> Quick answer: The pyroelectric materials in a PIR sensor generate a voltage spike when a 37°C human moves across its field of view, creating a temperature differential from a -10°C background [1]. This transient effect only occurs with changes in temperature, not static heat [1].
At the heart of every modern security system lies the Passive Infrared (PIR) sensor. These devices are designed to detect human movement by translating thermal signatures into electrical signals. But how do these sensors work at a physical level? Specifically, how does a moving 37°C human heat signature generate a voltage spike against a -10°C background?
How Pyroelectric Materials Generate Voltage Spikes
The pyroelectric materials in a PIR sensor translate the movement of a 37°C human into a voltage spike through their inherent ability to generate transient voltages in response to changes in temperature [1]. This property is crucial because static heat does not induce a net charge separation—otherwise, it would violate thermodynamic principles by generating energy without input [1].
The Principle of Differential Detection
A stationary body at 37°C will not trigger the sensor as equilibrium is reached and the voltage decays to zero [1]. However, when a human moves across the field of view, one half of the sensor detects the warmer IR radiation while the other remains exposed to the colder background (-10°C) [2]. This differential change in temperature creates a measurable voltage difference between the two halves [1].
Sensor Design and Differential Pair Configuration
The sensor is typically constructed with two or more pyroelectric elements arranged in opposing polarity, often in an H shape or paired configuration [9]. When both elements are uniformly heated by ambient IR from walls or distant heat sources, their equal but opposite voltages cancel out, producing no net output [14].
Movement and Voltage Pulses
When a warm body moves into the sensing area, IR radiation first hits one element, generating a positive voltage spike while the other remains at baseline, leading to a measurable differential signal [2]. As the body moves out of the field of view, this process reverses, with the first element cooling down and producing a negative voltage spike [2].
The Pyroelectric Effect
The physical mechanism relies on the pyroelectric effect: when the temperature of a non-centrosymmetric crystal changes, spontaneous polarization shifts, causing a temporary displacement of charge and thus a measurable voltage across the material [1]. This transient voltage decays once thermal equilibrium is reached [1].
Field of View and IR Flux
The sensor packaging geometry and lens ensure that only localized, directional changes in IR radiation are detected [2]. The movement of the human body creates a time-varying IR flux, which translates into dynamic signals rather than static ones.
Environmental Temperature Sensitivity
While PIR sensors do not detect absolute temperature but rather *changes in temperature over time*, their performance can vary with ambient temperature [3][12]. For instance, output voltage may drift with temperature changes, leading to false triggers or missed detections [18].
Compensation Mechanisms
To mitigate this, some systems incorporate a thermistor or temperature sensor to measure the ambient temperature and dynamically adjust the detection threshold [3][12][17][21]. The microcontroller recalibrates the comparator threshold using lookup tables or algorithms, ensuring consistent performance across varying conditions [21].
Key Takeaways
- PIR sensors detect moving humans through transient voltage generation in pyroelectric materials when a temperature differential is created.
- Differential detection ensures that only changes in IR flux trigger the sensor while filtering out uniform heating sources.
- Ambient temperature affects sensitivity and requires compensation mechanisms for reliable operation.
