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Passive infrared sensor
Passive infrared sensor
Passive infrared sensor
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Typical residential/commercial PIR-based motion detector (PID).

A passive infrared sensor (PIR sensor) is an electronic sensor that measures infrared (IR) light radiating from objects in its field of view. They are most often used in PIR-based motion detectors. PIR sensors are commonly used in security alarms and automatic lighting applications.

PIR sensors detect general movement, but do not give information on who or what moved. For that purpose, an imaging IR sensor is required.

PIR sensors are commonly called simply "PIR", or sometimes "PID", for "passive infrared detector". The term passive refers to the fact that PIR devices do not radiate energy for detection purposes. They work entirely by detecting infrared radiation (radiant heat) emitted by or reflected from objects.

Operating principles

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All objects with a temperature above absolute zero emit heat energy in the form of electromagnetic radiation. Usually this radiation isn't visible to the human eye because it radiates at infrared wavelengths, but it can be detected by electronic devices designed for such a purpose.

PIR-based motion detector

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A PIR motion detector used to control an outdoor, automatic light.
A camera trap with PIR motion detector.
An indoor light switch equipped with PIR-based occupancy sensor[1]

A PIR-based motion detector is used to sense movement of people, animals, or other objects. They are commonly used in burglar alarms and automatically activated lighting systems.

Operation

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A PIR sensor can detect changes in the amount of infrared radiation impinging upon it, which varies depending on the temperature and surface characteristics of the objects in front of the sensor.[2] When an object, such as a person, passes in front of the background, such as a wall, the temperature at that point in the sensor's field of view will rise from room temperature to body temperature, and then back again. The sensor converts the resulting change in the incoming infrared radiation into a change in the output voltage, and this triggers the detection. Objects of similar temperature but different surface characteristics may also have a different infrared emission pattern, and thus moving them with respect to the background may trigger the detector as well.[3]

PIRs come in many configurations for a wide variety of applications. The most common models have numerous Fresnel lenses or mirror segments, an effective range of about 10 meters (30 feet), and a field of view less than 180°. Models with wider fields of view, including 360°, are available, typically designed to mount on a ceiling. Some larger PIRs are made with single segment mirrors and can sense changes in infrared energy over 30 meters (100 feet) from the PIR. There are also PIRs designed with reversible orientation mirrors which allow either broad coverage (110° wide) or very narrow "curtain" coverage, or with individually selectable segments to "shape" the coverage.

Differential detection

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Pairs of sensor elements may be wired as opposite inputs to a differential amplifier. In such a configuration, the PIR measurements cancel each other so that the average temperature of the field of view is removed from the electrical signal; an increase of IR energy across the entire sensor is self-cancelling and will not trigger the device. This allows the device to resist false indications of change in the event of being exposed to brief flashes of light or field-wide illumination. (Continuous high energy exposure may still be able to saturate the sensor materials and render the sensor unable to register further information.) At the same time, this differential arrangement minimizes common-mode interference, allowing the device to resist triggering due to nearby electric fields. However, a differential pair of sensors cannot measure temperature in this configuration, and therefore is only useful for motion detection.

Practical Implementation

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When a PIR sensor is configured in a differential mode, it specifically becomes applicable as a motion detector device. In this mode, when a movement is detected within the "line of sight" of the sensor, a pair of complementary pulses[4] are processed at the output pin of the sensor. In order to implement this output signal for a practical triggering of a load such as a relay or a data logger, or an alarm, the differential signal is rectified using a bridge rectifier and fed to a transistorized relay driver circuit. The contacts of this relay close and open in response to the signals from the PIR, activating the attached load across its contacts, acknowledging the detection of a person within the predetermined restricted area.

Product design

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PIR motion sensor design

The PIR sensor is typically mounted on a printed circuit board containing the necessary electronics required to interpret the signals from the sensor itself. The complete assembly is usually contained within a housing, mounted in a location where the sensor can cover the area to be monitored.

The housing will usually have a plastic "window" through which the infrared energy can enter. Despite often being only translucent to visible light, infrared energy is able to reach the sensor through the window because the plastic used is transparent to infrared radiation. The plastic window reduces the chance of foreign objects (dust, insects, rain, etc.) from obscuring the sensor's field of view, damaging the mechanism, and/or causing false alarms. The window may be used as a filter, to limit the wavelengths to 8–14 micrometres, which is closest to the infrared radiation emitted by humans. It may also serve as a focusing mechanism; see below.

Focusing

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Different mechanisms can be used to focus the distant infrared energy onto the sensor surface.

Lenses

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The plastic window covering may have multiple facets molded into it, to focus the infrared energy onto the sensor. Each individual facet is a Fresnel lens.

  • Multi-Fresnel lens type of PIR
  • PIR motion detector housing with cylindrical faceted window. The animation highlights individual facets, each of which is a Fresnel lens, focusing light on the pyroelectric sensor element underneath.
    PIR motion detector housing with cylindrical faceted window. The animation highlights individual facets, each of which is a Fresnel lens, focusing light on the pyroelectric sensor element underneath.
  • PIR front cover only (electronics removed), with point light source behind, to show individual lenses.
    PIR front cover only (electronics removed), with point light source behind, to show individual lenses.
  • PIR with front cover removed, showing location of pyroelectric sensor (green arrow).
    PIR with front cover removed, showing location of pyroelectric sensor (green arrow).

