Read a Button

While digital output lets you control voltage on a pin, digital input lets you read voltage on a pin. You configure a pin as INPUT with pinMode(), then read it with digitalRead(), which returns either HIGH (button pressed, ~5V) or LOW (button released, 0V).

int buttonState = digitalRead(2);  // Returns HIGH or LOW

Without a resistor, a floating input pin picks up electrical noise from the environment, causing random HIGH/LOW readings even when nothing is pressed. A pull-down resistor (typically 10kΩ) connects the input pin to GND, forcing it to read LOW when the button is open.

When you press the button, 5V connects directly to the pin, overriding the resistor's pull to GND and reading HIGH.

• Button pin 1 → Pin 2 on Arduino AND one leg of 10kΩ resistor
• Other leg of 10kΩ resistor → GND
• Button pin 2 → 5V
• LED (with 220Ω) → Pin 13

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

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

void loop() {
  int state = digitalRead(buttonPin);
  if (state == HIGH) {
    digitalWrite(ledPin, HIGH);  // Button pressed — LED on
  } else {
    digitalWrite(ledPin, LOW);   // Button released — LED off
  }
}

Mechanical buttons don't cleanly switch — they 'bounce' between HIGH and LOW for a few milliseconds when pressed. This causes your code to register multiple presses for a single click.

The simplest fix: record the time of the last stable reading and ignore changes that happen within 50ms of it.

unsigned long lastDebounceTime = 0;
const unsigned long debounceDelay = 50;
int lastButtonState = LOW;
int buttonState = LOW;

void loop() {
  int reading = digitalRead(buttonPin);

  if (reading != lastButtonState) {
    lastDebounceTime = millis();
  }

  if ((millis() - lastDebounceTime) > debounceDelay) {
    buttonState = reading;
  }

  digitalWrite(ledPin, buttonState);
  lastButtonState = reading;
}