Comparison Table
| Aspect | Description |
|–––|––––-|
| Transient Voltage | Occurs with temperature change, not static heat [1] |
| Differential Design | Opposing polarity elements cancel out uniform heating [9][14] |
| Pyroelectric Effect | Spontaneous polarization shift generates transient voltage [1] |
Frequently Asked Questions
[
{„q”: „How does a PIR sensor detect movement?”, „a”: „A PIR sensor detects movement through the pyroelectric effect. When a human moves across its field of view, it creates a temperature differential that triggers a voltage spike [2].”},
{„q”: „What happens if there is uniform heating in the environment?”, „a”: „Uniform heating causes equal and opposite voltages in the opposing elements, canceling out any net output and preventing false triggers [14][9].”},
{„q”: „How does ambient temperature affect PIR sensor performance?”, „a”: „Ambient temperature can cause voltage drift, but sensors often include compensation mechanisms with thermistors to maintain consistent performance [21][18].”}
]
References
- [1] Technology_Connections__Passive_infrared_motion_sensors_a_two-bit_camera_powered_by_crystals__XRCprhlz4D8 — youtube
source passage
you glitched into existence right in front of a pyroelectric crystal [fwoop] it would produce a blip of voltage as your body heat raised its temperature but then… well it would stop. It’s only the change in temperature that produces the voltage, otherwise we’d have something of a free energy device on our hands. So once your body heat brought it to a new equilibrium temperature, the voltage would go away. But also, we don’t want that anyway. Such a sensor would react to anything near it that quickly changes temperature – say a heater switching on. And that’s not what we’re trying to detect. But what if you had… two of them? If one crystal warmed up but the other didn’t, there'd be an imbalance in voltage between the two which could be measured. So if you can figure out a way to cause a person moving in front of a device with a pair of pyroelectric crystals to heat only one of them at a time, you could detect that person’s movement. Which brings us to these: here I have two common types of passive infrared sensor, both used in lighting products. The translucent piece of plastic you find on the front of both sensors is actually a complicated lens. They look cloudy since they’re not transparent to visible light, but this material affects infrared light just as if it were glass. And sitting behind the lens (quite far behind it as a matter of fact) is our pair of pyroelectric crystals. They’re inside this metal can and hiding underneath a filter which rejects visible light so we c
- [2] WO2022192395A1_-_Passive_infrared_sensor_-_Google_Patents__72576e5c — patent
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silicon) that also protects the sensor elements. The PIR sensor 100 has a limited field of view 140 defined by the packaging geometry and whether it includes a lens. When the sensor is idle, both sensor elements 121 and 122 detect the same amount of IR, the ambient amount radiated from the room or walls or outdoors. When a warm body like a human or animal passes, the IR emissions from the body first strikes one half of the PIR sensor 100, which causes a positive differential change between the two sensor elements 121 and 122. When the warm body leaves the sensing area, the reverse happens, whereby the sensor generates a negative differential change. These change pulses are what is detected. [0005] The PIR sensor 100 further includes circuitry 150 coupled to the pyroelectric sensor elements 121 and 122 for generating an electrical output signal VIR in response to infrared radiation striking the sensor elements; the output signal VIR can have a typical peak-to-peak voltage of ~3.6 millivolts (mVpp). The sensor elements 121, 122 can be calibrated, for example, to be sensitive to human heat wavelengths (i.e., 8-14 pm); they detect a person, however, only if the human is moving. [0006] Referring now to FIG. 2, illustrated are the operational characteristics of PIR sensor 100 for a person moving relative to the sensor. The output signal characteristics of the PIR sensor 100 can vary as a function of the direction of movement of the person, distance of the person from the sensor, an
- [3] US10601244B2_-_Emergency_lighting_device_with_-_Google_Patents__2a8831b9 — patent
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lighting module light bulbs can be used by a consumer to replace the R30 incandescent bulbs in their house that would typically be used in recessed lighting fixtures at substantial savings on power consumption. In embodiments of wireless light bulbs or battery powered wireless lighting fixtures containing a PIR device for motion sensing, a thermal sensor may be present to provide a measurement of temperature to allow temperature compensation of the threshold for motion detection. In some embodiments, a temperature dependant voltage may be generated using a thermistor, a resistor network and a supply voltage where the output voltage is dependent on the resistance of the thermistor and that output voltage may be used to derive the threshold voltage used for motion detection. Thus, the change in sensitivity of the motion sensor over temperature may be compensated for by changing the threshold of the motion detection circuit. By way of an example, an operational amplifier used as a comparator at the output of the motion sensing circuitry has a threshold that the voltage that is a representation of the detected motion is compared against. Over temperature, the amplified output of the PIR sensor may vary to the point that false triggers may occur which would turn the light on when motion is not detected or has not been detected sufficiently to turn the light on. If the threshold at the comparator varies with temperature, the threshold may move higher or lower compensating for the c
