Introduction
Thermopile sensors are often confused with PIR sensors because both detect infrared. However, they operate on different principles and serve different purposes. This article clarifies the differences.
PIR (Passive Infrared) – Motion Detection
Principle: Pyroelectric effect – generates charge in response to changes in temperature. Differential dual-element design cancels common-mode signals.
Output: Typically digital (motion yes/no) or analog AC-coupled signal.
What it measures: Change in IR over time (derivative).
Applications: Motion detection, occupancy sensing.
Thermopile – Temperature Measurement
Principle: Seebeck effect – multiple thermocouples in series generate voltage proportional to temperature difference between target and sensor.
Output: Analog voltage (DC) proportional to absolute temperature of target (or temperature difference).
What it measures: Absolute IR intensity (temperature).
Applications: Non-contact thermometers, thermal cameras (array), oven temperature sensing.
Key Differences
| Aspect | PIR | Thermopile |
|---|---|---|
| Signal | AC-coupled (responds to change) | DC (responds to absolute) |
| Detects stationary objects? | No | Yes (temperature) |
| Typical use | Motion | Temperature |
| Elements | Dual (differential) | Single or array |
| Cost | Low | Moderate |
Can a Thermopile Detect Motion?
Yes, but indirectly. If a warm object moves across the field of view of a thermopile, the output voltage will change. However, thermopiles are slower and less sensitive to motion than PIR. They are not optimized for motion detection.
Can a PIR Measure Temperature?
No. PIR sensors are intentionally AC-coupled to ignore constant IR. They cannot give an absolute temperature reading.
Array Sensors: Thermal Cameras
Both technologies have array versions: PIR arrays (rare) and thermopile arrays (common, e.g., MLX90640, AMG8833). Thermopile arrays create low-resolution thermal images (8×8 to 32×24 pixels) used for presence detection, people counting, and fever screening.
Which One Do You Need?
- Need to know if someone is moving? PIR.
- Need to know the temperature of an object? Thermopile.
- Need to detect presence of a stationary person (e.g., sitting)? Neither alone; consider mmWave or a thermopile array with movement analysis (but limited).
- Need to count people in a room? Thermopile array or mmWave.
Conclusion
PIR and thermopile sensors are complementary. PIR excels at low-power motion detection; thermopile provides temperature data. Choose based on your application’s primary need.