Introduction
Few things are more frustrating than a motion sensor that triggers for no apparent reason. If your PIR sensor is acting up, here are the most likely culprits and how to address them.
1. Heat Sources
PIR sensors detect changes in infrared radiation. Any moving heat source can cause false triggers:
- HVAC vents: Warm or cold air blowing across the sensor can mimic motion.
- Radiators and space heaters: Rapid temperature changes.
- Direct sunlight: Sunlight contains IR and can cause false alarms, especially as shadows move.
- Reflections: Shiny surfaces may reflect IR from other sources.
Fix: Relocate the sensor away from these sources. If relocation isn’t possible, consider using a sensor with a narrower field of view or masking part of the lens.
2. Electrical Interference (EMI/RFI)
PIR sensors can be affected by electromagnetic interference from:
- Nearby power lines
- Switching power supplies
- Motors (fans, appliances)
- Radio transmitters
Fix: Use shielded cable for sensor connections, keep sensor away from large appliances, and ensure proper grounding.
3. Power Supply Noise
Unstable or noisy power can cause the sensor’s internal comparator to oscillate, leading to false triggers. This is common when powering sensors from the same supply as motors or relays.
Fix: Add a 100µF capacitor across VCC and GND near the sensor. Use a separate regulated power supply if possible.
4. Mechanical Vibration
Sensors mounted on vibrating surfaces (e.g., near a washing machine) may generate false signals due to microphonic effects.
Fix: Mount the sensor on a stable surface or use vibration damping material.
5. Insects or Small Animals
Small creatures crawling on or near the lens can trigger the sensor.
Fix: Use a pet-immune sensor or seal the sensor housing.
6. Dirty Lens
Dust, cobwebs, or moisture on the Fresnel lens can scatter IR and cause erratic behavior.
Fix: Gently clean the lens with a soft, dry cloth.
7. Sensor Sensitivity Too High
Excessive sensitivity can make the sensor pick up minor environmental changes.
Fix: Turn the sensitivity potentiometer counterclockwise to reduce range.
8. Wrong Trigger Mode
In repeatable trigger mode (H), the sensor may stay triggered for a long time if motion continues, which might be mistaken for false triggering.
Fix: Try non-repeatable mode (L) if you want a single pulse per event.
Conclusion
False triggers are usually traceable to one of these causes. Systematic troubleshooting—checking one potential cause at a time—will help you identify and fix the issue.