Occupancy sensor and motion sensor

The Occupancy sensor detects presence of people or animals in the target monitored area. The motion sensor responds to moving objects only. The difference between them is occupancy sensor produce signals whenever an object is stationary or not while motion sensor is sensitive to only moving objects. These type of sensors utilize some kind of a human body's property or body's actions. For instance, a sensor may be sensitive to body weight, heat, sounds, dielectric constant and so on.

•  Sensors which detects changes in the air pressure due to opening of the doors and also windows are referred as air pressure sensors.
•  The sensors which detects human body capacitance are referred as Capacitive Sensors.
•  Acoustic sensors utilizes the sound produced by the people.
•  Photoelectric sensor works on the pronciple of interruption of light beams by the moving objects
•  Optoelectric sensor uses detection of variations in the illumination. It also uses optical contrast in the region under target.
•  Pressure mat switches use the pressure sensitive long strips laid on the floors below the carpets to detect weight of an intruder.
•  Stress detectors use strain gauges imbedded into floor beams, staircases, and other structural components
•  Switch sensors utilizes electrical contacts connected to doors and windows.
•  Magnetic switches use a non-contact version of switch sensors.
•  Vibration detectors react to the vibration of walls or other building structures, also may be attached to doors or windows to detect movements.
•  Glass breakage detectors are sensors reacting to specific vibrations produced by shattered glass.
•  Infrared motion detectors are devices sensitive to heat waves emanated from warm or cold moving objects.
•  Microwave detectors are active sensors responsive to microwave electromagnetic signals reflected from objects.
•  Ultrasonic detectors are similar to microwaves except that ultrasonic waves are used instead of EM waves.
•  Video motion detectors are video equipment which compares a stationary image stored in memory with the current image from the protected area.
•  Video face recognition system uses image analyzers that compare facial features with a database.
•  Laser system detectors are similar to photoelectric detectors, except that they use narrow light beams and combinations of reflectors.
•  Triboelectric detectors are sensors capable of detecting static electric charges carried by moving objects

All of the above techniques are used in the design and development of occupancy sensor or motion sensor. These are the basic principles in the design of such sensors.

Capacitive Occupancy Sensor

occupancy sensor

Figure 1 depicts basic circuit, capacitance between the test plate and earth is equal to value C1. In the time when any person moves in the vicinity of the plate, it builds two additional capacitors; One between plate and body (Ca) and the other capacitor between body and earth (Cb). Hence the resulting total capacitor between plate and earth will become larger by ΔC.
C = C1 + ΔC

This type of sensor is referred as capacitive occupancy sensor. Being a conductive medium with a high dielectric constant, a human body develops a coupling capacitance to its surroundings. These capacitances Ca and Cb greatly depends on factors such as human body size, their clothing, carrying materials, type of surrounding objects, weather etc. The coupling capacitance will change due to movement of the persons in the target area. This will help the system discriminates static objects compare to the moving objects. Here all the objects form some degree of a capacitive coupling with respect to one another.

Optoelectronic Motion Sensor

optoelectronic motion sensor

The most popular intrusion sensors are the optoelectronic motion sensors. This type of motion sensor relies on EM radiation in the optical range. This Electromagnetic radiation will have wavelengths range from 0.4 to 20 µm. The sensor will have distance ranges upto hundred meters and used to find movement of people and animals.

The operating principle of the optical motion detectors is based on the detection of light (either in the visible or nonvisible spectrum) emanated from the surface of a moving object into the surrounding empty region. This radiation may be originated either by an external light source and later got reflected by some object or it may be produced by the object itself in the form of natural emission. The former sensor is referred as an active detector and the later one as a passive detector. as mentioned an active sensor requires an additional light source such as daylight, electric lamp an infrared LED etc. The passive detectors perceive mid and far infrared emission from objects having temperatures that are different from the surroundings region. Both of these type of detectors use an optical contrast as a means of object recognition and detection.

The optoelectronic motion sensors are very useful for indicating whether an object is moving or stationary. But they cannot distinguish one moving object from the another. They cannot be utilized to accurately measure the distance to a moving object or its velocity. The major application areas for the optoelectronic motion sensors are security systems, energy management etc. In the energy management it is used to switch light on and off. It is also used for making "smart homes", in which we can control various appliances such as air conditioners, cooling fans, stereo players and so on. This is also referred as home automation.

The most important advantages of an optoelectronic motion sensor are simplicity and low cost.


Also refer IoT compliant sensors which includes ambient light sensor, optical sensor, gesture sensor, proximity sensor, touch sensor, fingerprint sensor etc.

Refer Sensors and transducers article which covers types, basic functions and provide links to sensors and transducers. It include temperature sensor, humidity sensor, strain sensor, proximity or occupancy sensor, force or load sensor, voltage and current sensor, pressure sensor, speed sensor, resistance sensor, power sensor, level sensor, event or state sensor etc.

RF and Wireless Terminologies