Pyroelectric Infrared Sensor Structure Explained: PIR Operation Logic from Chip to Lens

In intrusion alarms, human-sensing lighting, smart homes, and industrial automation, pyroelectric infrared sensors (PIR sensors) are nearly standard devices. They are simple, ultra-low power, yet reliably detect human movement in complex environments, making them one of the most cost-effective human presence detection technologies.

The reason PIR sensors are "sensitive but stable" is not complex algorithms but highly engineered structural design. This article dissects how the internal structure distinguishes background thermal radiation from real moving targets.

1. Overall Structure Framework of PIR Sensors

A typical pyroelectric infrared sensor consists of three main systems:

2. Core Component: Internal Structure of Pyroelectric Chip

2.1 Infrared Filter: First Line of Interference Defense

The infrared filter on top of the chip is a band-pass structure, typically transmitting 5–14 μm, covering the 9–10 μm band where human radiation is concentrated. This design:

• Effectively blocks visible light

• Suppresses solar radiation, incandescent lamps, and other broadband interference

• Reduces the impact of slow environmental temperature changes

2.2 Pyroelectric Element: Converts Temperature Changes into Electrical Signals

Usually made of PZT or lithium tantalate, the small element (~2 × 4 mm) only generates signals when temperature changes. PIR detects moving humans because movement causes infrared radiation variation.

2.3 Dual-Element Structure: The Soul of PIR

Most practical PIR chips use two equal-area, opposite-polarity elements. Logic:

• Uniform environmental changes cancel each other out

• Moving heat source crossing both elements generates differential signals

• This design gives inherent resistance to temperature drift and slow-changing interference

2.4 JFET Preamplifier: From Weak to Usable Signals

The pA-level signal from the pyroelectric element is amplified using an internal JFET for impedance conversion, initial amplification, and noise reduction, ensuring signals leaving the chip are usable.

3. Optical Components: Fresnel Lens Creates Detectable Motion Signals

3.1 Focusing: Enhances Range and Sensitivity

Fresnel lens concentrates distant infrared onto the chip, improving signal-to-noise ratio at ultra-low power.

3.2 Zoning: Turns Space into Alternating Detection Fields

The lens divides space into alternating bright and dark regions:

• Moving human → periodic infrared changes

• Stationary human → stable signal

Lens shape affects detection patterns: fan-shaped, spherical, curtain-style.

4. Mechanical & Electronic Structure: Ensures Stable Operation

4.1 Metal Shielding Housing

Metal TO-5 packages provide physical protection and EMI shielding.

4.2 Pins & Peripheral Circuits

Typical module pins: Vcc, GND, OUT. RC filters and bias networks stabilize operating points and provide reliable input to comparators or MCU.

5. Typical Characteristics Resulting from Structure

• Ultra-low power (μA level)

• Insensitive to slow temperature changes

• Highly sensitive to moving heat sources

• Low algorithm requirement, high reliability

6. Nexisense Engineering Insights on PIR Selection

• Dual-element consistency

• Filter wavelength stability

• JFET noise characteristics

• Lens compatibility with application scenarios

7. Common FAQ (10)

1. Can PIR detect stationary people? No, only thermal changes.

2. Does sunlight cause false alarms? Dual-element and filters reduce this.

3. Can PIR detect through glass? Ordinary glass blocks 8–14 μm infrared.

4. What determines detection range? Lens design and SNR.

5. Does PIR require calibration? Usually not by users.

   
   

6. Analog vs digital PIR? Differences in signal processing and output.

7. High ambient temperature impact? Reduces contrast with human body.

8. Why use metal housing? For EMI shielding and thermal stability.

9. Can PIR distinguish humans from animals? Requires lens and algorithm; single chip cannot.

10. Suitable for outdoor use? Possible with special design.

8. Summary

PIR sensors are not simple devices but a highly optimized structural system:

• Fresnel lens creates triggerable infrared scene

• Dual-element chips detect moving heat sources differentially

• Filters and preamplifiers ensure clean, usable signals

This highly structured design explains why PIR remains reliable, energy-efficient, and a core sensing technology over decades.