Button reading with pull-up resistor in Arduino - Mini Project: Build & Apply

Button reading with pull-up resistor

📖 Scenario: You want to control a simple LED using a push button on your Arduino board. The button is connected with a pull-up resistor to ensure a stable reading.

🎯 Goal: Build a program that reads the button state using the internal pull-up resistor and turns an LED on when the button is pressed.

📋 What You'll Learn

Use pin 2 for the button input

Use pin 13 for the LED output

Enable the internal pull-up resistor for the button pin

Read the button state correctly considering the pull-up resistor

Turn the LED on when the button is pressed and off when released

💡 Why This Matters

🌍 Real World

Buttons with pull-up resistors are common in electronics to ensure stable input readings without extra hardware resistors.

💼 Career

Understanding button input reading and pull-up resistors is essential for embedded systems and hardware programming jobs.

1

Set up button and LED pins

Create two variables: buttonPin set to 2 and ledPin set to 13.

Arduino

# Create variables buttonPin and ledPin with values 2 and 13
# Your code here

Hint

Use const int to declare pin numbers.

2

Configure pin modes in setup

Write the setup() function. Inside it, set buttonPin as input with internal pull-up resistor using pinMode(buttonPin, INPUT_PULLUP). Set ledPin as output using pinMode(ledPin, OUTPUT).

Arduino

const int buttonPin = 2;
const int ledPin = 13;

void setup() {
  // Set buttonPin as input with pull-up resistor
  // Set ledPin as output
  // Your code here
}

Hint

Use pinMode to set pin modes. Use INPUT_PULLUP for the button pin.

3

Read button state and control LED

Write the loop() function. Inside it, read the button state into an int variable called buttonState using digitalRead(buttonPin). If buttonState is LOW, turn the LED on with digitalWrite(ledPin, HIGH). Otherwise, turn the LED off with digitalWrite(ledPin, LOW).

Arduino

const int buttonPin = 2;
const int ledPin = 13;

void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
  pinMode(ledPin, OUTPUT);
}

void loop() {
  // Read button state into buttonState
  // If buttonState is LOW, turn LED on
  // Else, turn LED off
  // Your code here
}

Hint

Remember that with pull-up resistor, button pressed means reading LOW.

4

Test and observe LED behavior

Add a delay(10) at the end of loop() to stabilize readings. Then upload the program and observe the LED. When you press the button connected to pin 2, the LED on pin 13 should turn on. When you release the button, the LED should turn off. Write Serial.println(buttonState) inside loop() to print the button state to the serial monitor.

Arduino

const int buttonPin = 2;
const int ledPin = 13;

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

void loop() {
  int buttonState = digitalRead(buttonPin);
  if (buttonState == LOW) {
    digitalWrite(ledPin, HIGH);
  } else {
    digitalWrite(ledPin, LOW);
  }
  // Print buttonState to serial monitor
  // Add a small delay
  // Your code here
}

Hint

Use Serial.begin(9600) in setup() to start serial communication.

Practice

1. What does setting pinMode(pin, INPUT_PULLUP) do in Arduino when reading a button?

easy

A. Enables the internal pull-up resistor to keep the pin HIGH when button is not pressed

B. Disables the pin to save power

C. Sets the pin as an output to drive an LED

D. Connects the pin directly to ground

Solution

1. Step 1: Understand pinMode with INPUT_PULLUP

   Using INPUT_PULLUP activates the internal pull-up resistor on the pin, so it reads HIGH by default.

2. Step 2: Effect on button reading

   When the button is pressed, it connects the pin to ground, making the reading LOW. When not pressed, the pull-up resistor keeps it HIGH.

3. Final Answer:

   Enables the internal pull-up resistor to keep the pin HIGH when button is not pressed -> Option A

4. Quick Check:

   INPUT_PULLUP means pin reads HIGH unless grounded [OK]

Hint: INPUT_PULLUP means pin is HIGH until button grounds it [OK]

Common Mistakes:

• Thinking INPUT_PULLUP sets pin as output
• Assuming pin reads LOW when button is not pressed
• Confusing pull-up with pull-down resistor

2. Which of the following is the correct syntax to set pin 7 as input with internal pull-up resistor in Arduino?

easy

A. pinMode(7, INPUT_PULLDOWN);

B. pinMode(7, OUTPUT_PULLUP);

C. pinMode(7, INPUT_PULLUP);

D. pinMode(7, INPUT); digitalWrite(7, LOW);

Solution

1. Step 1: Recall correct pinMode usage

   The correct way to enable internal pull-up resistor is pinMode(pin, INPUT_PULLUP);.

2. Step 2: Check each option

   pinMode(7, OUTPUT_PULLUP); uses OUTPUT_PULLUP which does not exist. pinMode(7, INPUT_PULLDOWN); uses INPUT_PULLDOWN which Arduino does not support internally. pinMode(7, INPUT); digitalWrite(7, LOW); sets pin as INPUT but digitalWrite LOW disables pull-up (equivalent to plain INPUT, floating pin), does not enable internal pull-up resistor.