Mirrors

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Some PIRs are manufactured with internal, segmented parabolic mirrors to focus the infrared energy. Where mirrors are used, the plastic window cover generally has no Fresnel lenses molded into it.

  • Segmented mirror type of PIR
  • Typical residential/commercial PID using an internal segmented mirror for focusing.
    Typical residential/commercial PID using an internal segmented mirror for focusing.
  • Cover removed. Segmented mirror at bottom with PC (printed circuit) board above it.
    Cover removed. Segmented mirror at bottom with PC (printed circuit) board above it.
  • Printed circuit board removed to show segmented mirror.
    Printed circuit board removed to show segmented mirror.
  • Segmented parabolic mirror removed from housing.
    Segmented parabolic mirror removed from housing.
  • Rear of circuit board which faces mirror when in place. Pyroelectric sensor indicated by green arrow.
    Rear of circuit board which faces mirror when in place. Pyroelectric sensor indicated by green arrow.

Beam pattern

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Passive Infrared motion detector with sensitivity beam pattern.
Motion detector with superimposed beam pattern. The length of the beams is a measure of the detector's sensitivity in that direction.

As a result of the focussing, the detector view is actually a beam pattern. Under certain angles (zones), the PIR sensor receives almost no radiation energy and under other angles the PIR receives concentrated amounts of infrared energy. This separation helps the motion detector to discriminate between field-wide illumination and moving objects.

When a person walks from one angle (beam) to another, the detector will only intermittently see the moving person. This results in a rapidly changing sensor signal which is used by the electronics to trigger an alarm or to turn on lighting. A slowly changing signal will be ignored by the electronics.

The number, shape, distribution and sensitivity of these zones are determined by the lens and/or mirror. Manufacturers do their best to create the optimal sensitivity beam pattern for each application.

Automatic lighting applications

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When used as part of a lighting system, the electronics in the PIR typically control an integral relay capable of switching mains voltage. This means the PIR can be set up to turn on lights that are connected to the PIR when movement is detected. This is most commonly used in outdoor scenarios either to deter criminals (security lighting) or for practical uses like the front door light turning on so you can find your keys in the dark.

Additional uses can be in public toilets, walk-in pantries, hallways or anywhere that automatic control of lights is useful. This can provide energy savings as the lights are only turned on when they are needed and there is no reliance on users remembering to turn the lights off when they leave the area.

Security applications

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When used as part of a security system, the electronics in the PIR typically control a small relay. This relay completes the circuit across a pair of electrical contacts connected to a detection input zone of the burglar alarm control panel. The system is usually designed such that if no motion is being detected, the relay contact is closed—a 'normally closed' (NC) relay. If motion is detected, the relay will open the circuit, triggering the alarm; or, if a wire is disconnected, the alarm will also operate.

Placement

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Manufacturers recommend careful placement of their products to prevent false alarms (i.e., any detection not caused by an intruder).

They suggest mounting the PIRs in such a way that the PIR cannot "see" out of a window. Although the wavelength of infrared radiation to which the chips are sensitive does not penetrate glass very well, a strong infrared source (such as from a vehicle headlight or sunlight) can overload the sensor and cause a false alarm. A person moving on the other side of the glass would not be "seen" by the PID. That may be good for a window facing a public sidewalk, or bad for a window in an interior partition.

It is also recommended that the PIR not be placed in such a position that an HVAC vent would blow hot or cold air onto the surface of the plastic which covers the housing's window. Although air has very low emissivity (emits very small amounts of infrared energy), the air blowing on the plastic window cover could change the plastic's temperature enough to trigger a false alarm.

Sensors are also often designed to "ignore" domestic pets, such as dogs or cats, by setting a higher sensitivity threshold, or by ensuring that the floor of the room remains out of focus.

Since PIR sensors have ranges of up to 10 meters (30 feet), a single detector placed near the entrance is typically all that is necessary for rooms with only a single entrance. PIR-based security systems are also viable in outdoor security and motion-sensitive lighting; one advantage is their low power draw, which allows them to be solar-powered.[5]

PIR remote-based thermometer

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Designs have been implemented in which a PIR circuit measures the temperature of a remote object.[6] In such a circuit, a non-differential PIR output is used. The output signal is evaluated according to a calibration for the IR spectrum of a specific type of matter to be observed. By this means, relatively accurate and precise temperature measurements may be obtained remotely. Without calibration to the type of material being observed, a PIR thermometer device is able to measure changes in IR emission which correspond directly to temperature changes, but the actual temperature values cannot be calculated.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Passive infrared sensor

