Button reading with pull-up resistor in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a pull-up resistor when reading a button in Arduino?

A pull-up resistor ensures the input pin reads a HIGH value when the button is not pressed, preventing it from floating and giving unreliable readings.

beginner

How does the Arduino internal pull-up resistor help when connecting a button?

Arduino has built-in pull-up resistors that can be enabled in code, so you don't need an external resistor. This simplifies wiring and ensures stable HIGH readings when the button is open.

beginner

What value does the Arduino digitalRead() function return when a button connected with a pull-up resistor is pressed?

It returns LOW (0) because pressing the button connects the pin to ground, overriding the pull-up resistor's HIGH state.

beginner

Why should you avoid leaving an input pin 'floating' when reading a button?

A floating pin can pick up electrical noise and randomly switch between HIGH and LOW, causing unreliable button readings.

beginner

Show the Arduino code snippet to enable the internal pull-up resistor on pin 2 and read the button state.

pinMode(2, INPUT_PULLUP); int buttonState = digitalRead(2);

What does enabling INPUT_PULLUP on a pin do?

A Connects the pin internally to ground

B Disables the pin

C Connects the pin internally to 5V through a resistor

D Makes the pin an output

INPUT_PULLUP connects the pin internally to 5V through a resistor, acting as a pull-up resistor.

When a button connected with a pull-up resistor is pressed, what value does digitalRead() return?

A LOW

B It depends on the button

C HIGH

D Analog value

Pressing the button connects the pin to ground, so digitalRead() returns LOW.

Why is it bad to leave a button input pin floating?

A It causes the Arduino to reset

B It can cause random HIGH or LOW readings

C It drains the battery quickly

D It makes the button press slower

Floating pins pick up noise and cause unstable readings.

Which Arduino function sets a pin as input with an internal pull-up resistor?

A digitalRead(pin)

B pinMode(pin, OUTPUT_PULLUP)

C digitalWrite(pin, HIGH)

D pinMode(pin, INPUT_PULLUP)

pinMode(pin, INPUT_PULLUP) enables the internal pull-up resistor.

If you want to use an external pull-up resistor, where should it be connected?

A Between the pin and 5V

B Between the button and 5V

C Between the button and ground

D Between the pin and ground

A pull-up resistor connects the input pin to 5V to keep it HIGH when the button is not pressed.

Explain how a pull-up resistor works when reading a button press on an Arduino.

Think about what happens to the pin voltage when the button is pressed or released.

Describe how to use Arduino's internal pull-up resistor to read a button and why it is useful.

Consider how internal pull-ups replace external resistors.

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