PIR / Motion Sensor
Sensor
Passive Infrared sensor that detects motion by sensing changes in infrared radiation (heat) from moving objects, primarily people.
What it looks like
PIR / Motion Sensor
A typical sensor — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Detection range
metersMaximum distance at which motion can be detected (typically 3-10m).
Detection angle
degreesField of view (typically 90-110°).
Sensitivity
adjustableHow much motion is required to trigger (potentiometer adjustment).
Time delay
secondsHow long output stays high after motion stops (adjustable 5-2000s).
Operating voltage
Volts (V)Typical 5V-12V DC. Check datasheet for exact range.
Markings
Usually no markings. Module may have 'OUT', 'VCC', 'GND' labels. Potentiometers marked 'SENS' (sensitivity) and 'TIME' (delay).
Standard values
Most modules are pre-assembled with fixed specs. Common range: 5-12V DC, 3-10m detection, 90-110° angle.
How to choose
1) Determine detection range needed. 2) Check field of view requirements. 3) Consider if adjustable sensitivity/delay needed. 4) Match operating voltage to your system. 5) Consider pet-immune if animals are present.
Pinout & package
HC-SR501 — 3-pin header under the dome
Check first: Pin order is printed on the underside of the board and is NOT the same on every clone — read the silkscreen before powering it. OUT is 3.3 V logic even when VCC is 5 V.
3-pin modules: VCC (power), OUT (digital output), GND (ground). Some have additional pins for sensitivity/delay adjustment. Package: PCB module with Fresnel lens.
Example circuits
- Motion-activated light
- Security alarm system
- Automatic door opener
- Motion-based data logger
- Energy-saving HVAC control
Common failures
Lens cracks or dirt blocking IR detection; output stuck high/low; sensitivity drifts over time; fails in extreme temperatures. No output when motion detected is most common failure.
How to test
1) Power module at correct voltage. 2) Use multimeter on OUT pin (should be LOW/GND). 3) Move hand in front of sensor. 4) Output should go HIGH/VCC. 5) Adjust pots if sensitivity/delay not working.
Substitutes
Ultrasonic sensors (for non-heat motion), microwave sensors (through-wall detection), video-based motion detection. No direct substitute for heat-based detection.