- [9] US20040169145A1_-_PIR_motion_sensor_-_Google_Patents__fc6ec8aa — patent
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formed first and 28, 30. In the embodiment shown in FIG. 2, thesecond PIR detectors first detector 28 has four, elements 32 (two pair of plus and minus polarity elements electrically connected together) and thesecond detector 30 has two elements 34 (one pair of plus and minus polarity elements), with each pair of 32, 34 being joined by an electrical connection, roughly forming an “H”. It is to be understood that theelements 28, 30 include, on the reverse side of thedetectors substrate 26 from that shown, complementary components (e.g. “plates” as explained below) which, together with those shown, form the 32, 34. Connections among these reverse-side plates are depicted by dashed lines.elements – The 28, 30 can be pyroelectric detectors that measure changes in far infrared radiation. Such detectors operate by the “piezoelectric effect”, which causes electrical charge migration in the presence of mechanical strain. Pyroelectric detectors take the form of a capacitor—two electrically conductive plates separated by a dielectric. The dielectric is often a piezoelectric ceramic, and is referred to herein as a “substrate”. When far infrared radiation causes a temperature change (and thus some mechanical strain) in the ceramic, electrical charge migrates from one plate to the other. If no external circuit is connected to the detector, then a voltage appears as the “capacitor” charges. If an external circuit is connected between the plates, then a current flows.detectors – In accordan
- [12] US8491159B2_-_Wireless_emergency_lighting_system_-_Google_Patents__b5668615 — patent
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recessed lighting fixtures at substantial savings on power consumption. – a thermal sensor may be present to provide a measurement of temperature to allow temperature compensation of the threshold for motion detection. – a temperature dependant voltage may be generated using a thermistor, a resistor network and a supply voltage where the output voltage is dependent on the resistance of the thermistor and that output voltage may be used to derive the threshold voltage used for motion detection. – the change in sensitivity of the motion sensor over temperature may be compensated for by changing the threshold of the motion detection circuit. – an operational amplifier used as a comparator at the output of the motion sensing circuitry has a threshold that the voltage that is a representation of the detected motion is compared against. – the amplified output of the PIR sensor may vary to the point that false triggers may occur which would turn the light on when motion is not detected or has not been detected sufficiently to turn the light on. If the threshold at the comparator varies with temperature, the threshold may move higher or lower compensating for the changes in performance of the PIR sensor and motion detector circuitry. – the temperature is measured, converted from analog to digital, read by a microcontroller and the microcontroller may set a threshold value through a digital to analog conversion based on the temperature reading. – the microcontroller may have an algori
- [14] Technology_Connections__Passive_infrared_motion_sensors_a_two-bit_camera_powered_by_crystals__XRCprhlz4D8 — youtube
source passage
say there was a bright flash of light – maybe from lightning. That will actually cause a voltage spike in a pyroelectric material because that visible light also becomes heat when it’s absorbed by it. But we don’t want the sensor to react to that. Thanks to the opposing polarity of the crystals it won’t. If the entire sensor area is heated uniformly, the equal and opposite voltages produced in the two halves of the pyroelectric element cancel each other out. But… If it’s only partially heated, then only one crystal produces voltage while the other doesn’t. That means there’s an imbalance, so there’s a voltage potential across the entire structure. And, since that structure is connected across ground and the gate of the transistor, that difference in heat (and thus difference in potential) will trigger the FET and it will in turn produce a blip on the signal pin. Which brings us back to the lens. I mean, lenses. If I remove this and we look at the backside, you’ll see that this is actually a bunch of Fresnel lenses. Counting each section with a circle bit as one lens means there are twenty-eight lenses in total. Unfortunately because this is designed to bend infrared light and not visible light I can’t demonstrate what this lens is actually doing all that well. But if I stick my phone’s flashlight behind it at roughly the same position the crystals sit, you might be able to see that only one section of the lens lights up brightly, and which section lights up changes as I move
- [17] US10601244B2_-_Emergency_lighting_device_with_-_Google_Patents__2a8831b9 — patent
source passage
