PIR motion sensor in Arduino

Introduction

A PIR motion sensor detects movement by sensing changes in infrared light. It helps your Arduino know when someone or something moves nearby.

To turn on a light automatically when someone enters a room.

To trigger an alarm if motion is detected in a restricted area.

To count people passing through a doorway.

To activate a camera when movement is sensed.

To save power by only running devices when motion is present.

Syntax

Arduino

int sensorPin = 2;  // PIR sensor output pin
int ledPin = 13;     // Built-in LED pin

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

void loop() {
  int sensorValue = digitalRead(sensorPin);
  if (sensorValue == HIGH) {
    digitalWrite(ledPin, HIGH);  // Turn LED on
    Serial.println("Motion detected!");
  } else {
    digitalWrite(ledPin, LOW);   // Turn LED off
    Serial.println("No motion.");
  }
  delay(500);
}

The PIR sensor output pin connects to a digital input pin on Arduino.

Use digitalRead() to check if motion is detected (HIGH) or not (LOW).

Examples

This example reads the PIR sensor on pin 7 and prints "Motion!" when movement is detected.

Arduino

int pirPin = 7;

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

void loop() {
  if (digitalRead(pirPin) == HIGH) {
    Serial.println("Motion!");
  }
  delay(1000);
}

This example uses the PIR sensor to turn a buzzer on when motion is detected.

Arduino

int pirPin = 3;
int buzzerPin = 9;

void setup() {
  pinMode(pirPin, INPUT);
  pinMode(buzzerPin, OUTPUT);
}

void loop() {
  if (digitalRead(pirPin) == HIGH) {
    digitalWrite(buzzerPin, HIGH);  // Sound buzzer
  } else {
    digitalWrite(buzzerPin, LOW);   // Stop buzzer
  }
}

Sample Program

This program reads the PIR sensor connected to pin 2. When motion is detected, it lights up the built-in LED and prints "Motion detected!" to the serial monitor. When no motion is detected, the LED turns off and it prints "No motion." The delay slows down the readings to twice per second.

Arduino

int pirPin = 2;       // PIR sensor output pin
int ledPin = 13;      // Built-in LED pin

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

void loop() {
  int motion = digitalRead(pirPin);
  if (motion == HIGH) {
    digitalWrite(ledPin, HIGH);  // Turn LED on
    Serial.println("Motion detected!");
  } else {
    digitalWrite(ledPin, LOW);   // Turn LED off
    Serial.println("No motion.");
  }
  delay(500);
}

Important Notes

Make sure the PIR sensor has time to calibrate after powering on (usually about 30 seconds).

Use the serial monitor in Arduino IDE to see printed messages.

The built-in LED on pin 13 is useful for quick testing without extra parts.

Summary

A PIR motion sensor detects movement by sensing infrared changes.

Connect the sensor output to a digital input pin and use digitalRead() to check for motion.

Use the sensor to trigger actions like turning on lights or sounding alarms.

Practice

1. What does a PIR motion sensor detect in an Arduino project?

easy

A. Light intensity changes

B. Sound levels in the environment

C. Movement by sensing infrared changes

D. Temperature variations

Solution

1. Step 1: Understand PIR sensor function

   A PIR sensor detects motion by sensing changes in infrared radiation emitted by objects.

2. Step 2: Compare with other options

   Sound, temperature, and light sensors detect different things, not motion via infrared.

3. Final Answer:

   Movement by sensing infrared changes -> Option C

4. Quick Check:

   PIR sensor = Motion detection [OK]

Hint: PIR sensors detect heat movement, not sound or light [OK]

Common Mistakes:

• Confusing PIR with sound or light sensors
• Thinking PIR detects temperature directly

2. Which Arduino code line correctly reads the PIR sensor connected to digital pin 7?

easy

A. int motion = digitalWrite(7);

B. int motion = digitalRead(7);

C. int motion = analogRead(7);

D. int motion = pinMode(7, INPUT);

Solution

1. Step 1: Identify correct function to read digital input

   digitalRead(pin) reads the digital value from a pin, suitable for PIR sensor output.

2. Step 2: Eliminate incorrect functions

   analogRead reads analog pins, digitalWrite sets output, pinMode sets pin mode but does not read value.

