Lesson 4: Digital I/O Basics

Understanding Digital Inputs and Outputs with Arduino

🎯 Learning Focus: Digital pins, LEDs, buttons, and basic control

📚 Learning Objectives

By the end of this Lesson, you will:

  • ✓ Understand digital HIGH and LOW states
  • ✓ Control LEDs with digitalWrite()
  • ✓ Read button inputs with digitalRead()
  • ✓ Implement pull-up and pull-down resistors

Key Concepts:

  • • Digital pin modes (INPUT/OUTPUT)
  • • Voltage levels (0V/5V)
  • • Current limiting with resistors
  • • Debouncing techniques

🔧 Section 1: Arduino IDE Setup & Hardware Connection

Installing the Arduino IDE

Now that you have your Arduino hardware, it's time to install the Arduino IDE (Integrated Development Environment) - the software we'll use to write and upload programs to your Arduino board.

What is the Arduino IDE?

  • • Text Editor: Where you write your Arduino code (called "sketches")
  • • Compiler: Translates your code into machine language
  • • Uploader: Sends your compiled program to the Arduino board
  • • Serial Monitor: View messages and data from your Arduino
  • • Library Manager: Install additional code libraries for sensors and components

Installation Steps:

  1. Download: Go to arduino.cc/software and download the Arduino IDE
  2. Install: Run the installer and follow the setup wizard
  3. Connect: Plug your Arduino into your computer using the USB cable
  4. Select Board: Go to Tools → Board → Arduino Uno (or your specific board)
  5. Select Port: Go to Tools → Port → Select the COM port (Windows) or /dev/cu.usbmodem (Mac)
  6. Test: Upload the built-in "Blink" example to verify everything works

🔍 Troubleshooting Common Issues:

  • • Port not found: Try different USB ports, restart the IDE, or install Arduino drivers
  • • Upload failed: Check that the correct board and port are selected
  • • Permission denied: Run Arduino IDE as administrator (Windows) or check permissions (Mac/Linux)
  • • Board not recognized: Install CH340 drivers for clone Arduino boards

💡 Section 2: Digital Output Basics

Understanding Digital Signals

Digital signals have only two states: HIGH (5V) and LOW (0V). This binary nature makes them perfect for controlling devices like LEDs, motors, and relays.

Key Functions:

  • pinMode(pin, OUTPUT) - Configure pin as output
  • digitalWrite(pin, HIGH) - Set pin to 5V
  • digitalWrite(pin, LOW) - Set pin to 0V

Basic LED Control

Let's start with the classic "Hello World" of electronics - blinking an LED.

Circuit Setup:

  • • LED connected to digital pin 13
  • • 220Ω resistor in series with LED
  • • Ground connection to complete circuit
// Basic LED Blink Program
const int ledPin = 13;

void setup() {
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
  Serial.println("LED Blink Program Started");
}

void loop() {
  digitalWrite(ledPin, HIGH);   // Turn LED on
  Serial.println("LED ON");
  delay(1000);                  // Wait 1 second
  
  digitalWrite(ledPin, LOW);    // Turn LED off
  Serial.println("LED OFF");
  delay(1000);                  // Wait 1 second
}

🔘 Section 2: Digital Input Basics

Reading Button States

Digital inputs allow us to read the state of switches, buttons, and sensors. The Arduino can detect whether a pin is HIGH or LOW.

Input Functions:

  • pinMode(pin, INPUT) - Configure pin as input
  • pinMode(pin, INPUT_PULLUP) - Input with internal pull-up
  • digitalRead(pin) - Read pin state (HIGH/LOW)

Pull-up Resistors

Pull-up resistors ensure that input pins have a defined state when not connected to anything. Arduino has built-in pull-up resistors we can enable. 

