PIR Motion
A passive infrared (PIR) sensor detects changes in the thermal-infrared flux across its dual-element pyroelectric sensor. When a warm body (a human, animal, or vehicle) moves within the sensor's field of view, the transition between sensor zones generates a voltage pulse that a comparator converts to a logic alarm.
Two pyroelectric elements are wired in opposing polarity. In a static scene, both elements receive the same background IR and cancel out. When a warm body moves, one element sees more IR than the other, producing a net signal. A Fresnel lens segments the field of view into alternating active and blind zones to increase sensitivity to lateral motion.
In plain terms
A camera that only sees temperature, filtered through a plastic lens that divides its field of view into zones: a warm body moving from one zone to another causes a change in IR flux, which the sensor converts to an alarm output.
Why designers use it
- Trigger a security light or alarm when a person enters a zone.
- Wake a microcontroller from deep-sleep when a user approaches a device.
- Automate building HVAC and lighting by occupancy.
- Detect intruders in retail environments and homes.
Best for
- Security
- Occupancy detection
- Wake-from-sleep
Key specifications
- Detection range: 3 m – 12 m
- Field of view: 60° – 180°
- Wavelength: 8 µm – 14 µm (Body-heat IR)
- Supply: 2.7 V – 12 V
- Quiescent current: 5 µA – 50 µA
When not to use it
- For precise location — PIR reports presence in a zone, not an X-Y position.
- For detecting cold objects or slow-moving objects that match background temperature.
Common mistakes
- Mounting the PIR behind glass — glass blocks the 8–14 µm thermal IR that the sensor responds to.
- Placing the sensor where HVAC airflow causes temperature gradients that trigger false alarms.
Where you will find it
- A household security system uses an HC-SR501 PIR module to detect intruders: the adjustable sensitivity and time-delay trimpots let the installer tune the sensor for a hallway without triggering on the family cat, whose smaller thermal signature can be gated out by reducing sensitivity.
- A smart thermostat uses a PIR sensor behind the display bezel to wake its screen when someone walks within 2 m: rather than running the display continuously (wasting power and shortening LED life), the MCU sleeps until the PIR's output pin rises, then wakes and shows the temperature.
- An automatic soap dispenser in an airport restroom uses a PIR to detect the approach of a user's hands: because pyroelectric sensors respond to moving heat sources, the hand-approach reliably triggers the motor pump without the false triggers that a static ambient-temperature sensor would cause.
A short history
Hans J. Keller's account distinguishes Herbert Berman's segmented mirror around 1970, used with thermistor sensors, from the later commercial availability of dual pyroelectric sensors in 1979. Optics and sensing elements are separate parts of the design; the early mirror should not be described as a pyroelectric sensor.
Good to know
- A conventional PIR motion detector responds to changes in received thermal radiation, so it may not reliably detect someone who remains still.
- The optics divide the field of view into detection zones; mirror and Fresnel-lens approaches both exist.
- Pyroelectric sensing responds to temperature changes in the sensing element. It is not the operating principle of a conventional moving-coil VU meter.