What is a Pyroelectric Sensor?
The pyroelectric sensor is the fundamental component that converts infrared radiation into an electrical signal. It consists of a crystalline material that exhibits the pyroelectric effect: a temporary voltage generated when the material’s temperature changes.
The Pyroelectric Effect
Certain materials (like lithium tantalate or lead zirconate titanate – PZT) have a spontaneous electric polarization that depends on temperature. When infrared radiation heats the material, the polarization changes, releasing charge on the surface electrodes. This charge is measured as a voltage across a load resistor.
The effect is pyroelectric, not piezoelectric (though the materials are often piezoelectric as well). It responds to changes in temperature, not absolute temperature, which is why PIR sensors detect motion (changing IR) rather than stationary warm objects.
Dual-Element Design
Most PIR sensors contain two pyroelectric elements connected in a differential (opposite polarity) configuration. This design provides two key benefits:
- Common-Mode Rejection: Both elements respond equally to uniform temperature changes (ambient temperature drift, sunlight), canceling the signal.
- Motion Detection: When a warm body moves across the field of view, it sequentially illuminates the two elements, producing a differential signal.
Typical Pyroelectric Materials
- Lithium Tantalate (LiTaO3): High pyroelectric coefficient, stable, used in high-quality sensors.
- PZT (Lead Zirconate Titanate): Lower cost, good performance, common in hobbyist modules.
- PVDF (Polyvinylidene Fluoride): Polymer film, flexible, used in some specialized sensors.
Electrical Equivalent Circuit
A pyroelectric sensor can be modeled as a current source in parallel with a capacitor and a very high resistance (10^10-10^14 Ω). The generated current is proportional to the rate of temperature change. In practice, a JFET or op-amp buffer is integrated to convert this high-impedance signal to a usable voltage.
Sensitivity and Noise
Pyroelectric sensors are extremely sensitive, capable of detecting temperature changes as small as 0.001°C. However, they also generate noise due to thermal fluctuations and material properties. The signal-to-noise ratio determines the minimum detectable motion.
Frequency Response
Pyroelectric sensors respond to frequencies from about 0.001 Hz to 100 Hz. The low-frequency cutoff is determined by the electrical time constant (sensor capacitance × load resistance). High-frequency response is limited by thermal diffusion time in the element.
Common Pyroelectric Sensor Models
- LHI 954: Used in many HC-SR501 modules, dual-element, TO-5 package.
- RE200B: Another common dual-element sensor.
- IRA-E700: From Murata, available with various filters.
- PIRL 5xx: Excelitas series.
Packaging and Filter Window
The pyroelectric element is hermetically sealed in a metal can (TO-5 or TO-39) with a window that transmits IR. The window material (silicon, germanium, or special coatings) determines the spectral response.
Conclusion
The pyroelectric sensor element is the heart of every PIR module. Understanding its operation helps in selecting the right sensor and troubleshooting issues.