Unauthorized access or intrusions into a protected premise are best detected by movement (or motion) sensors. These types of units utilize a variety of methods to sense motion of an individual. These methods are:
Infrared (PIRO ELEMENT) Detectors
Commonly referred to as PIR's (short for Passive Infrared Detector). The most common type of detector utilized today, and the most cost-effective. All are referred to as dual element detectors which, simply put, allows them to discriminate between commonly encountered background "noise" and the physical presence of an intruder. These detectors sense infrared light energy emitted by or reflected from an object. Infrared light is simply heat. All objects in a room emit some form of heat energy, some more than others. Humans and household pets (dogs & cats) emit a lot of infrared light in the form of body heat. We are surrounded by a "halo" of infrared energy. We can't see it ourselves, but these detectors are designed specifically to sense it. Some areas of a room can also emit a lot of infrared light or heat. A fireplace or a patch of floor illuminated by the sun for long periods of time produce a "bloom" of infrared light that is brighter than any living target. The way PIR's work is actually quite ingenious. The actual detection element is mounted behind a faceted lens usually made of an opaque plastic. This plastic is essentially a light filter. It allows free passage of infrared light, but restricts other bands of light (the middle oranges, yellows, greens, and blues (or ultraviolet). White light is composed of all the bands put together (including red and infrared) which is why the lens is often referred to as a "white light" filter. A moving target or object passes in front of the lens. It moves from one facet to another. Circuitry on-board the detector times the movement from facet to facet. When it falls into a preprogrammed set of variables (most often associated with a moving target), the alarm thresh hold is reached and the sensor is triggered. Unfortunately it is often difficult for the sensor to distinguish between an "animal" target and a human one (such as an intruder). Larger animals will often trigger "false alarms". Your security consultant should ask about pets and may even suggest restricting their movements (closing off the areas protected by motion sensors to the pet) to avoid the possibility of false alarms. New technology has advanced the capability of the PIR sensor, and "pet proof" detectors are now widely available. Pet immune or pet proof detectors are more expensive than standard motion sensors, but this is an option you may well wish to consider.
These are pictures of some of the more popular infrared sensors:
are motion sensors that are rarely utilized today because of the advances made in the PIR detector. These types of detectors utilize high frequency sound (a frequency well beyond human hearing) emitted by the detector. The Doppler Shift effect caused by a target or individual moving towards the sensor activates detection circuitry and causes the sensor to alarm. Unfortunately over time, the high frequency sound emitted by the sensor can become modulated to a lower frequency and begin to disturb pets in the home. Cats & dogs that refuse to enter certain rooms in the house or act differently when in the same room as one of these detectors are sure signs that the sensor may require adjustment or replacement (usually the latter). The major drawback with these types of sensors is the ease with which they are cheated. An intruder that recognizes the distinctive shape of the sensor has only to walk through the room PARALLEL to the sensor. As long as he/she doesn't approach the sensor, the "Doppler Shift" essential to the units activation will not occur.
are very similar to the ultra-sonic detector with one major exception. Instead of sound energy, this type of sensor utilizes radar energy to sense the Doppler Shift caused by a moving target. Because it utilizes radar (microwave) energy it can also sense side Doppler (that is movement parallel to the sensors line of sight), so the drawback encountered with the ultra-sonic detector is eliminated. Unfortunately and because it utilizes this technology, it can also virtually see through most walls or barriers. It will also sense the movement of liquids in household water and drain pipes. It's ability to be focused very tightly and it's uncommon range (some units can see three hundred feet or one hundred & eighty metres or more) makes it an excellent fence line protector. In the home or workplace, however, it is usually coupled to a PIR element and built into the same case. In this instance both detectors have to be in alarm before a signal is transmitted to the alarm control unit.
are not motion detectors in the strictest sense. Most often they utilize infra-red light energy, but instead of being passive (that is strictly an infra-red sensor), a beam system requires two separate elements to complete the circuit, namely an emitter and a receiver. Both these units are mounted facing each other. The emitter shines a beam of infra-red light at the receiver. When the beam is broken, the receiver trips the alarm. A less expensive cousin to this type of unit is usually seen in a lot of retail stores. When a customer enters the store, he usually breaks the beam and a chime or buzzer is sounded to alert the shopkeeper to the presence of the patron. In a security system, beam detectors can literally become invisible walls or curtains of protection, and since the detectors housings can be easily disguised or hidden, an intruder usually won't know where or when he breaks the beam.