// Button Input with Pull-up Resistor
const int buttonPin = 2;
const int ledPin = 13;

void setup() {
  pinMode(buttonPin, INPUT_PULLUP);  // Enable internal pull-up
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
  Serial.println("Button Input Program Started");
}

void loop() {
  int buttonState = digitalRead(buttonPin);
  
  if (buttonState == LOW) {  // Button pressed (pull-up inverts logic)
    digitalWrite(ledPin, HIGH);
    Serial.println("Button Pressed - LED ON");
  } else {
    digitalWrite(ledPin, LOW);
    Serial.println("Button Released - LED OFF");
  }
  
  delay(50);  // Debounce delay
}

🔄 Section 3: Combining Input and Output

🔧 Introduction to Breadboards

A breadboard is a reusable platform for building electronic circuits without soldering. It makes connecting components to your Arduino much easier!

How Breadboards Work:
  • Rows: Each numbered row (1-30) has 5 connected holes on each side
  • Power Rails: The long strips marked + and - run the full length
  • Center Gap: Separates left and right sides of each row
  • No Connection: Holes in different rows are not connected
Breadboard Setup Tips:
  • Connect Arduino 5V to the + power rail (red stripe)
  • Connect Arduino GND to the - power rail (blue/black stripe)
  • Use jumper wires to connect components to Arduino pins
  • Keep wires neat and use different colors for organization
Simple breadboard layout

Clean breadboard ready for components

🔌 Push Button Wiring with Breadboard

Step-by-Step Wiring:
  1. Set up power rails: Connect Arduino 5V to breadboard + rail, GND to - rail
  2. Place push button: Insert button across the center gap (e.g., rows 10-12)
  3. Connect to pin 2: Wire from Arduino digital pin 2 to one side of button
  4. Connect to ground: Wire from other side of button to - power rail (GND)
  5. No resistor needed: The code uses INPUT_PULLUP mode

💡 Why INPUT_PULLUP works: This mode connects an internal 20kΩ resistor between the pin and 5V. When the button is pressed, it connects the pin to ground (LOW). When released, the pull-up resistor keeps the pin at 5V (HIGH).

Note: The built-in button on the expansion board is connected to reset, not pin 2, so we need an external button for this exercise.

Push button wiring setup on breadboard

Push button connected to pin 2 with breadboard wiring

Interactive LED Control

Now let's create more interactive programs that respond to user input and control multiple outputs. 

// Toggle LED with Button Press
const int buttonPin = 2;
const int ledPin = 13;

bool ledState = false;
bool lastButtonState = HIGH;
bool currentButtonState = HIGH;

void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
  Serial.println("LED Toggle Program Started");
}

void loop() {
  currentButtonState = digitalRead(buttonPin);
  
  // Check for button press (HIGH to LOW transition)
  if (lastButtonState == HIGH && currentButtonState == LOW) {
    ledState = !ledState;  // Toggle LED state
    digitalWrite(ledPin, ledState);
    
    if (ledState) {
      Serial.println("LED Toggled ON");
    } else {
      Serial.println("LED Toggled OFF");
    }
    
    delay(50);  // Simple debounce
  }
  
  lastButtonState = currentButtonState;
}

Multiple LEDs and Patterns

Let's create more complex patterns using multiple LEDs and timing.

🔌 Multiple LED Wiring with Breadboard

Traffic Light LED Setup:
  1. Red LED: Long leg → 220Ω resistor → Arduino pin 11, Short leg → GND rail
  2. Yellow LED: Long leg → 220Ω resistor → Arduino pin 12, Short leg → GND rail
  3. Green LED: Long leg → 220Ω resistor → Arduino pin 13, Short leg → GND rail
  4. Push button: One side → Arduino pin 2, Other side → GND rail
  5. Power rails: Connect Arduino GND to breadboard - rail

💡 LED Polarity: LEDs have polarity - the long leg (anode) connects to the positive side through a resistor, and the short leg (cathode) connects to ground.

⚡ Why 220Ω resistors: These limit current to protect the LEDs from burning out. Each LED needs its own resistor.