View on Grokipedia
from Grokipedia
A passive infrared sensor (PIR sensor), also known as a pyroelectric detector, is an electronic device that detects motion by measuring changes in radiation emitted by warm objects, such as humans or , within its . Unlike active s, PIR sensors do not emit any energy themselves but passively absorb light in the 8–14 micrometer range, corresponding to around 300 . This makes them low-power, cost-effective, and suitable for battery-operated applications, with typical detection ranges of up to 15–20 feet and viewing angles of about 90 degrees. The working principle of a PIR sensor relies on the pyroelectric effect, where a pair of thin-film pyroelectric crystals—often made of materials like lithium tantalate or —generate an electrical charge proportional to rapid temperature fluctuations induced by incoming IR radiation. A or parabolic mirror focuses the IR energy from the environment onto these dual elements, creating a differential signal: when motion causes an imbalance (e.g., a warm body entering one detection zone but not the other), the sensor outputs a voltage to trigger an alarm or device. The sensor is most sensitive to wavelengths near 9.6 micrometers, as determined by (λ_max = 2.89777 × 10^6 nm·K / T), aligning with human body temperature emissions. However, PIR sensors primarily detect movement rather than static presence, and their performance can degrade in extreme temperatures or with obstructions. PIR sensors are widely deployed in security systems for intrusion detection, automatic and HVAC controls to enhance energy efficiency in enclosed spaces like offices and restrooms, and occupancy monitoring in smart buildings or IoT devices. Their integration with microcontrollers allows for adjustable sensitivity and delay times, making them versatile for applications ranging from to vehicular traffic monitoring. Despite advantages like low cost and non-intrusiveness, challenges include vulnerability to false triggers from heat sources like or heaters, often mitigated by dual-technology combinations with ultrasonic or sensors.

Operating Principles

Infrared Radiation Fundamentals

Infrared radiation encompasses electromagnetic waves with wavelengths ranging from approximately 700 nanometers to 1 millimeter, positioned between visible light and microwaves in the electromagnetic spectrum. This broad range is subdivided into near-infrared (0.7–1.4 μm), mid-infrared (1.4–3 μm), and far-infrared (3–1000 μm), but the thermal infrared band, particularly 8–15 μm, is most relevant for passive sensing applications as it aligns with emissions from terrestrial objects at ambient temperatures. All matter above absolute zero emits thermal infrared radiation due to the thermal agitation of its atoms and molecules, with the intensity and spectral distribution governed by the object's temperature. The principles of provide the foundational model for understanding infrared emission. A is an ideal absorber and emitter of radiation, and its spectral output is described by , which quantifies the radiance as a function of and : B(λ,T)=2hc2λ5(exp⁡(hcλkT)−1)−1B(\lambda, T) = \frac{2hc^2}{\lambda^5} \left( \exp\left(\frac{hc}{\lambda k T}\right) - 1 \right)^{-1} where hh is Planck's constant, cc is the , kk is Boltzmann's constant, λ\lambda is , and TT is absolute ./University_Physics_III_-Optics_and_Modern_Physics(OpenStax)/06%3A_Photons_and_Matter_Waves/6.02%3A_Blackbody_Radiation) further specifies that the of peak emission \lambda_\max shifts inversely with : \lambda_\max T = 2898 \, \mu\text{m} \cdot \text{K}. For a typical of 37°C (310 K), this places the peak emission around 9.3–10 μm, within the thermal infrared window where atmospheric absorption is minimal. Real objects approximate blackbody behavior through their , a dimensionless factor between 0 and 1 representing the of actual radiated power to that of a blackbody at the same temperature and . exhibits high emissivity of approximately 0.98 in the 8–14 μm range, enabling efficient thermal emission that contrasts with cooler environmental backgrounds, such as walls or air, which often have emissivities around 0.90–0.93. This difference allows warm bodies to stand out against surroundings at (typically 20–25°C or 293–298 K). The total radiated power from a surface follows the Stefan-Boltzmann law: P=ϵσAT4P = \epsilon \sigma A T^4 where σ=5.67×10−8 W/m2K4\sigma = 5.67 \times 10^{-8} \, \text{W/m}^2\text{K}^4 is the Stefan-Boltzmann constant, AA is surface area, and TT is in Kelvin; this quantifies the overall energy flux, emphasizing the strong temperature dependence (fourth-power scaling).

