Introduction
High-power radio transmitters like ham radio rigs (100W+) or CB radios can cause PIR sensors to false-trigger. The strong RF field can be rectified by the sensor’s electronics, creating false signals.
How RF Interference Happens
The high-gain amplifiers in PIR sensors can act as unintentional RF detectors. Strong RF fields can be rectified by semiconductor junctions, creating DC offsets; couple into long sensor wires acting as antennas; or cause internal oscillations.
Symptoms
- False triggers only when transmitter is keyed
- Triggers correlate with transmission (voice, CW, etc.)
- Sensor works fine when transmitter is off
Solutions
Physical Separation
Move the sensor as far from the transmitter and antenna as possible.
Shielding
Enclose the sensor in a metal shield (copper or aluminum) with a hole for the lens. Ground the shield.
Filtered Cables
Use shielded cable for sensor connections, with the shield grounded at one end. Add ferrite beads on power and signal wires near the sensor.
Input Protection
Add small capacitors (0.01µF) from each sensor pin to ground. Add series resistors (100-1000Ω) on signal lines.
Case Study: Ham Radio Shack
A ham radio operator’s PIR security light triggered every time he transmitted on 20 meters. Adding ferrite beads on the sensor wires and enclosing the sensor in a small metal box eliminated the problem.
Conclusion
RF interference from transmitters is manageable with proper shielding and filtering.