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Concept
Occupancy sensor
Summary
An occupancy sensor is an indoor device used to detect the presence of a person. Applications include automatic adjustment of lights or temperature or ventilation systems in response to the quantity of people present. The sensors typically use infrared, ultrasonic, microwave, or other technology. The term encompasses devices as different as PIR sensors, hotel room keycard locks and smart meters. Occupancy sensors are typically used to save energy, provide automatic control, and comply with building codes.
A vacancy sensor works like an occupancy sensor, however, lights must be manually turned ON, but will automatically turn OFF when motion is no longer detected.
Occupancy sensor types include:
PIR sensors, which work on heat difference detection, measuring infrared radiation. Inside the device is a pyroelectric sensor which can detect the sudden presence of objects (such as humans) who radiate a temperature different from the temperature of the background, such as the room temperature of a wall.
Environmental sensors, such as temperature, humidity and CO2 sensors, which detect the change in the environment due to the presence of a human.
Ultrasonic sensors, similar to radar. they work on the doppler shift principle. An ultrasonic sensor will send high frequency sound waves in area and will check for their reflected patterns. If the reflected pattern is changing continuously then it assumes that there is occupancy and the lighting load connected is turned on. If the reflected pattern is the same for a preset time then the sensor assumes there is no occupancy and the load is switched off.
Microwave sensors. Similar to the ultrasonic sensor, a microwave sensor also works on the doppler shift principle. A microwave sensor will send high frequency microwaves in an area and will check for their reflected patterns. If the reflected pattern is changing continuously then it assumes that there is occupancy and the lighting load connected is turned on. If the reflected pattern is the same for a preset time then the sensor assumes there is no occupancy and the load is switched off.
Official source
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