Detection Mechanism

Passive infrared (PIR) sensors operate passively by detecting variations in ambient infrared radiation emitted by thermal sources in the environment, such as warm objects or human bodies, without emitting any radiation of their own. This reliance on natural thermal emissions allows the sensors to identify changes in the infrared flux incident on their surface, which is crucial for applications like motion detection. Unlike active sensors that project beams, PIR devices respond solely to external heat signatures, making them energy-efficient and suitable for battery-powered systems. At the core of the detection mechanism is the pyroelectric effect, a transduction process in which certain ferroelectric materials, such as (PZT), generate an electrical charge in response to fluctuations. When infrared radiation is absorbed, it induces a change in the material, altering its spontaneous electric polarization and producing a proportional charge displacement across the crystal lattice. This effect is inherently dynamic, meaning the sensor outputs a signal only for changing temperatures, not steady-state conditions, which enhances sensitivity to transient events like moving heat sources. To facilitate infrared absorption, the pyroelectric element is typically coated with specialized absorbing layers, such as black coatings like gold black or polymer films infused with , which convert incoming IR photons into thermal energy via the photothermal effect. These coatings, often applied to one surface, maximize in the 8–14 μm relevant for terrestrial thermal detection, causing localized heating that stresses the pyroelectric material and generates charge. The resulting mechanical stress from this temperature rise directly contributes to the polarization change, enabling efficient signal generation without external power for the transduction step. The sensor can be modeled electrically as a in parallel with and resistance, where the pyroelectric current ipi_p is given by ip=p⋅A⋅dTdt,i_p = p \cdot A \cdot \frac{dT}{dt}, with pp as the pyroelectric (typically around 2–5 × 10^{-4} C/m²·K for PZT), AA the electrode area, and dTdt\frac{dT}{dt} the rate of temperature change induced by IR absorption. This model captures the sensor's response to rapid thermal transients, where higher dTdt\frac{dT}{dt} from sudden IR flux changes yields stronger currents, often in the picoampere to nanoampere range for typical motion events. The output current is then amplified and processed to produce a detectable voltage signal.

Sensor Response Characteristics

Passive infrared (PIR) sensors exhibit primarily within the long-wave atmospheric window to optimize detection of emissions from objects at ambient temperatures while minimizing interference from shorter wavelengths. Typical bandpass filters limit the response to approximately 7.5–13.5 μm, corresponding to the 8–14 μm range where atmospheric absorption is low and from human bodies (peaking around 9–10 μm at 300 K) is prominent. This filtering rejects and mid-wave beyond sources, ensuring selectivity for heat signatures. The temporal response of PIR sensors stems from their pyroelectric detection principle, producing an AC-coupled output that requires a non-zero rate of temperature change (dT/dt > 0) to generate a signal, thus necessitating motion or varying input for activation. Static heat sources do not trigger a response, as the pyroelectric material's polarization stabilizes without variation. Signal decay follows an exponential profile governed by and electrical time constants, typically ranging from 0.5 to 5 seconds in practical circuits, allowing the output to return to baseline after the stimulus ceases. Sensitivity in PIR sensors is quantified by metrics such as the noise-equivalent temperature difference (NETD), which represents the smallest detectable change amid , typically 0.1–1 for standard pyroelectric elements. This performance enables reliable detection of human-sized targets at distances up to several meters, though the effective range is modulated by the sensor's (FOV), which can extend up to 120° with appropriate . The of PIR sensors is tailored via bandpass filtering to emphasize signals from typical motion speeds of 0.5–3 m/s, commonly spanning 0.1–10 Hz to capture transient thermal changes while suppressing DC offsets from ambient conditions. This range aligns with the modulation frequencies of walking or gesturing, where lower rejects slow drifts and higher avoids from vibrations.

Hardware Design

Sensor Elements

The core of a passive infrared (PIR) sensor is the pyroelectric element, which converts incident radiation into an electrical charge via the pyroelectric effect. Primary materials used include ferroelectric ceramics such as (PZT), valued for their high pyroelectric coefficients typically ranging from 200 to 400 nC/cm²·K, enabling sensitive detection of changes. Alternatively, flexible (PVDF) films are employed, particularly in applications requiring thin, conformable structures, though their pyroelectric coefficients are generally lower, around 20-30 nC/cm²·K. These materials are selected for their ability to generate measurable charge in response to thermal fluctuations without requiring an external power source for the sensing mechanism itself. PIR sensors commonly feature single-element or dual-element configurations, with the latter being predominant for practical applications due to improved rejection. In a single-element , a solitary pyroelectric chip detects overall changes, but it is susceptible to baseline drifts from ambient variations. Dual-element sensors, by contrast, incorporate two adjacent pyroelectric elements—often identical in and —wired in electrical opposition to enable differential detection; this setup provides common-mode rejection, effectively canceling steady-state ambient drifts while amplifying signals from transient heat sources like moving objects. The elements are typically spaced 0.5-1 mm apart to align with the of human-body emissions around 10 μm. Electrodes on these pyroelectric elements consist of thin metal layers, such as or , deposited on opposing faces of the material to collect the generated charges efficiently. These electrodes form a capacitor-like structure with the pyroelectric layer as the , ensuring low (typically 1-10 pF) for high-speed response. The entire assembly is hermetically sealed within a metal can to protect against , , and mechanical stress, maintaining long-term stability in operational environments. Modern techniques allow these low-cost sensor elements to be produced for less than $1 per unit, facilitating widespread integration into consumer devices. The development of pyroelectric elements for PIR sensors traces back to the early with materials like , but significant advancements occurred in the 1950s, when detectors using materials like triglycine sulfate (TGS) emerged for military applications. Practical pyroelectric designs solidified in the post-war period. Commercialization accelerated in the 1970s with advancements in ceramic processing, leading to affordable, reliable units suitable for security and automation systems by the decade's end.