AC powered, battery backed wireless lighting module light bulbs can be used by a consumer to replace the R30 incandescent bulbs in their house that would typically be used in recessed lighting fixtures at substantial savings on power consumption. – a thermal sensor may be present to provide a measurement of temperature to allow temperature compensation of the threshold for motion detection. – a temperature dependant voltage may be generated using a thermistor, a resistor network and a supply voltage where the output voltage is dependent on the resistance of the thermistor and that output voltage may be used to derive the threshold voltage used for motion detection. – the change in sensitivity of the motion sensor over temperature may be compensated for by changing the threshold of the motion detection circuit. – an operational amplifier used as a comparator at the output of the motion sensing circuitry has a threshold that the voltage that is a representation of the detected motion is compared against. – the amplified output of the PIR sensor may vary to the point that false triggers may occur which would turn the light on when motion is not detected or has not been detected sufficiently to turn the light on. If the threshold at the comparator varies with temperature, the threshold may move higher or lower compensating for the changes in performance of the PIR sensor and motion detector circuitry. – the temperature is measured, converted from analog to digital, read by a micr
- [18] US8033686B2_-_Wireless_lighting_devices_and_applications__34ec7d33 — patent
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by changing the threshold of the motion detection circuit. By way of an example, an operational amplifier used as a comparator at the output of the motion sensing circuitry has a threshold that the voltage that is a representation of the detected motion is compared against. Over temperature, the amplified output of the PIR sensor may vary to the point that false triggers may occur which would turn the light on when motion is not detected or has not been detected sufficiently to turn the light on. If the threshold at the comparator varies with temperature, the threshold may move higher or lower compensating for the changes in performance of the PIR sensor and motion detector circuitry. In an alternate embodiment, the temperature is measured, converted from analog to digital, read by a microcontroller and the microcontroller may set a threshold value through a digital to analog conversion based on the temperature reading. In such a case, to determine the proper threshold level the microcontroller may have an algorithm programmed in it to calculate the required threshold based on the measured temperature, the microcontroller may contain a lookup table such that stored in memory a lookup using the read temperature will return the required threshold value and the like. In another embodiment, the wireless light bulb or battery powered wireless lighting fixture may have a communication interface such that a processor that has a measurement of temperature may send a command to the bu
- [21] US8491159B2_-_Wireless_emergency_lighting_system_-_Google_Patents__b5668615 — patent
source passage
If the threshold at the comparator varies with temperature, the threshold may move higher or lower compensating for the changes in performance of the PIR sensor and motion detector circuitry. In an alternate embodiment, the temperature is measured, converted from analog to digital, read by a microcontroller and the microcontroller may set a threshold value through a digital to analog conversion based on the temperature reading. In such a case, to determine the proper threshold level the microcontroller may have an algorithm programmed in it to calculate the required threshold based on the measured temperature, the microcontroller may contain a lookup table such that stored in memory a lookup using the read temperature will return the required threshold value and the like. In another embodiment, the wireless light bulb or battery powered wireless lighting fixture may have a communication interface such that a processor that has a measurement of temperature may send a command to the bulb or fixture to set the motion detection threshold for compensation. It is to be appreciated that any method of measuring temperature and using that information to modify the threshold based on the input temperature may be used. In embodiments of wireless light bulbs or battery powered wireless lighting fixtures containing any type of sensor, power circuitry, LED driver circuit or LED device that may change performance over temperature, a thermal sensor may be present to provide a measurement of
you glitched into existence right in front of a pyroelectric crystal [fwoop] it would produce a blip of voltage as your body heat raised its temperature but then… well it would stop. It’s only the change in temperature that produces the voltage, otherwise we’d have something of a free energy device on our hands. So once your body heat brought it to a new equilibrium temperature, the voltage would go away. But also, we don’t want that anyway. Such a sensor would react to anything near it that quickly changes temperature – say a heater switching on. And that’s not what we’re trying to detect. But what if you had… two of them? If one crystal warmed up but the other didn’t, there'd be an imbalance in voltage between the two which could be measured. So if you can figure out a way to cause a person moving in front of a device with a pair of pyroelectric crystals to heat only one of them at a time, you could detect that person’s movement. Which brings us to these: here I have two common types of passive infrared sensor, both used in lighting products. The translucent piece of plastic you find on the front of both sensors is actually a complicated lens. They look cloudy since they’re not transparent to visible light, but this material affects infrared light just as if it were glass. And sitting behind the lens (quite far behind it as a matter of fact) is our pair of pyroelectric crystals. They’re inside this metal can and hiding underneath a filter which rejects visible light so we c
