PIR motion sensor in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when the PIR sensor detects motion?

Consider this Arduino code snippet using a PIR motion sensor connected to pin 7 and an LED on pin 13. What will be printed to the Serial Monitor when motion is detected?

Arduino

int pirPin = 7;
int ledPin = 13;

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

void loop() {
  int motion = digitalRead(pirPin);
  if (motion == HIGH) {
    digitalWrite(ledPin, HIGH);
    Serial.println("Motion detected!");
  } else {
    digitalWrite(ledPin, LOW);
    Serial.println("No motion.");
  }
  delay(1000);
}

A Motion detected! (printed repeatedly every second while motion is present)

B No motion. (printed repeatedly every second regardless of motion)

C Motion detected! printed once, then no output

D No output at all

💡 Hint

Think about what happens inside the loop when the sensor reads HIGH.

When the PIR sensor detects motion, digitalRead returns HIGH. The code turns on the LED and prints "Motion detected!" every second while motion continues.

❓ Predict Output

intermediate

What happens if the PIR sensor pin is set as OUTPUT?

Given this Arduino code snippet, what will be the behavior of the PIR sensor reading?

Arduino

int pirPin = 7;

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

void loop() {
  int motion = digitalRead(pirPin);
  Serial.println(motion);
  delay(1000);
}

A The sensor reading will always be LOW (0)

B The sensor reading will always be HIGH (1)

C The sensor reading will be unpredictable and may cause errors

D The code will not compile due to pinMode error

💡 Hint

Consider what happens when you set a sensor input pin as OUTPUT.

Setting the PIR sensor pin as OUTPUT means the Arduino controls the pin voltage. digitalRead will read the output state, which defaults LOW unless set HIGH. Since the code never sets it HIGH, it reads LOW always.

🔧 Debug

advanced

Why does the LED stay on even without motion?

This Arduino code is supposed to turn on an LED when the PIR sensor detects motion, but the LED stays on even without motion. What is the cause?

Arduino

int pirPin = 7;
int ledPin = 13;

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

void loop() {
  int motion = digitalRead(pirPin);
  if (motion == HIGH) {
    digitalWrite(ledPin, HIGH);
    Serial.println("Motion detected!");
  } else {
    digitalWrite(ledPin, LOW);
    Serial.println("No motion.");
  }
  delay(1000);
}

A The LED pin is not set as INPUT

B The condition uses assignment (=) instead of comparison (==), so it always evaluates true

C The PIR sensor pin is not set as OUTPUT

D The Serial.begin baud rate is incorrect

💡 Hint

Check the if statement condition carefully.

The if condition uses a single equals sign (=), which assigns HIGH to motion instead of comparing. This makes the condition always true, so the LED stays on regardless of sensor input.

📝 Syntax

advanced

Which option fixes the syntax error in this PIR sensor code?

This code snippet has a syntax error. Which option fixes it?

Arduino

int pirPin = 7;
int ledPin = 13;

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

void loop() {
  int motion = digitalRead(pirPin);
  if (motion == HIGH) {
    digitalWrite(ledPin, HIGH);
    Serial.println("Motion detected!");
  } else {
    digitalWrite(ledPin, LOW);
    Serial.println("No motion.");
  }
  delay(1000);
}

A Remove the Serial.begin(9600) line

B Change pinMode(pirPin, INPUT) to pinMode(pirPin, OUTPUT)

C Add a semicolon after pinMode(pirPin, INPUT)

D Change delay(1000) to delay(1,000)

💡 Hint

Look for missing punctuation in the setup function.

The line 'pinMode(pirPin, INPUT)' is missing a semicolon at the end, causing a syntax error. Adding the semicolon fixes it.

🚀 Application

expert

How to debounce PIR sensor signals in Arduino code?

PIR sensors sometimes give noisy signals causing flickering LED output. Which code snippet best implements a simple debounce to stabilize the LED output?

A

Use an interrupt on pirPin to toggle LED state:

volatile bool ledState = false;

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

void loop() {}

void toggleLED() {
  ledState = !ledState;
  digitalWrite(ledPin, ledState);
  Serial.println(ledState ? "Motion detected!" : "No motion.");
}

B

Add a delay(5000) inside loop to slow down sensor reading:

void loop() {
  int motion = digitalRead(pirPin);
  if (motion == HIGH) {
    digitalWrite(ledPin, HIGH);
    Serial.println("Motion detected!");
  } else {
    digitalWrite(ledPin, LOW);
    Serial.println("No motion.");
  }
  delay(5000);
}

C

Use analogRead on pirPin and check if value > 512:

void loop() {
  int motion = analogRead(pirPin);
  if (motion > 512) {
    digitalWrite(ledPin, HIGH);
    Serial.println("Motion detected!");
  } else {
    digitalWrite(ledPin, LOW);
    Serial.println("No motion.");
  }
}

D

Use a delay after detecting motion before reading sensor again:

int pirPin = 7;
int ledPin = 13;
unsigned long lastMotionTime = 0;
const unsigned long debounceDelay = 2000;

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

void loop() {
  int motion = digitalRead(pirPin);
  unsigned long currentTime = millis();
  if (motion == HIGH && (currentTime - lastMotionTime) > debounceDelay) {
    digitalWrite(ledPin, HIGH);
    Serial.println("Motion detected!");
    lastMotionTime = currentTime;
  } else if ((currentTime - lastMotionTime) > debounceDelay) {
    digitalWrite(ledPin, LOW);
    Serial.println("No motion.");
  }
}

💡 Hint

Debouncing means ignoring rapid repeated signals for a short time.

Option D uses a timer to ignore repeated motion signals within 2 seconds, stabilizing the LED output and avoiding flicker caused by noisy sensor signals.