Introduction
Almost all PIR sensors use a dual-element design. This is not arbitrary – it’s essential for the sensor’s function. This article explains why two elements are better than one.
The Problem with Single-Element Sensors
A single pyroelectric element responds to any change in infrared radiation, including:
- Ambient temperature changes (sun coming out, heater turning on).
- Vibration (which can change the element’s temperature).
- Electrical noise.
This would cause constant false triggers. A single-element sensor cannot distinguish between a moving person and a changing background.
The Dual-Element Solution
A dual-element sensor contains two pyroelectric elements connected in series or parallel with opposite polarity. This configuration provides common-mode rejection:
- Both elements see the same ambient temperature changes → signals cancel (output zero).
- Both elements see the same vibration → signals cancel.
- A warm body moving across the field illuminates one element then the other → differential signal (output non-zero).
Element Arrangements
Series Opposed (Most Common)
The two elements are connected in series with opposite polarity. Output is taken from the junction. This gives high common-mode rejection.
Parallel Opposed
Elements are connected in parallel with opposite orientation. This gives higher signal current but lower impedance.
Dual with Common Electrode
Some sensors have a common electrode between the two elements, simplifying connection.
How It Detects Motion Direction
The order in which the elements are illuminated determines the polarity of the output signal:
- Left-to-right motion: positive pulse then negative pulse.
- Right-to-left motion: negative pulse then positive pulse.
This allows direction detection with appropriate signal processing.
Element Geometry
The physical arrangement of the elements affects sensitivity and directionality:
- Side-by-side: Sensitive to horizontal motion.
- Top-bottom: Sensitive to vertical motion (less common).
- Quad elements: Four elements arranged for both horizontal and vertical sensitivity.
Common-Mode Rejection Ratio (CMRR)
CMRR quantifies how well the sensor rejects common signals. A high CMRR (e.g., >30 dB) means the sensor is good at ignoring ambient changes.
Effect of Element Matching
For good CMRR, the two elements must be well-matched in sensitivity and thermal characteristics. Manufacturing variations affect matching, which is why high-quality sensors cost more.
Limitations of Dual-Element Design
- Motion directly toward or away from the sensor (radial) may not produce a strong differential signal.
- Very slow motion may not create a detectable difference.
- If both elements are simultaneously illuminated (e.g., a large warm object moving directly toward), signal may be reduced.
Conclusion
The dual-element design is the key innovation that makes PIR sensors practical. It enables reliable motion detection in real-world environments with changing backgrounds.