silicon) that also protects the sensor elements. The PIR sensor 100 has a limited field of view 140 defined by the packaging geometry and whether it includes a lens. When the sensor is idle, both sensor elements 121 and 122 detect the same amount of IR, the ambient amount radiated from the room or walls or outdoors. When a warm body like a human or animal passes, the IR emissions from the body first strikes one half of the PIR sensor 100, which causes a positive differential change between the two sensor elements 121 and 122. When the warm body leaves the sensing area, the reverse happens, whereby the sensor generates a negative differential change. These change pulses are what is detected. [0005] The PIR sensor 100 further includes circuitry 150 coupled to the pyroelectric sensor elements 121 and 122 for generating an electrical output signal VIR in response to infrared radiation striking the sensor elements; the output signal VIR can have a typical peak-to-peak voltage of ~3.6 millivolts (mVpp). The sensor elements 121, 122 can be calibrated, for example, to be sensitive to human heat wavelengths (i.e., 8-14 pm); they detect a person, however, only if the human is moving. [0006] Referring now to FIG. 2, illustrated are the operational characteristics of PIR sensor 100 for a person moving relative to the sensor. The output signal characteristics of the PIR sensor 100 can vary as a function of the direction of movement of the person, distance of the person from the sensor, an
lighting module light bulbs can be used by a consumer to replace the R30 incandescent bulbs in their house that would typically be used in recessed lighting fixtures at substantial savings on power consumption. In embodiments of wireless light bulbs or battery powered wireless lighting fixtures containing a PIR device for motion sensing, a thermal sensor may be present to provide a measurement of temperature to allow temperature compensation of the threshold for motion detection. In some embodiments, a temperature dependant voltage may be generated using a thermistor, a resistor network and a supply voltage where the output voltage is dependent on the resistance of the thermistor and that output voltage may be used to derive the threshold voltage used for motion detection. Thus, the change in sensitivity of the motion sensor over temperature may be compensated for by changing the threshold of the motion detection circuit. By way of an example, an operational amplifier used as a comparator at the output of the motion sensing circuitry has a threshold that the voltage that is a representation of the detected motion is compared against. Over temperature, the amplified output of the PIR sensor may vary to the point that false triggers may occur which would turn the light on when motion is not detected or has not been detected sufficiently to turn the light on. If the threshold at the comparator varies with temperature, the threshold may move higher or lower compensating for the c
formed first and 28, 30. In the embodiment shown in FIG. 2, thesecond PIR detectors first detector 28 has four, elements 32 (two pair of plus and minus polarity elements electrically connected together) and thesecond detector 30 has two elements 34 (one pair of plus and minus polarity elements), with each pair of 32, 34 being joined by an electrical connection, roughly forming an “H”. It is to be understood that theelements 28, 30 include, on the reverse side of thedetectors substrate 26 from that shown, complementary components (e.g. “plates” as explained below) which, together with those shown, form the 32, 34. Connections among these reverse-side plates are depicted by dashed lines.elements – The 28, 30 can be pyroelectric detectors that measure changes in far infrared radiation. Such detectors operate by the “piezoelectric effect”, which causes electrical charge migration in the presence of mechanical strain. Pyroelectric detectors take the form of a capacitor—two electrically conductive plates separated by a dielectric. The dielectric is often a piezoelectric ceramic, and is referred to herein as a “substrate”. When far infrared radiation causes a temperature change (and thus some mechanical strain) in the ceramic, electrical charge migrates from one plate to the other. If no external circuit is connected to the detector, then a voltage appears as the “capacitor” charges. If an external circuit is connected between the plates, then a current flows.detectors – In accordan
recessed lighting fixtures at substantial savings on power consumption. – a thermal sensor may be present to provide a measurement of temperature to allow temperature compensation of the threshold for motion detection. – a temperature dependant voltage may be generated using a thermistor, a resistor network and a supply voltage where the output voltage is dependent on the resistance of the thermistor and that output voltage may be used to derive the threshold voltage used for motion detection. – the change in sensitivity of the motion sensor over temperature may be compensated for by changing the threshold of the motion detection circuit. – an operational amplifier used as a comparator at the output of the motion sensing circuitry has a threshold that the voltage that is a representation of the detected motion is compared against. – the amplified output of the PIR sensor may vary to the point that false triggers may occur which would turn the light on when motion is not detected or has not been detected sufficiently to turn the light on. If the threshold at the comparator varies with temperature, the threshold may move higher or lower compensating for the changes in performance of the PIR sensor and motion detector circuitry. – the temperature is measured, converted from analog to digital, read by a microcontroller and the microcontroller may set a threshold value through a digital to analog conversion based on the temperature reading. – the microcontroller may have an algori
