Operating Principles and Applications of PIR Sensors
– All objects above absolute zero emit heat energy as electromagnetic radiation
– Radiation is usually invisible to the human eye but can be detected by electronic devices
– Heat energy radiates at infrared wavelengths
– PIR-based motion detectors are designed to detect this infrared radiation
– PIR sensors are commonly used in burglar alarms and automatic lighting systems
– Used to sense movement of people, animals, or objects
– Can be used outdoors or indoors
– Examples include controlling outdoor lights, camera traps, and indoor occupancy sensors
– Detects changes in infrared radiation caused by moving objects
– PIR sensors detect changes in infrared radiation
– Temperature and surface characteristics of objects affect the infrared radiation detected
– Passing objects cause a rise in temperature, triggering the sensor
– Objects with similar temperature but different surface characteristics can also trigger the sensor
– PIRs come in various configurations with different ranges and fields of view
– PIR electronics control a relay capable of switching mains voltage
– PIR can turn on lights connected to it when movement is detected
– Commonly used for security lighting and practical purposes like front door lights
– Used in public toilets, walk-in pantries, hallways, and other areas
– Provides convenience and energy efficiency by activating lights only when needed
Differential Detection and Product Design of PIR Sensors
– Pairs of sensor elements wired as opposite inputs to a differential amplifier
– PIR measurements cancel each other, removing average temperature from the signal
– Increases in infrared energy across the sensor do not trigger the device
– Differential arrangement minimizes common-mode interference from electric fields
– Differential pair of sensors is only useful for motion detection, not temperature measurement
– PIR sensor mounted on a printed circuit board
– Electronics on the board interpret signals from the sensor
– Complete assembly contained within a housing for monitoring areas
– Housing usually has a plastic window transparent to infrared radiation
– Window reduces chance of foreign objects obscuring the sensor’s view
Focusing and Beam Pattern of PIR Sensors
– Different mechanisms used to focus distant infrared energy onto the sensor surface
– Plastic window may have multiple facets acting as Fresnel lenses
– Mirrors can also be used to focus the infrared energy
– Mirrored PIRs do not have Fresnel lenses on the window
– Focusing helps the motion detector discriminate between field-wide illumination and moving objects
– Focusing creates a beam pattern in the detector’s view
– PIR sensor receives concentrated amounts of infrared energy in certain angles
– Under other angles, the sensor receives almost no radiation energy
– This separation helps discriminate between field-wide illumination and moving objects
– Optimal sensitivity beam pattern determined by lens and/or mirror design
Security Applications and Placement of PIR Sensors
– PIR electronics control a relay in a security system
– Relay opens circuit if motion is detected, triggering the alarm
– Alarm can also be triggered if a wire is disconnected
– Manufacturers recommend careful placement to prevent false alarms
– PIRs should not be placed where they can see out of a window
– PIRs should not be placed where HVAC vents blow air onto the plastic cover, as it can trigger false alarms
– PIRs are designed to ignore domestic pets by setting a higher sensitivity threshold or keeping the floor out of focus
– For rooms with a single entrance, a single PIR detector near the entrance is sufficient
– PIR-based security systems can be solar-powered due to their low power draw
PIR Remote-based Thermometer
– PIR circuits can be used to measure the temperature of remote objects
– Non-differential PIR output is used in this design
– Calibration is necessary for accurate and precise temperature measurements
– PIR thermometers can measure changes in IR emission corresponding to temperature changes
– Actual temperature values cannot be calculated without calibrationSources: https://en.wikipedia.org/wiki/Passive_infrared_sensor
