in tandem about 14 centimetres apart and both these beams must be broken at the same time to activate an alarm.

Beams can be hard wired or wireless depending on the requirements. Wireless beams can be varying distances although trees, hills and buildings will affect range.

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Usage

These beams are used mainly in gateways or as perimeter security and are very effective.

4 beam sets placed around a property can be used with the one receiver unit. Anything breaking any one of the beams will be detected and the receiver will respond as required.

Passive Infrared driveway sensors

These sensors are placed across driveways to detect cars, people and often animals entering or exiting the property. They are not as precise as the beam sets above but do offer an affordable alternative. The positioning is very important and these units can be configured for sensitivity and distance up to 10 metres. The battery powered, wireless sensors transmit to a receiver in the property when activated.

The receiver beeps to notify the owner of the property that a vehicle, person or animal has passed the sensor. This alert can also be attached to a SmartAlarm which will send you a text message and call you on your mobile phone.

As with the beam sets the signal range is affected by hill, trees and buildings.

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How PIRs work

A passive InfraRed sensor (PIR sensor) is an electronic device which measures infrared light radiating from objects in its field of view. PIRs are the detectors of choice in most homes and businesses. Apparent motion is detected when an infrared source with one temperature, such as a human, passes in front of an infrared source with another temperature, such as a wall.

All objects emit what is known as black body radiation. This energy is invisible to the human eye but can be detected by electronic devices designed for such a purpose. The term 'passive' in this instance means the PIR does not emit energy of any type but merely accepts incoming infrared radiation.

In a PIR-based motion detector, the PIR sensor is typically mounted on a printed circuit board which also contains the necessary electronics required to interpret the signals from the chip. The complete circuit is contained in a housing which is then mounted in a location where the sensor can view the area to be monitored. Infrared energy is able to reach the sensor through the window because the plastic used is transparent to infrared radiation (but only translucent to visible light). This plastic sheet prevents the introduction of dust and insects which could obscure the sensor's field of view.

The PIR device can be thought of as a kind of infrared 'camera' which remembers the amount of infrared energy focused on its surface. Once power is applied to the PIR the electronics in the PIR shortly settle into a quiescent state and energize a small relay. This relay controls a set of electrical contacts which are usually connected to the detection input of an alarm control panel. If the amount of infrared energy focused on the sensor changes within a configured time period, the device will switch the state of the alarm output relay.

Manufacturers recommend careful placement of their products to prevent false alarms. They suggest mounting the PIRs in such a way that the PIR cannot 'see' out of a window. Although the wavelength of infrared radiation to which the chips are sensitive does not penetrate glass very well, a strong infrared source (a vehicle headlight, sunlight reflecting from a vehicle window) can overload the chip with enough infrared energy to fool the electronics and cause a false (non-intruder caused) alarm. A person moving on the other side of the glass however would not be 'seen' by the PIR.

They also recommended that the PIR not be placed near air conditioner vents that blow hot or cold air onto the surface of the plastic which covers the PIR. Although air has very low emissivity (emits very small amounts of infrared energy), the air blowing on the plastic window cover could change the plastic's temperature enough to, once again, fool the electronics.

PIRs come in many configurations for a wide variety of applications. The most common used in home security systems have an effective range of 10 - 12 metres and cover an area up to 110°. Other and often more expensive PIRs are used for longer distances or specialist requirements.

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