say there was a bright flash of light – maybe from lightning. That will actually cause a voltage spike in a pyroelectric material because that visible light also becomes heat when it’s absorbed by it. But we don’t want the sensor to react to that. Thanks to the opposing polarity of the crystals it won’t. If the entire sensor area is heated uniformly, the equal and opposite voltages produced in the two halves of the pyroelectric element cancel each other out. But… If it’s only partially heated, then only one crystal produces voltage while the other doesn’t. That means there’s an imbalance, so there’s a voltage potential across the entire structure. And, since that structure is connected across ground and the gate of the transistor, that difference in heat (and thus difference in potential) will trigger the FET and it will in turn produce a blip on the signal pin. Which brings us back to the lens. I mean, lenses. If I remove this and we look at the backside, you’ll see that this is actually a bunch of Fresnel lenses. Counting each section with a circle bit as one lens means there are twenty-eight lenses in total. Unfortunately because this is designed to bend infrared light and not visible light I can’t demonstrate what this lens is actually doing all that well. But if I stick my phone’s flashlight behind it at roughly the same position the crystals sit, you might be able to see that only one section of the lens lights up brightly, and which section lights up changes as I move
AC powered, battery backed wireless lighting module light bulbs can be used by a consumer to replace the R30 incandescent bulbs in their house that would typically be used in recessed lighting fixtures at substantial savings on power consumption. – a thermal sensor may be present to provide a measurement of temperature to allow temperature compensation of the threshold for motion detection. – a temperature dependant voltage may be generated using a thermistor, a resistor network and a supply voltage where the output voltage is dependent on the resistance of the thermistor and that output voltage may be used to derive the threshold voltage used for motion detection. – the change in sensitivity of the motion sensor over temperature may be compensated for by changing the threshold of the motion detection circuit. – an operational amplifier used as a comparator at the output of the motion sensing circuitry has a threshold that the voltage that is a representation of the detected motion is compared against. – the amplified output of the PIR sensor may vary to the point that false triggers may occur which would turn the light on when motion is not detected or has not been detected sufficiently to turn the light on. If the threshold at the comparator varies with temperature, the threshold may move higher or lower compensating for the changes in performance of the PIR sensor and motion detector circuitry. – the temperature is measured, converted from analog to digital, read by a micr
by changing the threshold of the motion detection circuit. By way of an example, an operational amplifier used as a comparator at the output of the motion sensing circuitry has a threshold that the voltage that is a representation of the detected motion is compared against. Over temperature, the amplified output of the PIR sensor may vary to the point that false triggers may occur which would turn the light on when motion is not detected or has not been detected sufficiently to turn the light on. If the threshold at the comparator varies with temperature, the threshold may move higher or lower compensating for the changes in performance of the PIR sensor and motion detector circuitry. In an alternate embodiment, the temperature is measured, converted from analog to digital, read by a microcontroller and the microcontroller may set a threshold value through a digital to analog conversion based on the temperature reading. In such a case, to determine the proper threshold level the microcontroller may have an algorithm programmed in it to calculate the required threshold based on the measured temperature, the microcontroller may contain a lookup table such that stored in memory a lookup using the read temperature will return the required threshold value and the like. In another embodiment, the wireless light bulb or battery powered wireless lighting fixture may have a communication interface such that a processor that has a measurement of temperature may send a command to the bu
If the threshold at the comparator varies with temperature, the threshold may move higher or lower compensating for the changes in performance of the PIR sensor and motion detector circuitry. In an alternate embodiment, the temperature is measured, converted from analog to digital, read by a microcontroller and the microcontroller may set a threshold value through a digital to analog conversion based on the temperature reading. In such a case, to determine the proper threshold level the microcontroller may have an algorithm programmed in it to calculate the required threshold based on the measured temperature, the microcontroller may contain a lookup table such that stored in memory a lookup using the read temperature will return the required threshold value and the like. In another embodiment, the wireless light bulb or battery powered wireless lighting fixture may have a communication interface such that a processor that has a measurement of temperature may send a command to the bulb or fixture to set the motion detection threshold for compensation. It is to be appreciated that any method of measuring temperature and using that information to modify the threshold based on the input temperature may be used. In embodiments of wireless light bulbs or battery powered wireless lighting fixtures containing any type of sensor, power circuitry, LED driver circuit or LED device that may change performance over temperature, a thermal sensor may be present to provide a measurement of