Optical Systems

Optical systems in passive infrared (PIR) sensors are designed to collect and focus emitted by warm objects, such as humans, onto the small pyroelectric detection element, which typically has an active area of approximately 1 mm², thereby increasing the effective and defining the field of view (FOV) to enhance detection range and selectivity. The most common optical component is the Fresnel lens, a low-cost, molded plastic array featuring concentric grooves that enable thin profiles while providing focusing power for transmission in the long-wave infrared (LWIR) band of 8–14 μm. These lenses are typically fabricated from polyethylene (PE) materials, such as high-density PE or specialized formulations like POLY IR®, with a thickness of about 0.38–0.46 mm, achieving high transmittance—often exceeding 80%—in the target wavelength range due to minimal absorption losses. Typical focal lengths for these lenses range from 10 to 50 mm, allowing compact designs suitable for consumer motion detectors. As alternatives to lenses, segmented parabolic mirrors are employed, particularly in designs requiring broader coverage or where transmission losses must be minimized; these are often constructed from aluminized plastic or metallized Mylar substrates, offering reflective efficiencies up to 90–95% in the IR spectrum without introducing chromatic distortion. Such mirror systems are favored in larger installations, like commercial security setups, to achieve wider FOVs—up to 360°—while maintaining focus on the sensor. To enable differential motion sensing, which helps discriminate true movement from , both Fresnel lenses and mirrors are segmented into multiple facets—typically dozens to hundreds—each corresponding to a distinct detection zone that projects a narrow beam onto the sensor, often aligned with dual-element detectors for balanced signal comparison. This faceted arrangement creates an array of overlapping or alternating zones, improving selectivity for transverse motion across the FOV.

Signal Processing Circuits

The signal processing circuits in passive infrared (PIR) sensors are essential for converting the weak, noisy electrical signals generated by pyroelectric elements into reliable outputs suitable for motion detection or other applications. These circuits typically follow the sensor element and include stages for amplification, filtering, thresholding, and power optimization to handle the inherently low-level outputs, which are on the order of picoamperes to nanoamperes of current corresponding to charge variations of tens to hundreds of picocoulombs from flux changes. Amplification is the first critical stage, employing low-noise preamplifiers such as source-follower buffers or operational amplifiers to boost the minuscule currents from the pyroelectric detector. preamps are favored for their high (>10^12 Ω) and low gate leakage, which minimize loading on the and preserve , converting the nA-level currents to initial voltage signals in the microvolt range before further amplification. Subsequent stages use dual-channel op amps, like the TSU102 or similar low-power devices, configured in two-stage topologies with total gains ranging from 10^4 to 10^6 (approximately 80-120 dB) to elevate these to millivolt levels for downstream processing. For instance, a common design achieves 69 dB gain across two stages (35 dB first, 34 dB second) using precision op amps with gain-bandwidth products exceeding 2.7 kHz to ensure faithful reproduction of motion-induced signals without distortion. Filtering follows amplification to isolate the relevant components associated with moving warm objects while suppressing and offsets. A , typically set at 0.1-0.6 Hz, removes DC biases and very low-frequency drifts from ambient temperature variations, emphasizing transient changes from motion. This is paired with a around 5-10 Hz to attenuate high-frequency electrical , creating a bandpass response (e.g., 0.7-10 Hz) tailored to walking speeds of 0.5-5 Hz. These filters are implemented using RC networks around the op amps, such as 0.6 Hz high-pass and 5 Hz low-pass in each amplification stage. A window comparator then processes the filtered signal, comparing it against upper and lower thresholds (typically 50-200 μV post-amplification, or equivalent voltage levels like 0.53 V low and 2.77 V high at 3.3 V supply) to generate a clean digital trigger output, preventing false activations from minor fluctuations. Power management in these circuits prioritizes for battery-powered or low-energy systems, operating at supply voltages of 3-12 V DC with quiescent currents under 50 μA. The pyroelectric sensor itself consumes about 19 μA, while the amplification and filtering stages add minimal draw (e.g., 1.2-2.4 μA for op amps like TSU102/104), yielding total consumption around 24 μA at 3.3 V. Integration with microcontrollers is facilitated by direct digital output from the , often compatible with ADC inputs for advanced processing, and designs like those using single 5 V supplies ensure broad compatibility. To adapt to varying environments, integration time adjustment is incorporated via variable resistors that tune the effective of the high-pass filters or bias networks, allowing sensitivity from 10-30 seconds post-power-up. This prevents saturation from steady sources like , which could otherwise overwhelm the amplifiers by shifting the DC baseline, and enables optimization for indoor versus outdoor use without altering core hardware.

