There are many different systems available to protect homes and business premises from unwanted intruders. Typically there may be many different sensors, connected to a central control unit. The most common types of sensor are simply used to detect the opening of a door or window, or to detect motion using passive infrared technology (PIR). Systems may simply sound an alarm if an intruder is detected, or they may be connected by telephone to a police station or to a central monitoring office. Homes and businesses fitted with properly maintained intruder alarms can benefit from reduced insurance rates.
Some sensors are designed for indoor use only. These would not be used outdoors because they would be excessively sensitive, causing many false alarms. Indoor sensors use various technologies including passive infrared (PIR), ultrasound, microwave, and photo electricity. Of these types PIR is the most popular in home and small business environments. It is passive in the sense that it does not generate its own infrared signal. It simply detects the infrared signal (heat) being given off by an intruder.
The detectors are able to measure the temperature of the room (or other protected space), and they then detect any change of temperature caused by the presence of a person. PIRs are popular as they are both cheap and reliable. Other indoor detectors such as ultrasound detectors, microwave detectors and photoelectric beams are all active systems. They all generate some sort of wave or beam signal, and detect when that signal is broken by an intruder. Ultrasonic systems operate in the frequency range from 15 to 75 kHz which is just above the audible hearing range for humans. The Doppler shift principle is used, meaning that they detect motion, as the ultrasound reflected from a moving intruder will change slightly in frequency.
There are also a number of outdoor sensor systems. Usually these would be deployed around the perimeter of the premises to be protected. Some sensors (vibration or inertia sensors) are mounted on fences, and can detect the motion when someone tries to climb the fence. These are very reliable, and moderately priced. False alarms are very rare from these sensors.
Another reliable outdoor detection system is based on the detection of magnetic anomalies. Two wires run around the perimeter (for example on the top of a wall) and are driven electrically so as to produce an electromagnetic field between the wires. If a person were to climb the wall, then the presence of his body would change the electromagnetic field, which would be detected by a signal processor unit which continually monitors the two wires. Again this is a very reliable system, although it can be affected by interference from high-voltage power lines. Regardless of the sensor technology used, the sensor units are normally linked back to a central control unit. These connections can be via low voltage wiring, or they can be wireless. Some systems are linked either to a central office, or to the local police station. Most insurance companies will offer a discount for monitored intruder alarm systems which are regularly maintained in homes and businesses.