Introduction
Refrigerators and air conditioners cycle on and off, causing temperature changes and electrical noise that can trigger PIR sensors. This is a common problem in home automation.
How They Cause False Triggers
1. Temperature Changes
When a refrigerator compressor runs, it releases heat. If the sensor is near the refrigerator, this rapid temperature change can mimic motion.
2. Electrical Noise
Compressor motors generate electrical noise when starting and stopping. This noise can couple into sensor wiring or power lines, causing false triggers.
3. Vibration
Compressor vibration can physically shake the sensor, creating a signal.
Symptoms
- False triggers correlated with refrigerator or AC cycling.
- Triggers often occur in bursts when compressor starts.
- Sensor works fine when appliance is off.
Solutions
1. Relocate Sensor
Move the sensor away from the appliance. Even 1-2 meters can make a big difference.
2. Add EMI Filtering
Add a 0.1µF ceramic capacitor and a 10-100µF electrolytic capacitor across the sensor’s power pins. Add ferrite beads on power and signal wires.
3. Use Shielded Cable
If the sensor is near the appliance, use shielded cable for connections, with shield grounded at one end.
4. Isolate Power Supplies
Power the sensor from a different circuit than the appliance. Use a battery or separate wall wart.
5. Add Software Filtering
In your code, ignore triggers that occur within a few seconds of each other, or require multiple detections within a time window.
6. Use a Different Sensor Technology
Microwave sensors are less susceptible to electrical noise from motors.
Case Study: Kitchen Motion Sensor
A motion sensor in a kitchen triggered every time the refrigerator compressor started. Adding a 0.1µF capacitor at the sensor and moving it 1 meter away from the refrigerator eliminated the problem.
Conclusion
Appliance interference is common but manageable. Start with relocation and power filtering.