Motion Detection Applications

Core Operation

Passive infrared (PIR) sensors detect motion by identifying changes in radiation emitted by warm objects, such as humans, within their . When a warm object enters the sensor's , it disrupts the balance of incident on the dual pyroelectric elements, which are typically arranged to receive radiation from opposing zones; this asymmetry causes a temperature differential in the elements, generating a small (AC) voltage proportional to the rate of change in IR intensity. As the object moves across the , the IR alternates between the elements, producing a series of positive and negative voltage pulses that represent the motion. The raw voltage pulses from the pyroelectric elements are amplified and filtered through signal processing circuitry to isolate motion-induced changes while suppressing noise and steady-state IR. To confirm valid motion and minimize false positives from transient environmental fluctuations, trigger logic often employs pulse counting, requiring multiple pulses—typically 2 to 4 within a 1- to 2-second window—to activate the detection threshold, mimicking the pattern of human or animal movement. Single-pulse modes may be used for higher sensitivity applications but increase false alarm risk. Upon trigger confirmation, the produces an output signal to interface with external systems, commonly in the form of a contact closure for high-power loads, an open-collector output for sinking current, or a TTL-compatible digital high/low signal for integration. This output remains asserted for an adjustable hold time, ranging from 1 to 300 seconds post-detection, allowing sustained activation of connected devices like lights or alarms. The core operation can be illustrated by a basic timing sequence:
  • IR Flux Change: Warm object enters one detection zone, causing initial voltage spike (e.g., positive differential).
  • Voltage Pulse Generation: As object moves to opposing zone, negative spike follows, forming pulse train.
  • Signal Processing: Pulses amplified, filtered, and counted to meet trigger threshold.
  • Output Activation: Logic asserts output high/low for programmed duration, then resets until next motion event.
This sequence ensures reliable motion detection without emitting energy, relying solely on passive IR reception.

Differential Detection Techniques

Differential detection techniques in passive infrared (PIR) sensors employ multiple sensing elements or zones to compare signals, thereby isolating motion-induced changes from uniform environmental variations. In the dual-element configuration, two pyroelectric elements are arranged with opposing polarities, producing an output voltage Vout=A1−A2V_{out} = A_1 - A_2, where A1A_1 and A2A_2 represent the infrared irradiances incident on each element. This differential setup effectively cancels common-mode signals, such as gradual shifts in ambient temperature or steady sunlight exposure, which affect both elements equally, enhancing detection reliability by focusing solely on transient differences caused by moving objects. For broader coverage, multi-zone arrays utilize Fresnel lenses divided into 20 to 100 segments that alternate in polarity, creating a pattern of positive and negative detection zones across the field of view. As a warm object, such as a human, traverses these zones, it generates a pulsed output signal only upon crossing boundaries, for instance, from a positive to a negative zone, which can also indicate directionality in motion. This alternating polarity design ensures that static heat sources produce no net signal, while movement yields a characteristic oscillating response. In practical implementations, integrated circuits like the BISS0001 process these differential signals through operational amplifiers and bi-directional level detectors for noise-immune amplification. The chip incorporates software-like algorithms for pulse integration, where output pulse width is controlled by timing components (Tx≈24576×R10×C6T_x \approx 24576 \times R_{10} \times C_6), and via inhibit timing (Ti≈24×R9×C7T_i \approx 24 \times R_9 \times C_7) to prevent oscillations from minor fluctuations. These features stabilize the output by requiring sustained signal changes before triggering, while briefly referencing signal amplification as handled in dedicated circuits. These techniques provide advantages including rejection of steady thermal noise, as the differential comparison suppresses uniform background radiation.

Coverage and Beam Patterns

Standard passive infrared (PIR) sensors typically exhibit a horizontal ranging from 90° to 120°, which allows for broad lateral coverage in indoor environments such as rooms or hallways. The vertical is narrower, generally between 70° and 80°, focusing detection on movement at human height while minimizing sensitivity to ground-level disturbances. For standard units, the detection range extends from 5 to 12 meters, depending on factors like mounting height and environmental conditions, enabling effective monitoring of spaces up to medium-sized areas. Beam patterns in PIR sensors are shaped primarily through the design of , which divide the field of view into multiple facets or segments to create discrete detection zones. These facets refract infrared radiation to form fan-shaped patterns, suitable for wall-mounted applications where coverage fans out horizontally from the sensor, or volumetric patterns that provide three-dimensional detection volumes for more comprehensive area surveillance. In pet-immune designs, the lens incorporates a narrow vertical beam configuration, typically limiting sensitivity to zones above 1.1 meters to ignore the infrared signatures of small animals moving at lower heights, thus reducing false alarms from pets weighing up to 40 kg. To achieve full 360° coverage and minimize blind spots, multi-sensor arrays or specialized ceiling-mounted units employ overlapping detection zones from multiple lens segments or dual pyroelectric elements. Sensitivity in these patterns exhibits a cosine-squared falloff away from the central axis, resulting in gradually reduced responsiveness at the edges of the field of view, which helps prioritize detections in the primary monitoring area. Overlaps between zones ensure uniform coverage without gaps, particularly in circular or spherical lens designs that extend up to 12 meters in diameter. Testing for uniform response across beam patterns follows voluntary guidelines such as the NEMA WD 7-2011 standard, which defines protocols for measuring horizontal and vertical fields of view through grid-based activation tests in controlled environments. This standard ensures consistent performance by evaluating detection in discrete cells, typically within a 30 ft × 30 ft area, and verifies that sensitivity remains reliable without significant degradation over time or under stress conditions.

