Introduction
Ceiling fans are a common source of false triggers for PIR sensors, especially when the sensor is mounted on the ceiling near the fan. The moving blades create changing IR patterns that mimic motion.
How Ceiling Fans Cause False Triggers
1. Moving Blades
The rotating fan blades create moving shadows and reflections. Even though the blades themselves may be at room temperature, they modulate the IR from the ceiling and walls behind them.
2. Air Movement
The fan creates air currents that can carry warm or cool air across the sensor, creating temperature changes.
3. Vibration
An unbalanced fan can cause vibrations that transmit to the sensor, especially if they share the same mounting surface.
Symptoms
- False triggers only when fan is on.
- Triggers at regular intervals corresponding to fan speed.
- Sensor works fine when fan is off.
Solutions
1. Relocate Sensor
The best solution: move the sensor away from the fan. If both are ceiling-mounted, place the sensor at least 2-3 meters from the fan.
2. Change Fan Speed
Sometimes false triggers occur only at certain speeds. Changing the fan speed may help.
3. Use a Different Mounting Location
Instead of ceiling mounting, consider wall mounting the sensor away from the fan’s air stream.
4. Add a Physical Barrier
A small shield can block the sensor’s view of the fan while still covering the room. This is tricky but possible.
5. Use a Sensor with Narrower Field of View
A curtain lens that doesn’t “see” the fan area may help.
6. Software Filtering
If the fan speed is constant, you may be able to filter out the regular signal in software (advanced).
Case Study: Conference Room Sensor
A conference room had a PIR sensor that triggered randomly. The culprit was a ceiling fan. Relocating the sensor to a wall away from the fan eliminated the false triggers.
Conclusion
Ceiling fan interference is common but usually solved by relocating the sensor. If that’s not possible, try changing fan speed or using a different sensor type.