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.
- Cause internal oscillations in the amplifier.
Symptoms
- False triggers only when transmitter is keyed.
- Triggers correlate with transmission (voice, CW, etc.).
- Sensor may work fine when transmitter is off.
Solutions
1. Physical Separation
Move the sensor as far from the transmitter and antenna as possible. Even a few meters can make a big difference.
2. Shielding
Enclose the sensor in a metal shield (copper or aluminum) with a hole for the lens. The shield must be grounded. This is very effective.
3. 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.
4. Input Protection
Add small capacitors (0.01µF) from each sensor pin to ground to shunt RF. Add series resistors (100-1000Ω) on signal lines.
5. Use a Sensor with Better RF Immunity
Some industrial sensors are designed with better EMI/RFI immunity. Look for sensors that mention RF immunity in datasheets.
6. Low-Pass Filtering
Add an LC low-pass filter on the power input to block RF from entering via power 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 (with lens exposed) eliminated the problem.
Conclusion
RF interference from transmitters is challenging but manageable with proper shielding and filtering. Start with ferrite beads and shielded cable.