Specialized Applications

Security Systems

Passive infrared (PIR) sensors are integral to modern security systems, serving as primary motion detectors in burglar alarms for both residential and commercial settings. These sensors identify unauthorized entry by detecting changes in infrared radiation emitted by warm-bodied intruders, triggering alerts upon sensing movement within their field of view. In standalone configurations, a PIR sensor can directly activate an audible siren or silent notification to a monitoring center; in networked setups, it integrates seamlessly with CCTV cameras for visual verification, automated sirens for immediate deterrence, and central control panels for rapid response coordination. This versatility makes PIR sensors a cornerstone of intrusion detection, enabling proactive protection against break-ins in homes and businesses. To bolster reliability and minimize erroneous triggers, PIR sensors are frequently paired with technology in hybrid detectors, a that employs "and" logic—requiring confirmation from both and Doppler-shift microwave detection before alarming. This anti-masking approach counters vulnerabilities like heat sources or small animals, significantly lowering rates; tuned hybrid systems can achieve fewer than one false activation per year, enhancing user confidence and reducing response fatigue for authorities. Such integration has become standard in professional installations, where dual verification ensures alarms reflect genuine threats. Compliance with rigorous commercial standards is essential for PIR-based devices, particularly UL 639, which certifies intrusion-detection units for use in burglary-protection signaling systems by evaluating sensitivity, environmental resilience, and operational . Under UL 639, certified units must deliver response times of 2 seconds or less from detection to alarm indication at the control panel, ensuring swift intervention while maintaining low nuisance rates. This , mandated in many building codes and insurance policies, underscores the sensors' role in verifiable, high-performance security. The widespread adoption of PIR sensors in systems dates to the , when stabilized pyroelectric elements enabled reliable commercial deployment. FBI data indicates a 59% reduction in U.S. property crimes, including break-ins, from 1993 to 2022. A 2013 at Charlotte study of 422 convicted burglars reinforces the deterrent impact of alarm systems, revealing that 60% would seek an alternative target upon spotting an alarm system and more than 50% would abandon an attempt if discovering one mid-entry.

Automatic Lighting Control

Passive infrared (PIR) sensors play a key role in automatic lighting control systems by detecting human occupancy through motion, enabling energy-efficient activation of lights only when spaces are in use. These sensors trigger upon detecting changes indicative of movement, typically turning lights on immediately and maintaining them for a programmable duration of 5 to 30 minutes if no further motion is detected, after which the lights automatically turn off to prevent unnecessary . This occupancy-based operation is often enhanced by integration with timers and photocells, which support daylight harvesting by measuring ambient levels and dimming or deactivating artificial lights when natural illumination exceeds a set threshold, further optimizing use in varied lighting conditions. In commercial buildings, PIR-enabled lighting controls have demonstrated significant energy savings, with studies indicating reductions of up to 60% in lighting energy compared to always-on systems, particularly when paired with energy-efficient LEDs that respond seamlessly to sensor signals without compatibility issues. These savings stem from the sensors' ability to align lighting with actual occupancy patterns, reducing waste in intermittently used areas like offices, hallways, and restrooms. System variants include wall-mounted PIR units suited for enclosed rooms, providing targeted coverage up to 1,200 square feet, and ceiling-mounted models for open spaces, offering 360-degree detection patterns to monitor larger areas effectively. Many designs incorporate adjustable lux thresholds, allowing users to set activation points from 5 to 1,000 lux, ensuring lights engage only in low-light conditions tailored to specific environments. The evolution of PIR sensors in smart homes has accelerated since the 2010s, driven by IoT integration that enables wireless connectivity and remote management. Protocols like allow PIR sensors to communicate with hubs and apps, facilitating automations such as app-controlled scenes or synchronization with other devices for enhanced user convenience and further energy optimization. This proliferation has made PIR-based systems a standard for residential occupancy detection, with devices like Zigbee-compatible motion sensors triggering lights via ecosystems such as or Google Home.

Non-Contact Thermometry

Passive infrared (PIR) sensors adapted for non-contact thermometry employ focused to concentrate from a single target zone onto the detector, enabling measurement of steady-state IR flux rather than transient changes. This configuration typically utilizes thermopiles, which provide a stable DC response to continuous , or specialized pyroelectric elements modified for DC-coupled operation to overcome the inherent AC sensitivity of standard pyroelectrics. The , often comprising or lenses, define a narrow to isolate the target's emission, minimizing interference from surrounding areas. Accuracy in applications, such as forehead temperature scanning, reaches ±0.5°C within typical operating ranges, achieved through calibration that compensates for ambient temperature variations and assumptions for human skin (around 0.98). FDA-cleared models, available since the early 2000s, incorporate to adjust for environmental factors like and , ensuring reliability for clinical use. These devices undergo rigorous validation against reference thermometers, meeting standards like ASTM E1965 for expanded limits. Key applications include fever screening, which surged during the 2020 for rapid, hygienic assessment in public spaces, and industrial hot-spot monitoring to detect overheating in machinery or electrical systems before failures occur. Operational ranges for these uses typically span 0.5 to 2 meters, balancing precision with practicality in handheld or fixed installations. In industrial settings, such sensors integrate with systems to alert on anomalies exceeding safe thresholds. Handheld units, such as the ThermoWorks Wand or Tenergy Non-Contact Forehead Thermometer, feature LCD displays for immediate readout and process the detector voltage VV into target TT using the simplified relation T=Vk+TambientT = \frac{V}{k} + T_{\text{ambient}}, where kk is a device-specific factor derived from empirical radiance-temperature correlations. These FDA-cleared examples often include dual modes for body and surface measurements, with audible alerts for elevated readings, facilitating user-friendly deployment in diverse scenarios.

