Introduction
Electric motors generate electrical noise when starting and running. This noise can couple into PIR sensor circuits, causing false triggers. This is common with fans, pumps, garage door openers, and power tools.
How Motors Cause Interference
1. Conducted Noise
When a motor starts, it draws a high current, causing voltage spikes on the power line. If the PIR sensor shares the same power supply, these spikes can affect it.
2. Radiated Noise
Motors generate electromagnetic fields that can induce currents in nearby sensor wiring.
3. Arc Noise
Brush-type motors (like in some power tools) create electrical arcs that generate broadband RF noise.
Symptoms
- False triggers when motors start or run.
- Triggers correlated with specific appliances (e.g., garage door opening).
- Sensor works when motors are off.
Solutions
1. Separate Power Supplies
Power the PIR sensor from a different circuit than the motor. Use a battery or a separate wall wart with its own transformer.
2. Add Power Line Filtering
Install a line filter (like a common-mode choke) on the motor’s power input. Add capacitors and ferrites to the sensor’s power lines.
3. Use Shielded Cable
Use shielded twisted pair for sensor connections, with shield grounded at one end.
4. Physical Separation
Move the sensor and its wiring away from motor cables. Keep sensor wires short.
5. Add Snubber on Motor
For AC motors, add an RC snubber (resistor-capacitor) across the motor terminals to suppress spikes. For DC motors, add a flyback diode.
6. Ferrite Beads
Add ferrite beads on sensor power and signal wires near the sensor.
Case Study: Garage Door Sensor
A PIR sensor in a garage false-triggered every time the garage door opener ran. The opener’s motor was on the same electrical circuit. Adding a line filter on the opener and moving the sensor’s power supply to a different circuit solved the problem.
Conclusion
Motor interference is common but manageable with proper power isolation and filtering.