HC-SR501 PIR Motion Sensor

HC-SR501 PIR Motion Sensor
The sensor is topped with a white, dome-shaped Fresnel lens. This lens is actually a collection of many small lenses that focus infrared light onto a pyroelectric sensor hidden underneath. When a warm body moves across the sensor's field of view, it creates a "hot" spot that the circuitry identifies as motion.

 
Why it’s a Home Automation Essential:
• Energy Efficiency: Because it is passive, it draws very little power, making it ideal for battery-operated security cameras or smart lights that only turn on when someone enters the room.
• Highly Customizable: Unlike simpler modules, the HC-SR501 allows you to manually tune exactly how it behaves using two onboard potentiometers and a jumper.
• Large Detection Area: With a 120° wide-angle view and a range of up to 7 meters, one sensor can easily monitor an entire small room or a hallway.
Technical Capabilities:
• Operating Voltage: 4.5V to 20V (It has an onboard voltage regulator, but the output signal is always a clean 3.3V digital HIGH).
• Adjustable Time Delay: You can set how long the output stays HIGH after it stops seeing motion (from 3 seconds up to 5 minutes).
• Adjustable Sensitivity: You can shrink the detection distance down to 3 meters for small spaces or crank it up to 7 meters for driveways.

 
The Two Trigger Modes (The Yellow Jumper):
On the corner of the board, there is a 3-pin header with a yellow jumper. This determines how the sensor handles continuous movement:
1. L (Single Trigger): If you keep moving in front of the sensor, the output will go HIGH, then LOW when the timer ends, even if you are still moving. It won't go HIGH again until it "resets."
2. H (Repeatable Trigger - Default): As long as you keep moving, the output stays HIGH. The timer only starts counting down once you stop moving. This is much better for automatic room lighting.

 
Best For:
The HC-SR501 is the gold standard for automatic hallway lights, burglar alarms, interactive museum exhibits, and energy-saving office monitors.