Button reading with pull-up resistor in Arduino - Step-by-Step Execution

Concept Flow - Button reading with pull-up resistor

Setup pinMode INPUT_PULLUP

↓

Read digital pin state

↓

Is button pressed?

↓

Button LOW

↓

Take action

↓

Repeat

The Arduino sets the button pin as input with pull-up resistor, reads the pin state, and checks if the button is pressed (LOW) or not (HIGH).

Execution Sample

Arduino

const int buttonPin = 2;
void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
}
void loop() {
  int buttonState = digitalRead(buttonPin);
}

This code sets up a button on pin 2 with an internal pull-up resistor and reads its state continuously.

Execution Table

Step	Action	Pin State Read	Button Pressed?	Output
1	Setup pinMode INPUT_PULLUP	N/A	N/A	Pin 2 configured with pull-up resistor
2	Read digital pin	HIGH	No	Button not pressed
3	Read digital pin	LOW	Yes	Button pressed
4	Read digital pin	HIGH	No	Button not pressed
5	Read digital pin	LOW	Yes	Button pressed
6	Loop continues	N/A	N/A	Repeat reading pin state

💡 Loop runs indefinitely, reading button state repeatedly.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
buttonState	undefined	HIGH	LOW	HIGH	LOW	varies each loop

Key Moments - 3 Insights

Why does the button read HIGH when not pressed?

Because the internal pull-up resistor connects the pin to 5V, so when the button is not pressed, the pin reads HIGH as shown in execution_table step 2.

Why is the button pressed state LOW, not HIGH?

Pressing the button connects the pin to ground, making the reading LOW, as seen in execution_table step 3 and 5.

What happens if we don't use INPUT_PULLUP?

Without INPUT_PULLUP, the pin might float and read random values, causing unreliable button readings.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of buttonState at step 3?

A undefined

B LOW

C HIGH

D 5V

💡 Hint

Check the 'Pin State Read' column at step 3 in execution_table.

At which step does the button read as pressed?

A Step 3

B Step 2

C Step 4

D Step 6

💡 Hint

Look for 'Button Pressed?' column marked 'Yes' in execution_table.

If the button is never pressed, what will buttonState be in the variable_tracker?

A Alternates LOW and HIGH

B Always LOW

C Always HIGH

D Undefined

💡 Hint

Refer to variable_tracker and execution_table steps where button is not pressed.

Concept Snapshot

Use pinMode(pin, INPUT_PULLUP) to enable internal pull-up resistor.
Button not pressed reads HIGH because of pull-up.
Button pressed connects pin to ground, reads LOW.
Read button state with digitalRead(pin).
Check for LOW to detect press.
Loop reading for continuous detection.

Full Transcript

This example shows how to read a button using Arduino's internal pull-up resistor. First, the pin is set as INPUT_PULLUP, which connects it internally to 5 volts. When the button is not pressed, the pin reads HIGH. Pressing the button connects the pin to ground, so the reading becomes LOW. The code continuously reads the pin state in the loop. The variable buttonState changes between HIGH and LOW depending on the button press. This method avoids floating pin issues and gives reliable button readings.

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