Introduction
PIR sensors seem simple, but there are common pitfalls that lead to frustration. Learn from others’ mistakes.
Mistake 1: Not Allowing Warm-Up Time
The problem: You power up the sensor and immediately test, but it doesn’t work or triggers randomly. You think it’s broken.
The fix: Always allow 30-60 seconds for the sensor to stabilize after power-up. Mention this in your code comments or user manual.
Mistake 2: Ignoring the Jumper/Trigger Mode
The problem: In repeatable mode (H), the output stays high as long as motion continues. In non-repeatable (L), it doesn’t. Using the wrong mode leads to unexpected behavior (e.g., lights staying on too long).
The fix: Understand your application: for occupancy, use repeatable; for single-shot events, use non-repeatable.
Mistake 3: Incorrect Voltage Assumptions
The problem: Connecting a 5V-only sensor to a 3.3V microcontroller (or vice versa) without level shifting. The sensor may not work or may be damaged.
The fix: Check datasheet voltage requirements. Use level shifters or choose compatible sensors.
Mistake 4: Poor Placement Near Heat Sources
The problem: Mounting the sensor facing a window, heater, or air vent causes constant false triggers.
The fix: Plan placement carefully. Test in location before final mounting.
Mistake 5: Overlooking the Lens
The problem: The Fresnel lens gets dirty, scratched, or removed, severely reducing range.
The fix: Keep lens clean. Handle with care. Ensure it’s properly clipped on.
Bonus Mistake: Not Testing with a Multimeter
Many issues are hardware-related. Test the sensor alone before blaming code.
Avoid these mistakes and your PIR projects will be much more reliable.