A motion detector is an electrical device that utilizes a

sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

to detect nearby

motion In physics, motion is the phenomenon in which an object changes its position with respect to time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed and frame of reference to an observer and mea ...

. Such a device is often integrated as a

component Circuit Component may refer to: •Are devices that perform functions when they are connected in a circuit. In engineering, science, and technology Generic systems *System components, an entity with discrete structure, such as an assemb ...

of a system that automatically performs a task or alerts a user of motion in an area. They form a vital component of security, automated lighting control, home control, energy efficiency, and other useful systems.

Overview

An ''active'' electronic motion detector contains an optical, microwave, or acoustic sensor, as well as a transmitter. However, a ''passive'' contains only a sensor and only senses a signature from the moving object via emission or reflection. Changes in the optical, microwave or acoustic field in the device's proximity are interpreted by the electronics based on one of several technologies. Most low-cost motion detectors can detect motion at distances of about . Specialized systems are more expensive but have either increased sensitivity or much longer ranges. Tomographic motion detection systems can cover much larger areas because the radio waves it senses are at frequencies which penetrate most walls and obstructions, and are detected in multiple locations. Motion detectors have found wide use in commercial applications. One common application is activating automatic door openers in businesses and public buildings. Motion sensors are also widely used in lieu of a true

occupancy sensor An occupancy sensor is an indoor motion detecting device used to detect the presence of a person to automatically control lights or temperature or ventilation systems. The sensors use infrared, ultrasonic, microwave, or other technology. The ter ...

in activating street lights or indoor lights in walkways, such as lobbies and staircases. In such ''

smart lighting A lighting control system is an intelligent network based lighting control solution that incorporates communication between various system inputs and outputs related to lighting control with the use of one or more central computing devices. Light ...

'' systems, energy is conserved by only powering the lights for the duration of a timer, after which the person has presumably left the area. A motion detector may be among the sensors of a

burglar alarm A security alarm is a system designed to detect intrusion, such as unauthorized entry, into a building or other areas such as a home or school. Security alarms used in residential, commercial, industrial, and military properties protect against ...

that is used to alert the home owner or security service when it detects the motion of a possible intruder. Such a detector may also trigger a

security camera A closed-circuit television camera can produce images or recordings for surveillance or other private purposes. Cameras can be either video cameras, or digital stills cameras. Walter Bruch was the inventor of the CCTV camera. The main purpose o ...

to record the possible intrusion.

Sensor technology

Several types of motion detection are in wide use:

Passive infrared (PIR)

Passive infrared (PIR) sensors are sensitive to a person's skin temperature through emitted

black-body radiation Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific, continuous spect ...

mid-infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

wavelengths, in contrast to background objects at room temperature. No energy is emitted from the sensor, thus the name ''passive infrared''. This distinguishes it from the

electric eye An electric eye is a photodetector used for detecting obstruction of a light beam. An example is the door safety system used on garage door openers that use a light transmitter and receiver at the bottom of the door to prevent closing if there is ...

for instance (not usually considered a ''motion detector''), in which the crossing of a person or vehicle interrupts a visible or infrared beam. These devices can detect objects, people, or animals by picking up one's infrared radiation.

Microwave

These detect motion through the principle of

Doppler radar A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the f ...

, and are similar to a

radar speed gun A radar speed gun (also radar gun and speed trap gun) is a device used to measure the speed of moving objects. It is used in law-enforcement to measure the speed of moving vehicles and is often used in professional spectator sport, for things su ...

. A

continuous wave A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration. It may refer to e.g. a laser or particle ...

microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...

radiation is emitted, and phase shifts in the reflected microwaves due to motion of an object toward (or away from) the receiver result in a

heterodyne A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is us ...

signal at a low

audio frequency An audio frequency or audible frequency (AF) is a periodic vibration whose frequency is audible to the average human. The SI unit of frequency is the hertz (Hz). It is the property of sound that most determines pitch. The generally accepted ...

Ultrasonic

ultrasonic transducer Ultrasonic transducers and ultrasonic sensors are devices that generate or sense ultrasound energy. They can be divided into three broad categories: transmitters, receivers and transceivers. Transmitters convert electrical signals into ultrasoun ...

emits an

ultrasonic wave Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...

(sound at a frequency higher than a human ear can hear) and receives reflections from nearby objects. Exactly as in

, heterodyne detection of the received field indicates motion. The detected

doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...

is also at low audio frequencies (for walking speeds) since the ultrasonic

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

of around a centimeter is similar to the wavelengths used in microwave motion detectors. One potential drawback of ultrasonic sensors is that the sensor can be sensitive to motion in areas where coverage is undesired, for instance, due to reflections of sound waves around corners. Such extended coverage may be desirable for lighting control, where the goal is the detection of any occupancy in an area, but for opening an automatic door, for example, a sensor selective to traffic in the path toward the door is superior.

Tomographic motion detector

These systems sense disturbances to radio waves as they pass from node to node of a mesh network. They have the ability to detect over large areas completely because they can sense through walls and other obstructions. RF tomographic motion detection systems may use dedicated hardware, other wireless-capable devices or a combination of the two. Other wireless capable devices can act as nodes on the mesh after receiving a software update.

Video camera software

With the proliferation of low-cost

digital camera A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile device ...

s able to shoot video, it is possible to use the output of such a camera to detect motion in its field of view using

software Software is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and which actually performs the work. At the lowest programming level, executable code consists ...

. This solution is particularly attractive when the intent is to record video triggered by motion detection, as no hardware beyond the camera and computer is needed. Since the observed field may be normally illuminated, this may be considered another ''passive'' technology. However, it can also be used together with

near-infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

illumination to detect motion in the ''dark'', that is, with the illumination at a wavelength undetectable by a human eye.

Gesture detector

Photodetectors and infrared lighting elements can support digital screens to detect hand motions and gestures with the aid of machine learning algorithms.

Dual-technology motion detectors

Many modern motion detectors use combinations of different technologies. While combining multiple sensing technologies into one detector can help reduce false triggering, it does so at the expense of reduced detection probabilities and increased vulnerability. For example, many dual-tech sensors combine both a PIR sensor and a microwave sensor into one unit. For motion to be detected, both sensors must trip together. This lowers the probability of a false alarm since heat and light changes may trip the PIR but not the microwave, or moving tree branches may trigger the microwave but not the PIR. If an intruder is able to fool either the PIR or microwave, however, the sensor will not detect it. Often, PIR technology is paired with another model to maximize accuracy and reduce energy use. PIR draws less energy than emissive microwave detection, and so many sensors are calibrated so that when the PIR sensor is tripped, it activates a microwave sensor. If the latter also picks up an intruder, then the alarm is sounded.

References

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