Button reading with pull-up resistor in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when the button is pressed?

Consider this Arduino code that reads a button connected with a pull-up resistor. What will be printed to the Serial Monitor when the button is pressed?

Arduino

const int buttonPin = 2;
void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {
  int buttonState = digitalRead(buttonPin);
  if (buttonState == LOW) {
    Serial.println("Button pressed");
  } else {
    Serial.println("Button not pressed");
  }
  delay(500);
}

A Button pressed

B Button not pressed

C No output

D Error: pinMode not set

💡 Hint

Remember that with a pull-up resistor, the button pin reads LOW when pressed.

Using INPUT_PULLUP sets the pin HIGH by default. When the button is pressed, it connects the pin to ground, making it LOW. So the code prints "Button pressed".

🧠 Conceptual

intermediate

Why use INPUT_PULLUP for button reading?

Why is it recommended to use INPUT_PULLUP mode when reading a button in Arduino?

A It automatically debounces the button

B It makes the button output HIGH when pressed

C It disables the internal resistor to save power

D It provides an internal pull-up resistor to avoid floating input

💡 Hint

Think about what happens if the input pin is not connected to anything.

Using INPUT_PULLUP activates the internal pull-up resistor, preventing the input pin from floating and reading random values.

🔧 Debug

advanced

Why does the button always read as pressed?

This Arduino code always prints "Button pressed" even when the button is not pressed. What is the cause?

Arduino

const int buttonPin = 2;
void setup() {
  pinMode(buttonPin, INPUT);
  Serial.begin(9600);
}

void loop() {
  int buttonState = digitalRead(buttonPin);
  if (buttonState == LOW) {
    Serial.println("Button pressed");
  } else {
    Serial.println("Button not pressed");
  }
  delay(500);
}

A The buttonPin number is wrong

B The pinMode should be INPUT_PULLUP to avoid floating input

C Serial.begin is missing baud rate

D digitalRead should be replaced with analogRead

💡 Hint

Think about what happens to an input pin without a pull-up or pull-down resistor.

Without INPUT_PULLUP, the input pin floats and may randomly read LOW, causing the code to think the button is pressed.

📝 Syntax

advanced

Identify the syntax error in this button reading code

Which option contains the correct syntax to set the button pin as input with pull-up resistor?

Arduino

const int buttonPin = 2;
void setup() {
  // Set button pin mode here
}
void loop() {}

A pinMode(buttonPin, INPUT_PULLUP);

B pinMode(buttonPin, PULLUP_INPUT);

C pinMode(buttonPin INPUT_PULLUP);

D pinMode(buttonPin, INPUT-PULLUP);

💡 Hint

Check the syntax of the pinMode function and the constant name.

The correct syntax is pinMode(pin, INPUT_PULLUP); with a comma and the constant INPUT_PULLUP.

🚀 Application

expert

How many times will "Button pressed" print in 5 seconds?

Given this Arduino code, how many times will "Button pressed" be printed to the Serial Monitor in 5 seconds if the button is held down continuously?

Arduino

const int buttonPin = 2;
void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {
  int buttonState = digitalRead(buttonPin);
  if (buttonState == LOW) {
    Serial.println("Button pressed");
  }
  delay(500);
}

A Cannot determine

B 5 times

C 10 times

D 20 times

💡 Hint

Calculate how many 500 ms delays fit into 5 seconds.

The loop prints "Button pressed" once every 500 ms. In 5 seconds, 5 / 0.5 = 10 times.

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