3. Final Answer:

   int motion = digitalRead(7); -> Option B

4. Quick Check:

   Use digitalRead for digital sensor input [OK]

Hint: Use digitalRead to get sensor state from digital pins [OK]

Common Mistakes:

• Using analogRead on digital pin
• Using digitalWrite instead of digitalRead
• Confusing pinMode with reading value

3. What will be the output on the Serial Monitor when the following Arduino code runs and the PIR sensor detects motion?

const int pirPin = 8;
void setup() {
  pinMode(pirPin, INPUT);
  Serial.begin(9600);
}
void loop() {
  int motion = digitalRead(pirPin);
  if (motion == HIGH) {
    Serial.println("Motion detected!");
  } else {
    Serial.println("No motion.");
  }
  delay(1000);
}

medium

A. Motion detected! printed every second when motion is present

B. No output because Serial.begin is missing

C. Motion detected! printed once and then stops

D. Syntax error due to missing semicolon

Solution

1. Step 1: Analyze the loop behavior

   The loop reads the PIR sensor pin every second and prints "Motion detected!" if motion is HIGH, else "No motion.".

2. Step 2: Check Serial setup and output

   Serial.begin(9600) is correctly called in setup, so output will appear on Serial Monitor continuously.

3. Final Answer:

   Motion detected! printed every second when motion is present -> Option A

4. Quick Check:

   Serial prints motion status each second [OK]

Hint: Loop prints sensor state every second with delay(1000) [OK]

Common Mistakes:

• Thinking Serial.begin is missing
• Assuming output prints only once
• Confusing syntax errors

4. Identify the error in this Arduino code snippet using a PIR sensor on pin 9:

void setup() {
  pinMode(9, OUTPUT);
  Serial.begin(9600);
}
void loop() {
  int motion = digitalRead(9);
  if (motion == HIGH) {
    Serial.println("Motion detected");
  }
  delay(500);
}

medium

A. Pin 9 should be set as INPUT, not OUTPUT

B. Serial.begin should be in loop()

C. digitalRead cannot be used on pin 9

D. Missing semicolon after Serial.println

Solution

1. Step 1: Check pinMode for sensor input

   PIR sensor output is digital input, so pin 9 must be set as INPUT, not OUTPUT.

2. Step 2: Verify other code parts

   Serial.begin is correctly in setup, digitalRead works on pin 9, and semicolon is present.

3. Final Answer:

   Pin 9 should be set as INPUT, not OUTPUT -> Option A

4. Quick Check:

   Input pins must use pinMode(INPUT) [OK]

Hint: Set sensor pins as INPUT, not OUTPUT [OK]

Common Mistakes:

• Setting sensor pin as OUTPUT
• Moving Serial.begin to loop
• Ignoring pinMode settings

5. You want to turn on an LED connected to pin 13 only when the PIR sensor on pin 2 detects motion. Which Arduino code snippet correctly implements this behavior?

hard

A. pinMode(2, INPUT);
pinMode(13, OUTPUT);
if (digitalRead(2) == LOW) {
digitalWrite(13, HIGH);
}

B. pinMode(2, OUTPUT);
pinMode(13, INPUT);
if (digitalRead(13) == HIGH) {
digitalWrite(2, HIGH);
}

C. pinMode(2, INPUT);
pinMode(13, OUTPUT);
if (digitalRead(2) == HIGH) {
digitalWrite(13, HIGH);
}

D. pinMode(2, INPUT);
pinMode(13, OUTPUT);
digitalWrite(13, digitalRead(2));

Solution

1. Step 1: Set correct pin modes

   Pin 2 is sensor input, so INPUT; pin 13 is LED output, so OUTPUT.

2. Step 2: Use digitalRead to control LED

   digitalWrite(13, digitalRead(2)) sets LED HIGH when sensor reads HIGH, LOW otherwise.

3. Step 3: Eliminate other options

   pinMode(2, OUTPUT); pinMode(13, INPUT); if (digitalRead(13) == HIGH) { digitalWrite(2, HIGH); } has wrong pin modes and logic. pinMode(2, INPUT); pinMode(13, OUTPUT); if (digitalRead(2) == HIGH) { digitalWrite(13, HIGH); } turns LED on for motion but lacks else clause to turn off when no motion. pinMode(2, INPUT); pinMode(13, OUTPUT); if (digitalRead(2) == LOW) { digitalWrite(13, HIGH); } reverses logic.

4. Final Answer:

   pinMode(2, INPUT); pinMode(13, OUTPUT); digitalWrite(13, digitalRead(2)); -> Option D

5. Quick Check:

   LED state = PIR sensor state [OK]

Hint: Write LED pin to sensor pin state directly for simplicity [OK]

Common Mistakes:

• Setting sensor pin as OUTPUT
• Reversing HIGH/LOW logic
• Confusing input/output pin modes
• Omitting else clause to turn off LED