PIR motion sensor in Arduino - Step-by-Step Execution

Concept Flow - PIR motion sensor

Start

↓

Initialize PIR sensor pin

↓

Read PIR sensor value

↓

Is motion detected?

No→Wait and read again

Yes↓

Turn ON LED or output

↓

Wait some time

↓

Turn OFF LED or output

↓

Loop back to read sensor

The program initializes the PIR sensor pin, reads its value repeatedly, and if motion is detected, it turns on an LED for a short time before turning it off and continuing to check.

Execution Sample

Arduino

int pirPin = 2;
int ledPin = 13;

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

void loop() {
  int val = digitalRead(pirPin);
  if (val == HIGH) {
    digitalWrite(ledPin, HIGH);
    delay(1000);
    digitalWrite(ledPin, LOW);
  }
}

This code reads the PIR sensor on pin 2 and turns on the LED on pin 13 for 1 second when motion is detected.

Execution Table

Step	Action	pirPin Value	Condition (val == HIGH)	LED State	Delay
1	Read PIR sensor	LOW	False	OFF	0
2	Check condition	LOW	False	OFF	0
3	No motion detected, loop again	LOW	False	OFF	0
4	Read PIR sensor	HIGH	True	OFF	0
5	Turn LED ON	HIGH	True	ON	0
6	Wait 1000 ms	HIGH	True	ON	1000
7	Turn LED OFF	HIGH	True	OFF	0
8	Loop back to read sensor	UNKNOWN	Unknown	OFF	0

💡 The loop runs forever, reading sensor and turning LED on/off when motion is detected.

Variable Tracker

Variable	Start	After Step 1	After Step 4	After Step 5	After Step 7	Final
val	undefined	LOW	HIGH	HIGH	HIGH	undefined
LED State	OFF	OFF	OFF	ON	OFF	OFF

Key Moments - 3 Insights

Why does the LED turn on only when val == HIGH?

Because the PIR sensor outputs HIGH when motion is detected, as shown in execution_table rows 4 and 5 where val is HIGH and LED turns ON.

What happens if the PIR sensor reads LOW?

The condition val == HIGH is false (rows 1-3), so the LED stays OFF and the program loops to read the sensor again.

Why is there a delay after turning the LED ON?

The delay keeps the LED ON for 1 second so the motion detection is visible, as shown in step 6 before turning LED OFF in step 7.

Visual Quiz - 3 Questions

Look at the execution_table, what is the LED state at step 5?

A Blinking

B OFF

C ON

D Undefined

💡 Hint

Check the LED State column at step 5 in the execution_table.

At which step does the PIR sensor detect motion (val == HIGH)?

A Step 4

B Step 7

C Step 1

D Step 3

💡 Hint

Look at the pirPin Value and Condition columns in the execution_table.

If the delay(1000) is removed, what changes in the execution_table?

A LED stays ON indefinitely

B LED turns ON and OFF immediately without visible delay

C PIR sensor stops detecting motion

D No change at all

💡 Hint

Consider the Delay column and how it affects LED State timing.

Concept Snapshot

PIR motion sensor code:
- Initialize sensor pin as INPUT
- Read sensor value in loop
- If value is HIGH (motion detected), turn LED ON
- Wait some time (delay)
- Turn LED OFF
- Repeat forever

Full Transcript

This program uses a PIR motion sensor connected to pin 2 and an LED on pin 13. It reads the sensor value repeatedly. When the sensor detects motion, it outputs HIGH. The program checks if the sensor value is HIGH. If yes, it turns the LED ON, waits 1 second, then turns the LED OFF. If no motion is detected (sensor LOW), the LED stays OFF. This loop runs forever, continuously checking for motion and lighting the LED when motion is detected.