The principles by which motion detector alarms operate are complex. Type motion alarm detector into any search engine and you will be overwhelmed by the information returned. In this article we will look at just the very basic principles behind the operation of these devices.
First, there are active motion detector alarms and passive ones. An active motion detector sensor actively sends out some sort of signal and monitors the way that signal is reflected back to the sensor. Some of these types of sensors emit ultrasonic signals, far above your range of hearing. More commonly they operate by emitting microwaves, similar to the way a police radar gun works. These sensors constantly monitor the signal that is reflected back to the detector. Any change in the reflected signal is interpreted as motion and triggers some sort of response.
There is also a passive motion sensor alarm. This type does not emit any signal at all, but simple measures any signal that is present. They usually monitor infrared signals. Infrared light is most commonly associated with heat, and what these4 sensors are designed to do is look for the heat signature of a human body.
The two types of detectors are not mutually exclusive. In fact, dual technology sensors are quite common. Microwave sensors are the most precise and desirable, but consume energy by constantly generating and emitting a microwave signal. They are often combined with a passive infrared detector, and are not powered until the passive detector, which uses much less energy, is triggered. The passive detector then turns on the active detector, which makes a more reliable determination. This combining of the two technologies, together with careful placement of the detectors in the first place allows a very high degree of discrimination and prevents false alarms. This is the basic principle behind a pet-proof motion detector burglar alarm. A simple IR detector activates a microwave sensor when a heat signature is detected. The more precise microwave detector, however, is able to see the difference between a walking person and a dog or cat.
There are multiple options on the handling of the signal once a sensor detects motion. One option is to activate a recording device. Many video surveillance cameras in stores, for example, do not constantly record footage of an empty area. They only begin recording when some sort of motion sensor detects movement in the area and signals them to begin recording.
The more common options are, of course, to notify either the local authorities or a home security company. If a home security company is notifies, they typically call the home and ask if there is a problem. The customer has a pre-selected code word to tell the company. If they cannot get through, or if the person answering the phone does not know the code word, then the security company notifies the local authorities, who then respond. Although this approach loses some time, it is a safeguard against sending a false alarm to the local police. Many communities charge a steep fee for false alarm responses.
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