How does a motion sensor work?

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How does a motion sensor work?
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There are two basic kinds of motion detector sensors. They are categorised based on how they detect motion. The first type is called an active sensor. Active sensors emit energy, usually ultrasonic sound waves, into the immediate area. They are sometimes called radar-based motion detectors. Passive sensors do not emit any energy, but instead read changes in the energy, using a predetermined baseline, in the surrounding area.

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Two Kinds of Motion Sensors

There are two basic kinds of motion detector sensors. They are categorised based on how they detect motion. The first type is called an active sensor. Active sensors emit energy, usually ultrasonic sound waves, into the immediate area. They are sometimes called radar-based motion detectors. Passive sensors do not emit any energy, but instead read changes in the energy, using a predetermined baseline, in the surrounding area.

How Active Sensors Work

Active motion sensors work by sending out bursts of ultrasonic sound waves, after which the sensor waits for the energy to be reflected back. A good example of an active motion sensor is an automatic garage door opener. If there is nobody in the immediate area, the waves will return in the same pattern in which they were released. If there is someone, however, the energy will bounce back in a disturbed pattern. Active motion detectors are created with sensors that will send an alarm signal in the event that the pattern is disturbed. In the example of the garage door opener, the disturbed pattern (disturbed by the appearance of the car) triggers the sensor to open the garage door.

How Passive Sensors Work

Passive motion detectors are more commonly used to secure businesses and residential homes than active motion sensors. They are also known as passive infrared sensors (or PIR sensors) because they detect and measure incoming infrared energy. They are sometimes also called pyroelectric detectors. Any body, including animals and humans, will emit infrared energy because it creates heat. The amount emitted depends on the temperature of the body, but in humans, it is usually between 9 and 10 micrometers.

Most passive infrared sensors can actually detect emissions in the range of 8 to 12 micrometers. They do this with use of a photo detector. The photo detector coverts light in these wavelengths into an electrical current, which is run through a tiny computer housed in the unit. The alarm is triggered when the photo detector detects large or fast variations in the distribution of the emitted infrared energy. Normal movement in humans will naturally create such variations. Smaller variations are ignored by the computer to allow for naturally occurring events in the supervised area, such as the slow rise of heat as the sun rises for the day.

Oftentimes, PIR systems are combined with a basic photo-sensor motion detector. You frequently see these at a shopping mall, near the entrance to a store. When someone walks between them, the motion detector will sometimes emit a tone. Photo sensors usually consist of a laser beam and a light sensor. When the light is interrupted by the motion of a person crossing it, the sensor notices the brief drop in the light level, and alerts the control box, which will either sound an alarm or emit that bell sound you often hear in stores.

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