The Passive Infra-Red detectors are an essential part of the system. These components work on the principle of heat radiated by the human body. By using infra-red waves, the sensors will detect the moving heat emanated from the human body. Once this is detected, sensors will alert the main panel. These PIRs work on a basic system are known as Quad PIRs.
More accurate PIRs are known as Dual-Tech PIRs, which use microwaves together with IR waves in order to double check any movement. Additionally, even more accurate PIRs are called Tri-Tech PIRs. Apart from IRs and microwaves, they have an in-built processor so that apart from analysing waves, they can also detect an intrusion through computer software. These Dual and Tri-Tech PIRs are mainly used in places where the temperature can change drastically in a few moments, such as when a sensor is located opposite a window or in the washroom of a house.
The number of PIRs required for your alarm will vary in accordance to the building as not every property is the same. Normally, a PIR is placed at a direct point of access from the outside. The PIR is never placed facing a direct entrance but is usually placed in a way that, when entering a building, the intruder will find the PIR behind their back. The PIRs have a definite range for which they can detect intrusion, varying on the type and brand. To determine the amount of sensors that should be used, a site inspection is highly advisable.