3. Final Answer:

   pinMode(7, INPUT_PULLUP); -> Option C

4. Quick Check:

   Use INPUT_PULLUP to enable pull-up resistor [OK]

Hint: Use INPUT_PULLUP exactly to enable pull-up resistor [OK]

Common Mistakes:

• Using OUTPUT_PULLUP which is invalid
• Trying INPUT_PULLDOWN which Arduino lacks
• digitalWrite(7, LOW) after INPUT (floating input, no pull-up)

3. What will be the output on the Serial Monitor when the button connected to pin 4 is pressed, given this code?

void setup() {
  pinMode(4, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {
  int state = digitalRead(4);
  Serial.println(state);
  delay(500);
}

medium

A. Prints 1 when button pressed, 0 when released

B. Always prints 1 regardless of button state

C. Always prints 0 regardless of button state

D. Prints 0 when button pressed, 1 when released

Solution

1. Step 1: Understand INPUT_PULLUP behavior

   With INPUT_PULLUP, the pin reads HIGH (1) when button is not pressed and LOW (0) when pressed because button connects pin to ground.

2. Step 2: Analyze Serial output

   The code prints the pin state every 500ms. When pressed, it prints 0; when released, it prints 1.

3. Final Answer:

   Prints 0 when button pressed, 1 when released -> Option D

4. Quick Check:

   Pressed = LOW (0), Released = HIGH (1) [OK]

Hint: Pressed button reads LOW (0) with pull-up resistor [OK]

Common Mistakes:

• Assuming pressed reads HIGH (1)
• Confusing pull-up with pull-down logic
• Ignoring that button grounds the pin when pressed

4. The following code is intended to read a button with internal pull-up resistor on pin 2, but it does not work correctly. What is the error?

void setup() {
  pinMode(2, INPUT);
  Serial.begin(9600);
}

void loop() {
  int val = digitalRead(2);
  Serial.println(val);
  delay(200);
}

medium

A. Missing INPUT_PULLUP mode to enable pull-up resistor

B. Serial.begin should be in loop, not setup

C. digitalRead cannot be used on pin 2

D. delay(200) is too short for button reading

Solution

1. Step 1: Check pinMode configuration

   The code uses pinMode(2, INPUT); which does not enable the internal pull-up resistor, so the pin may float and give unreliable readings.

2. Step 2: Correct usage for button with pull-up

   To use the internal pull-up resistor, the pinMode should be INPUT_PULLUP. This prevents floating and ensures stable readings.

3. Final Answer:

   Missing INPUT_PULLUP mode to enable pull-up resistor -> Option A

4. Quick Check:

   Use INPUT_PULLUP to avoid floating input [OK]

Hint: Always use INPUT_PULLUP for button input to avoid floating [OK]

Common Mistakes:

• Using INPUT without pull-up resistor
• Moving Serial.begin to loop unnecessarily
• Thinking delay affects button reading correctness

5. You want to detect a button press using pin 8 with internal pull-up resistor. Which code snippet correctly reads the button and turns on an LED on pin 13 only when the button is pressed?

hard

A. pinMode(8, INPUT_PULLUP);
pinMode(13, OUTPUT);
if(digitalRead(8) == HIGH) digitalWrite(13, HIGH); else digitalWrite(13, LOW);

B. pinMode(8, INPUT_PULLUP);
pinMode(13, OUTPUT);
if(digitalRead(8) == LOW) digitalWrite(13, HIGH); else digitalWrite(13, LOW);

C. pinMode(8, INPUT);
pinMode(13, OUTPUT);
if(digitalRead(8) == LOW) digitalWrite(13, HIGH); else digitalWrite(13, LOW);

D. pinMode(8, OUTPUT);
pinMode(13, INPUT_PULLUP);
if(digitalRead(8) == LOW) digitalWrite(13, HIGH); else digitalWrite(13, LOW);

Solution

1. Step 1: Set pin modes correctly

   Pin 8 must be input with internal pull-up resistor: INPUT_PULLUP. Pin 13 is output for LED.

2. Step 2: Understand button logic with pull-up

   Button press connects pin 8 to ground, so digitalRead(8) returns LOW when pressed.

3. Step 3: Write correct if condition

   Turn LED on when button is pressed (pin reads LOW), so condition is if(digitalRead(8) == LOW).

4. Final Answer:

   pinMode(8, INPUT_PULLUP); pinMode(13, OUTPUT); if(digitalRead(8) == LOW) digitalWrite(13, HIGH); else digitalWrite(13, LOW); -> Option B

5. Quick Check:

   Pressed = LOW, LED ON when LOW [OK]

Hint: Button pressed reads LOW; turn LED ON when reading is LOW [OK]

Common Mistakes:

• Checking for HIGH instead of LOW on button press
• Setting button pin as OUTPUT
• Not enabling INPUT_PULLUP resistor