Digital Inputs and Outputs

Understanding Digital I/O

Digital I/O (Input/Output) is the foundation of Arduino. Digital signals have only two states: ON or OFF (HIGH or LOW).

Input vs Output

Digital Output

We've already seen digital output with LEDs. Now let's expand:

Multiple LEDs

Connect and control multiple LEDs:

int led1 = 10;
int led2 = 11;
int led3 = 12;
void setup() {
  pinMode(led1, OUTPUT);
  pinMode(led2, OUTPUT);
  pinMode(led3, OUTPUT);
}
void loop() {
  digitalWrite(led1, HIGH);
  digitalWrite(led2, LOW);
  digitalWrite(led3, LOW);
  delay(500);
  
  digitalWrite(led1, LOW);
  digitalWrite(led2, HIGH);
  digitalWrite(led3, LOW);
  delay(500);
  
  digitalWrite(led1, LOW);
  digitalWrite(led2, LOW);
  digitalWrite(led3, HIGH);
  delay(500);
}

Digital Input

Reading buttons and switches:

Connecting a Button

Connect a push button:

1. One leg to Arduino pin 2 2. Other leg to GND through 10kΩ resistor (pull-down) 3. Connect 5V to button through internal pull-up

Button Circuit

5V ---[Button]--- Pin 2 ---[10kΩ]--- GND

Reading Button State

int buttonPin = 2;
int ledPin = 13;
void setup() {
  pinMode(buttonPin, INPUT);
  pinMode(ledPin, OUTPUT);
}
void loop() {
  int buttonState = digitalRead(buttonPin);
  
  if (buttonState == HIGH) {
    digitalWrite(ledPin, HIGH);  // LED on
  } else {
    digitalWrite(ledPin, LOW);   // LED off
  }
}

Using Internal Pull-Up

Arduino has built-in pull-up resistors:

void setup() {
  pinMode(2, INPUT_PULLUP);  // Enable internal pull-up
}

With INPUT_PULLUP:

• Button pressed = LOW
• Button released = HIGH

Complete Example with Pull-Up

int buttonPin = 2;
int ledPin = 13;
void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
  pinMode(ledPin, OUTPUT);
}
void loop() {
  if (digitalRead(buttonPin) == LOW) {
    digitalWrite(ledPin, HIGH);
  } else {
    digitalWrite(ledPin, LOW);
  }
}

Debouncing

Buttons can cause multiple readings due to mechanical bounce:

Without Debounce (Problem)

// This can cause multiple triggers!
if (digitalRead(buttonPin) == LOW) {
  count++;
}

With Debounce (Solution)

int buttonPin = 2;
int ledState = LOW;
int lastButtonState = HIGH;
unsigned long lastDebounceTime = 0;
unsigned long debounceDelay = 50;
void loop() {
  int reading = digitalRead(buttonPin);
  
  if (reading != lastButtonState) {
    lastDebounceTime = millis();
  }
  
  if ((millis() - lastDebounceTime) > debounceDelay) {
    if (reading == LOW && ledState == LOW) {
      ledState = HIGH;
    } else if (reading == LOW && ledState == HIGH) {
      ledState = LOW;
    }
  }
  
  digitalWrite(13, ledState);
  lastButtonState = reading;
}

PWM Output

PWM (Pulse Width Modulation) simulates analog output:

int ledPin = 9;  // Must be PWM pin
void setup() {
  pinMode(ledPin, OUTPUT);
}
void loop() {
  // Fade LED in
  for (int brightness = 0; brightness <= 255; brightness++) {
    analogWrite(ledPin, brightness);
    delay(10);
  }
  
  // Fade LED out
  for (int brightness = 255; brightness >= 0; brightness--) {
    analogWrite(ledPin, brightness);
    delay(10);
  }
}

Practical Project: Traffic Light

Create a simple traffic light:

int redPin = 10;
int yellowPin = 11;
int greenPin = 12;
void setup() {
  pinMode(redPin, OUTPUT);
  pinMode(yellowPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
}
void loop() {
  digitalWrite(redPin, HIGH);
  delay(5000);
  digitalWrite(redPin, LOW);
  
  digitalWrite(yellowPin, HIGH);
  delay(2000);
  digitalWrite(yellowPin, LOW);
  
  digitalWrite(greenPin, HIGH);
  delay(5000);
  digitalWrite(greenPin, LOW);
}

Summary

Digital I/O is essential:

• pinMode configures input or output
• digitalRead reads button/switch state
• digitalWrite controls LEDs and other outputs
• INPUT_PULLUP uses internal pull-up resistor
• analogWrite creates PWM for variable output

Next Lesson

In the next lesson, you'll learn how to read from sensors, specifically a distance sensor.