Limitations and Practical Considerations

Environmental Influences

Passive infrared (PIR) sensors detect motion through changes in infrared radiation (dT/dt) emitted by warm objects, but environmental heat sources can induce unintended variations, leading to false triggers. Direct sunlight, radiant heaters, or air drafts from HVAC systems often cause such changes by altering background temperatures or creating transient IR patterns, resulting in significant false alarm rates in systems without proper tuning. For instance, environmental noises like sunlight reflections or shadows can elevate false positive detections, as documented in outdoor surveillance studies. Additional interference arises from biological or mechanical sources that mimic human motion signatures. Insects or small pets moving within the detection zone emit detectable IR heat, while vibrations from nearby machinery or wind can indirectly affect sensor stability by shifting the optics. High humidity exacerbates issues through lens condensation, which scatters incoming IR rays and reduces overall detection accuracy. Extreme ambient temperatures further degrade PIR performance due to the pyroelectric material's properties and diminished contrast between targets and surroundings. Sensitivity is affected at extreme temperatures, but detection improves in conditions due to greater thermal contrast between the target and background, while degrading above approximately 35°C as ambient approaches , reducing the differential signal; typical operating ranges are -20°C to +60°C. To mitigate these influences, PIR designs incorporate spectral filters within the to selectively pass long-wave (8-14 μm) while rejecting shorter wavelengths from , thereby reducing false triggers from solar heating. However, comprehensive solutions often require integration with techniques addressed elsewhere.

Installation Guidelines

For effective deployment of passive infrared (PIR) sensors in motion detection applications, the optimal mounting height for human detection is typically 1.8 to 2.5 meters above the floor, as this range allows the sensor to reliably capture heat signatures from walking adults while minimizing detection of smaller movements. Sensors should be installed away from direct sources of heat variation, such as air vents, HVAC ducts (at least 1.2 to 1.8 meters distant), windows exposed to , or appliances like stoves and refrigerators, to prevent interference with the detection zones. Proper orientation involves positioning the sensor to face primary entry points or pathways, ensuring that potential targets cross multiple detection zones rather than approaching directly, which enhances triggering reliability. A downward tilt of 10 to 15 degrees is recommended to align the field of view with typical human movement heights, particularly in indoor settings. For comprehensive coverage in larger areas, multiple s should be installed with overlapping detection patterns to eliminate blind spots and ensure uniform sensitivity across the space. Post-installation testing is essential to verify performance, using a walk-test protocol where an individual moves through the detection area at various speeds and distances to confirm consistent triggering without gaps. During commissioning, sensitivity can be adjusted via onboard potentiometers—typically a or jumper settings—to balance detection range and reduce false alarms from environmental factors like air currents, starting at a medium setting and fine-tuning based on test results. Installations involving low-voltage wiring for PIR sensors in automated systems must comply with the () Article 725, which governs Class 2 and Class 3 remote-control circuits to ensure safe power-limited operation below 30 volts AC or 60 volts DC, including requirements for conductor sizing, separation from high-voltage lines, and overcurrent protection. This adherence helps mitigate risks such as electrical hazards in setups. Recent advancements in passive infrared (PIR) sensor technology have focused on , enabling seamless integration into compact devices. Developments include pet-immune designs that ignore small sources below a certain or , reducing false positives in residential settings. enhancements have significantly improved PIR sensor performance by incorporating algorithms to differentiate between human and animal movements, thereby reducing false positives in detection systems. In applications, such as smart building monitoring, integrated with PIR sensors achieves high accuracy in , with reported precision rates exceeding 97% for human presence detection. These AI-driven systems analyze binary PIR signals to estimate more reliably, minimizing errors from non-human triggers like pets. Emerging applications of PIR sensors extend beyond traditional uses into and . In conservation efforts, PIR-equipped camera traps enable non-invasive monitoring of animal populations, capturing motion-triggered images to support assessments and studies. For HVAC optimization, PIR sensors detect occupancy in real-time to adjust ventilation and heating dynamically; studies show such systems can contribute to savings of 10–30% depending on implementation. The global PIR sensor market reflects this growth, valued at approximately $0.8 billion as of 2025 and projected to expand at a (CAGR) of 15% through 2035, driven by demand in IoT and smart systems. Sustainability initiatives have led to the development of low-power PIR designs tailored for Internet of Things (IoT) deployments, featuring standby currents as low as microamperes to extend battery life in remote sensors. Solar-compatible variants, often paired with energy-harvesting modules, enable off-grid operation for outdoor applications, reducing reliance on replaceable batteries. Ongoing research into graphene-based PIR sensors promises higher sensitivity, with uncooled thermal detectors demonstrating ultrahigh responsivity for improved infrared detection at room temperature.

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