PIR Motion Sensor

The PIR (Passive Infrared) motion sensor detects movement by sensing changes in infrared radiation emitted by warm objects like humans and animals. The HC-SR501 module is the most widely used variant, offering adjustable sensitivity and delay time, making it ideal for automation, security systems, and occupancy detection.

For this interfacing you need the following components:

• Arduino board (Uno, Nano, Mega, etc.)
• HC-SR501 PIR motion sensor module
• Breadboard and jumper wires
• USB cable to connect Arduino to your computer

Schematic

Connect the PIR sensor to the Arduino as follows:

HC-SR501              Arduino
--------              -------
VCC           -->     5V
OUT           -->     Digital Pin 2
GND           -->     GND

No external components are required. The module operates at 5V and its output is a standard 5V logic signal.

Pin Map

On the HC-SR501, pins are typically labeled on the board. The order varies by manufacturer — verify the labels before connecting.

HC-SR501 Adjustments

The module has two potentiometers and a jumper:

Install necessary Library

No external library is required. PIR motion detection uses standard digitalRead() built into Arduino.

Code with complete explanation

This sketch reads the PIR sensor output and prints a message to the Serial Monitor whenever motion is detected or the area becomes clear.

const int pirPin = 2;

int calibrationTime = 30; // seconds for warm-up

void setup()
{
  Serial.begin(9600);
  pinMode(pirPin, INPUT);
  Serial.println("PIR Motion Sensor Test");
  Serial.print("Calibrating for ");
  Serial.print(calibrationTime);
  Serial.println(" seconds...");

  // Warm-up period: the sensor needs time to stabilize
  for (int i = 0; i < calibrationTime; i++)
  {
    delay(1000);
    Serial.print(".");
  }

  Serial.println();
  Serial.println("Sensor ready - move to trigger");
  Serial.println();
}

void loop()
{
  int sensorValue = digitalRead(pirPin);

  if (sensorValue == HIGH)
  {
    Serial.println("Motion detected!");
    delay(100); // Debounce: avoid repeated triggers from same event
  }
  else
  {
    // Optional: uncomment to see clear state
    // Serial.println("No motion");
  }

  delay(100);
}

Code breakdown

• digitalRead(pirPin) — reads the PIR output pin. It returns HIGH when motion is detected and LOW when the area is clear.
• The calibration loop waits 30 seconds after power-up. During this time the sensor stabilizes its internal reference level to the ambient infrared environment.
• The delay(100) after detection prevents rapid repeated triggers from the same motion event. For repeatable triggering, set the module jumper to H mode.
• The PIR output stays HIGH for the duration set by the time delay potentiometer (Tx), then returns LOW.

Adding an LED indicator

To visually indicate motion detection with an LED:

const int pirPin = 2;
const int ledPin = 13; // built-in LED

void setup()
{
  Serial.begin(9600);
  pinMode(pirPin, INPUT);
  pinMode(ledPin, OUTPUT);

  delay(30000); // 30-second calibration
  Serial.println("Ready");
}

void loop()
{
  if (digitalRead(pirPin) == HIGH)
  {
    digitalWrite(ledPin, HIGH);
    Serial.println("Motion detected");
    delay(100);
  }
  else
  {
    digitalWrite(ledPin, LOW);
  }
}

Steps to perform this interfacing

1. Connect the PIR sensor to the Arduino as shown in the schematic.
2. Set the HC-SR501 jumper to H (repeatable) or L (single) depending on your application.
3. Adjust the sensitivity and time delay potentiometers to your desired settings.
4. Copy the code into the Arduino IDE.
5. Select the correct board and port (Tools > Board and Tools > Port).
6. Upload the sketch to the Arduino.
7. Open the Serial Monitor (Tools > Serial Monitor, set baud rate to 9600).
8. Wait 30 seconds for calibration — do not move during this period.
9. Move in front of the sensor and observe Motion detected! messages.

Caution

• The PIR sensor requires a calibration (stabilization) period of 30-60 seconds after power-up. During this time, avoid moving in its field of view for best results.
• The sensor detects changes in infrared radiation, not static heat. A stationary person or object will not trigger it after the initial detection.
• The detection range is affected by ambient temperature. In very hot weather (close to body temperature, ~37 C), sensitivity decreases because the thermal contrast is smaller.
• The HC-SR501 has a 110-120 degree detection angle and a range of up to 7 meters. The Fresnel lens on the module shapes this pattern — do not remove or modify it.
• Do not mount the sensor near windows, air conditioning vents, heaters, or direct sunlight. Rapid ambient temperature changes can cause false triggers.
• The sensor works best when the moving object crosses the detection zones (perpendicular to the sensor), not when moving directly toward it.
• The output is active HIGH — most modules do not have an inverted output option. If you need active LOW, use a transistor or logic inverter.