Burglar alarms are contrived so as to alert the user about particular danger. They are divided into two fields: home burglar alarms and industrial burglar alarms that also include roof coatings. Specific types of sensors are designed for indoor use. Due to weather durability and false alarm vulnerability outdoor use is not advised.
Different types of indoor burglar alarms are: ultrasonic detectors, passive infrared detectors and roofing coatings detectors. Frequency range between 25-75 kHz ultrasonic sound waves are transmitted which are not audible to human beings. Doppler shift is the basic principle used, in which frequency change is detected because of object motion. Two conditions must occur for successful detection of Doppler Effect: change in the ultrasonic frequency must occur due to the motion of object. Passive infrared detectors are commonly seen in the households and small business environments. It has an affordable and reliable functionality. In this the detector is able to function without the need to radiate and generate own energy. Glass break detectors are used in the internal perimeter of building protection. The glass breaks generate a sound in wide band of frequencies. The frequency ranges from infrasonic which is below 20 hertz to 20 hertz above.
Outdoor burglar alarms also come in wide variety: passive magnetic field detection, microwave barriers, micro phonic systems and E-field. Passive magnetic detectors are based on the principle of magnetic anomaly. It uses electromagnetic field generator powered by two wires running in parallel. They are further connected to a signal processor which detects the change in magnetic field. Next is the microwave barrier, whose operation is simple. It is a device in which sensible and visible wall of protection is produced through an electromagnetic beam using high frequency waves that pass from transmitter to the receiver. For the detection of intruder attempting to climb or cutting the chain-wire fence, micro-phonic systems are used. They are installed as sensor cables attached to rigid chain-wire fence. The system is based on electromagnetic or co-axial sensor cable with the controller having the ability to differentiate between signals from the chain-wire being cut or climbed over.
E-field system may be installed on fences, building perimeters and walls. It uses electromagnetic field generator powering one wire with the other wire sensing and running parallel to it. The sensing wire is further connected to a signal processor that analyses movement of the intruder, mass of the intruder and the time intruder is in pattern. So, depending upon the application burglar alarms must be used.