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.

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