Digital I/O on Arduino

Digital Signals

Digital pins have only two states: HIGH (5 V on Uno) and LOW (0 V). This maps directly to 1 and 0 in your code.

You use digital I/O to:

• Output: control LEDs, buzzers, relays, motors via a driver
• Input: read buttons, switches, IR receivers, limit sensors

Configuring a Pin ߀” pinMode()

Before using a pin you must declare its direction:

pinMode(pin, mode);

Mode	Constant	When to use
Output	OUTPUT	Driving components (LED, buzzer)
Input	INPUT	Reading a switch (floating when open)
Input with pull-up	INPUT_PULLUP	Reading a switch with no external resistor

Always call pinMode() inside setup().

Writing a Pin ߀” digitalWrite()

digitalWrite(pin, value);

digitalWrite(9, HIGH);   // Set pin 9 to 5 V
digitalWrite(9, LOW);    // Set pin 9 to 0 V

Reading a Pin ߀” digitalRead()

int state = digitalRead(pin);  // Returns HIGH or LOW

Wiring an LED

Always add a current-limiting resistor (220 ?©ß€“470 ?©) in series with an LED ߀” without one the LED draws too much current and burns out, or damages the pin.

Arduino pin 9 ß”€ß”€ß”€ß”€ 220 ?© ß”€ß”€ß”€ß”€ LED (+) ß”€ß”€ß”€ß”€ LED (ß?’) ß”€ß”€ß”€ß”€ GND

void setup() {
  pinMode(9, OUTPUT);
}

void loop() {
  digitalWrite(9, HIGH);
  delay(500);
  digitalWrite(9, LOW);
  delay(500);
}

Wiring a Button

A button connects a pin to either HIGH or LOW when pressed. Using INPUT_PULLUP is the easiest approach ߀” the pin reads HIGH when the button is open, LOW when pressed.

Arduino pin 2 ß”€ß”€ß”€ß”€ Button ß”€ß”€ß”€ß”€ GND

void setup() {
  pinMode(2, INPUT_PULLUP);  // Internal pull-up resistor active
  pinMode(9, OUTPUT);
}

void loop() {
  int btn = digitalRead(2);

  if (btn == LOW) {           // Button is pressed (pulled to GND)
    digitalWrite(9, HIGH);    // LED on
  } else {
    digitalWrite(9, LOW);     // LED off
  }
}

Debouncing

Mechanical buttons produce brief electrical noise when pressed ("bounce"). A simple software debounce:

unsigned long lastDebounceTime = 0;
const unsigned long debounceDelay = 50;
int lastButtonState = HIGH;

void loop() {
  int reading = digitalRead(2);
  if (reading != lastButtonState) {
    lastDebounceTime = millis();
  }
  if ((millis() - lastDebounceTime) > debounceDelay) {
    // reading is stable ߀” use it here
  }
  lastButtonState = reading;
}