Rising, falling, and change triggers in Arduino - Mini Project: Build & Apply

Rising, falling, and change triggers

📖 Scenario: You are building a simple Arduino project to detect button presses. The button can be pressed down or released, and you want to know when the button is pressed (rising edge), released (falling edge), or changed state (either pressed or released).

🎯 Goal: Create an Arduino sketch that reads a button input and prints messages when the button state rises (goes from LOW to HIGH), falls (goes from HIGH to LOW), or changes state.

📋 What You'll Learn

Use a variable called buttonPin set to pin 2

Use a variable called buttonState to read the current button state

Use a variable called lastButtonState to store the previous button state

Detect rising edge (LOW to HIGH) and print "Button pressed"

Detect falling edge (HIGH to LOW) and print "Button released"

Detect any change in button state and print "Button state changed"

💡 Why This Matters

🌍 Real World

Detecting button presses and releases is common in devices like remote controls, game controllers, and user interfaces.

💼 Career

Understanding how to detect input changes is important for embedded systems programming and hardware interface development.

1

Set up the button input pin

Create an integer variable called buttonPin and set it to 2. In the setup() function, set buttonPin as an input using pinMode(buttonPin, INPUT). Also, start serial communication at 9600 baud with Serial.begin(9600).

Arduino

int buttonPin = 2;

void setup() {
  // Set buttonPin as input
  // Start serial communication
  // Your code here
}

void loop() {
  // Your code here
}

Hint

Use pinMode to set the pin mode and Serial.begin(9600) to start serial communication.

2

Create variables to track button state

Create two integer variables called buttonState and lastButtonState. Initialize lastButtonState to LOW before setup(). In the loop() function, read the current button state from buttonPin using digitalRead(buttonPin) and store it in buttonState.

Arduino

int buttonPin = 2;
int buttonState;
int lastButtonState = LOW;

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

void loop() {
  // Read the button state
  // Your code here
}

Hint

Use digitalRead(buttonPin) to get the current button state.

3

Detect rising, falling, and any change in button state

In the loop() function, use if statements to detect when the button state changes compared to lastButtonState. If buttonState is HIGH and lastButtonState is LOW, print "Button pressed". If buttonState is LOW and lastButtonState is HIGH, print "Button released". If buttonState is different from lastButtonState, print "Button state changed". Finally, update lastButtonState to the current buttonState.

Arduino

int buttonPin = 2;
int buttonState;
int lastButtonState = LOW;

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

void loop() {
  buttonState = digitalRead(buttonPin);
  // Detect rising edge
  // Detect falling edge
  // Detect any change
  // Update lastButtonState
  // Your code here
}

Hint

Compare buttonState and lastButtonState to detect changes and print messages accordingly.

4

Print the button state changes

Add Serial.println statements inside the if blocks to print the exact messages: "Button pressed" for rising edge, "Button released" for falling edge, and "Button state changed" for any change. Run the program and observe the serial monitor output when pressing and releasing the button.

Arduino

int buttonPin = 2;
int buttonState;
int lastButtonState = LOW;

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

void loop() {
  buttonState = digitalRead(buttonPin);

  if (buttonState != lastButtonState) {
    // Print "Button state changed"
    // Print "Button pressed" if rising edge
    // Print "Button released" if falling edge
    // Update lastButtonState
    // Your code here
  }
}

Hint

Press and release the button to see the messages in the serial monitor.

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