Rising, falling, and change triggers in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Detecting a Rising Edge Trigger

What will be the output on the serial monitor when the button connected to pin 2 is pressed (assuming it was previously not pressed)?

Arduino

const int buttonPin = 2;
int buttonState = 0;
int lastButtonState = 0;

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

void loop() {
  buttonState = digitalRead(buttonPin);
  if (buttonState == HIGH && lastButtonState == LOW) {
    Serial.println("Rising edge detected");
  }
  lastButtonState = buttonState;
  delay(50);
}

A Change detected

B Falling edge detected

C Rising edge detected

D No output

💡 Hint

Think about when the button state changes from LOW to HIGH.

The code prints "Rising edge detected" only when the button state changes from LOW to HIGH, which is a rising edge trigger.

❓ Predict Output

intermediate

Detecting a Falling Edge Trigger

What will be printed on the serial monitor when the button connected to pin 3 is pressed (assuming it was previously not pressed)?

Arduino

const int buttonPin = 3;
int buttonState = 0;
int lastButtonState = HIGH;

void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {
  buttonState = digitalRead(buttonPin);
  if (buttonState == LOW && lastButtonState == HIGH) {
    Serial.println("Falling edge detected");
  }
  lastButtonState = buttonState;
  delay(50);
}

A No output

B Rising edge detected

C Change detected

D Falling edge detected

💡 Hint

Focus on when the button state changes from HIGH to LOW.

The code prints "Falling edge detected" only when the button state changes from HIGH to LOW, which is a falling edge trigger.

❓ Predict Output

advanced

Detecting Any Change Trigger

What will be printed on the serial monitor when the button connected to pin 4 changes state (pressed or released)?

Arduino

const int buttonPin = 4;
int buttonState = 0;
int lastButtonState = 0;

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

void loop() {
  buttonState = digitalRead(buttonPin);
  if (buttonState != lastButtonState) {
    Serial.println("Change detected");
  }
  lastButtonState = buttonState;
  delay(50);
}

A Change detected

B No output

C Falling edge detected

D Rising edge detected

💡 Hint

Think about any difference between current and last button state.

The code prints "Change detected" whenever the button state changes, either from LOW to HIGH or HIGH to LOW.

🧠 Conceptual

advanced

Understanding Interrupts with Rising and Falling Edges

Which statement correctly describes how to use interrupts to detect a falling edge on pin 2 in Arduino?

A attachInterrupt(digitalPinToInterrupt(2), ISR, RISING);

B attachInterrupt(digitalPinToInterrupt(2), ISR, FALLING);

C attachInterrupt(2, ISR, CHANGE);

D attachInterrupt(digitalPinToInterrupt(2), ISR, HIGH);

💡 Hint

Look for the correct interrupt mode for falling edge detection.

To detect a falling edge, the interrupt mode must be FALLING. The function digitalPinToInterrupt converts the pin number to the interrupt number.

❓ Predict Output

expert

Output of Combined Edge Detection with Interrupts

Given the code below, what will be the output on the serial monitor if the button connected to pin 3 is pressed and then released?

Arduino

volatile int count = 0;

void ISR() {
  count++;
}

void setup() {
  pinMode(3, INPUT_PULLUP);
  Serial.begin(9600);
  attachInterrupt(digitalPinToInterrupt(3), ISR, CHANGE);
}

void loop() {
  static int lastCount = 0;
  if (count != lastCount) {
    Serial.print("Interrupt count: ");
    Serial.println(count);
    lastCount = count;
  }
}

A Interrupt count: 2

B Interrupt count: 1

C Interrupt count: 0

D No output

💡 Hint

Each press and release triggers one interrupt with CHANGE mode.

The CHANGE mode triggers an interrupt on both rising and falling edges. Pressing and releasing the button causes two interrupts, so count increments twice.

Practice

1. Which interrupt trigger type activates when a signal changes from LOW to HIGH on an Arduino pin?

easy

A. RISING

B. FALLING

C. CHANGE

D. LOW

Solution

1. Step 1: Understand the meaning of RISING trigger

   The RISING trigger activates when the signal goes from LOW (0) to HIGH (1).

2. Step 2: Compare with other triggers

   FALLING triggers on HIGH to LOW, CHANGE triggers on any change, LOW triggers when signal is LOW.

