PIR & Motion Sensors

Motion sensors switch on lights, sound alarms and count people. They all give a simple digital output that goes HIGH when something moves.

PIR (passive infrared) sensors watch for changes in body heat crossing their field of view, so a person sitting perfectly still can eventually stop triggering them.

  • The HC-SR501 is the classic module: longer range (up to about 7 m), a 4.5-20V supply, and two potentiometers for sensitivity and hold time.
  • Mini PIRs such as the HC-SR505 are tiny, fixed and very low power, which suits battery builds.
  • The RCWL-0516 is different: a microwave radar that senses movement through plastic, glass and thin wood, so it still works sealed inside a box.

How to choose:

  • Pick the HC-SR501 when you want to tune it.
  • Pick a mini PIR for battery or compact builds.
  • Pick radar when the sensor must be hidden.

Every PIR needs a 30-60 second warm-up after power-on, and false triggers during that window are normal. Keep PIRs away from curtains, direct sun and air vents. All of these work with Arduino and ESP32 using a single digitalRead().

Frequently asked questions

Why does my PIR trigger right after power-on?

It needs 30-60 seconds to settle to the room's infrared level.

Why does a still person stop triggering it?

A PIR detects changes in infrared, not heat itself. A motionless body gives a flat signal.

HC-SR501 or a mini PIR?

The HC-SR501 if you need adjustable range and delay. A mini PIR for small, low-power builds.

Can a motion sensor work inside a box?

A PIR cannot see through plastic. The RCWL-0516 radar can.

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