3. Final Answer:

   RISING -> Option A

4. Quick Check:

   LOW to HIGH = RISING [OK]

Hint: RISING means signal goes up from LOW to HIGH [OK]

Common Mistakes:

• Confusing RISING with FALLING
• Thinking CHANGE triggers only on RISING
• Mixing LOW trigger with RISING

2. Which of the following is the correct syntax to attach an interrupt on pin 2 for a FALLING trigger in Arduino?

easy

A. attachInterrupt(2, ISR, RISING);

B. attachInterrupt(digitalPinToInterrupt(2), ISR, FALLING);

C. attachInterrupt(2, ISR, CHANGE);

D. attachInterrupt(digitalPinToInterrupt(2), ISR, LOW);

Solution

1. Step 1: Identify correct function usage

   attachInterrupt requires pin number converted by digitalPinToInterrupt for portability.

2. Step 2: Match trigger type FALLING

   attachInterrupt(digitalPinToInterrupt(2), ISR, FALLING); uses digitalPinToInterrupt(2) and FALLING correctly.

3. Final Answer:

   attachInterrupt(digitalPinToInterrupt(2), ISR, FALLING); -> Option B

4. Quick Check:

   Use digitalPinToInterrupt with FALLING [OK]

Hint: Use digitalPinToInterrupt(pin) with trigger type [OK]

Common Mistakes:

• Using pin number directly without digitalPinToInterrupt
• Mixing trigger types with wrong syntax
• Using LOW trigger incorrectly

3. What will be the output on the serial monitor if the following Arduino code is run and the input pin changes from LOW to HIGH, then HIGH to LOW?

volatile int count = 0;
void ISR() { count++; }
void setup() {
  Serial.begin(9600);
  pinMode(2, INPUT);
  attachInterrupt(digitalPinToInterrupt(2), ISR, CHANGE);
}
void loop() {
  Serial.println(count);
  delay(1000);
}

medium

A. 0 0 0 ... (no change)

B. 1 1 1 ... (count increments once)

C. 2 2 2 ... (count increments twice)

D. Error: ISR cannot increment variable

Solution

1. Step 1: Understand CHANGE trigger behavior

   CHANGE triggers on any signal change: LOW to HIGH and HIGH to LOW, so ISR runs twice.

2. Step 2: Count increments in ISR

   Each ISR call increments count by 1, so after two changes count = 2.

3. Final Answer:

   2 2 2 ... (count increments twice) -> Option C

4. Quick Check:

   CHANGE triggers twice, count = 2 [OK]

Hint: CHANGE triggers on both edges, count increments twice [OK]

Common Mistakes:

• Assuming CHANGE triggers only once
• Forgetting volatile keyword importance
• Thinking ISR cannot modify variables

4. Identify the error in this Arduino code snippet that tries to attach an interrupt on pin 3 for a RISING trigger:

void ISR() {
  // interrupt code
}
void setup() {
  attachInterrupt(3, ISR, RISING);
}

medium

A. ISR function must have parameters

B. attachInterrupt must be called in loop(), not setup()

C. RISING is not a valid trigger type

D. Pin number 3 should be converted using digitalPinToInterrupt()

Solution

1. Step 1: Check attachInterrupt pin argument

   Pin number must be converted with digitalPinToInterrupt(pin) for correct interrupt mapping.

2. Step 2: Validate other parts

   ISR has correct signature (no parameters), RISING is valid, attachInterrupt is correctly called in setup.

3. Final Answer:

   Pin number 3 should be converted using digitalPinToInterrupt() -> Option D

4. Quick Check:

   Use digitalPinToInterrupt for pin numbers [OK]

Hint: Always convert pin with digitalPinToInterrupt() [OK]

Common Mistakes:

• Passing raw pin number directly
• Thinking ISR needs parameters
• Calling attachInterrupt in loop()

5. You want to count how many times a button connected to pin 4 changes state (pressed or released). Which interrupt trigger should you use and why?

hard

A. CHANGE, because it counts both presses and releases

B. FALLING, because it counts only releases

C. RISING, because it counts only presses

D. LOW, because it triggers when button is held down

Solution

1. Step 1: Understand button state changes

   A button press changes signal from HIGH to LOW or LOW to HIGH depending on wiring.

2. Step 2: Choose trigger to count all changes

   CHANGE triggers on any signal change, so it counts both presses and releases.

3. Final Answer:

   CHANGE, because it counts both presses and releases -> Option A

4. Quick Check:

   Count all changes = use CHANGE trigger [OK]

Hint: Use CHANGE to count all button state changes [OK]

Common Mistakes:

• Using RISING or FALLING and missing half the events
• Confusing LOW trigger with change detection
• Not considering both press and release