Multiple LEDs and control buttons wired on breadboard

Traffic light LEDs with control buttons - complete interactive setup

// Traffic Light Simulator
const int redPin = 11;
const int yellowPin = 12;
const int greenPin = 13;
const int buttonPin = 2;

void setup() {
  pinMode(redPin, OUTPUT);
  pinMode(yellowPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
  pinMode(buttonPin, INPUT_PULLUP);
  Serial.begin(9600);
  Serial.println("Traffic Light Simulator Started");
}

void loop() {
  // Green light
  digitalWrite(greenPin, HIGH);
  digitalWrite(yellowPin, LOW);
  digitalWrite(redPin, LOW);
  Serial.println("GREEN - Go!");
  delay(3000);
  
  // Yellow light
  digitalWrite(greenPin, LOW);
  digitalWrite(yellowPin, HIGH);
  digitalWrite(redPin, LOW);
  Serial.println("YELLOW - Caution!");
  delay(1000);
  
  // Red light
  digitalWrite(greenPin, LOW);
  digitalWrite(yellowPin, LOW);
  digitalWrite(redPin, HIGH);
  Serial.println("RED - Stop!");
  delay(3000);
}

🛠️ Hands-On Activity: Interactive Control Panel

Project: Build a Simple Control Panel

Create an interactive control panel with multiple buttons and LEDs that demonstrates digital I/O concepts.

Required Components:

  • • Arduino Uno
  • • 3 LEDs (Red, Yellow, Green)
  • • 2 Push buttons
  • • 3 × 220Ω resistors (for LEDs)
  • • Breadboard and jumper wires

Challenge Tasks:

  1. Wire the circuit according to the schematic
  2. Program button 1 to cycle through LED colors
  3. Program button 2 to create a blinking pattern
  4. Add Serial Monitor feedback for all actions
  5. Implement proper button debouncing
// Control Panel Challenge Solution
const int redPin = 11;
const int yellowPin = 12;
const int greenPin = 13;
const int button1Pin = 2;
const int button2Pin = 3;

int currentLED = 0;  // 0=red, 1=yellow, 2=green
bool blinkMode = false;
bool lastButton1State = HIGH;
bool lastButton2State = HIGH;

void setup() {
  pinMode(redPin, OUTPUT);
  pinMode(yellowPin, OUTPUT);
  pinMode(greenPin, OUTPUT);
  pinMode(button1Pin, INPUT_PULLUP);
  pinMode(button2Pin, INPUT_PULLUP);
  Serial.begin(9600);
  Serial.println("Control Panel Ready!");
}

void loop() {
  // Check button 1 for LED cycling
  bool button1State = digitalRead(button1Pin);
  if (lastButton1State == HIGH && button1State == LOW) {
    currentLED = (currentLED + 1) % 3;
    Serial.print("Switched to LED: ");
    Serial.println(currentLED);
    delay(50);  // Debounce
  }
  lastButton1State = button1State;
  
  // Check button 2 for blink mode
  bool button2State = digitalRead(button2Pin);
  if (lastButton2State == HIGH && button2State == LOW) {
    blinkMode = !blinkMode;
    Serial.print("Blink mode: ");
    Serial.println(blinkMode ? "ON" : "OFF");
    delay(50);  // Debounce
  }
  lastButton2State = button2State;
  
  // Control LEDs
  updateLEDs();
}

void updateLEDs() {
  // Turn off all LEDs first
  digitalWrite(redPin, LOW);
  digitalWrite(yellowPin, LOW);
  digitalWrite(greenPin, LOW);
  
  // Turn on current LED
  if (blinkMode && (millis() % 500 < 250)) {
    // Blink current LED
    switch(currentLED) {
      case 0: digitalWrite(redPin, HIGH); break;
      case 1: digitalWrite(yellowPin, HIGH); break;
      case 2: digitalWrite(greenPin, HIGH); break;
    }
  } else if (!blinkMode) {
    // Solid current LED
    switch(currentLED) {
      case 0: digitalWrite(redPin, HIGH); break;
      case 1: digitalWrite(yellowPin, HIGH); break;
      case 2: digitalWrite(greenPin, HIGH); break;
    }
  }
}

📝 Assessment & Homework

📊 Lesson 4 Quiz Topics:

  • • Digital HIGH and LOW states
  • • pinMode() configurations
  • • digitalWrite() and digitalRead()
  • • Pull-up resistor concepts
  • • Button debouncing techniques

🏠 Homework Assignments:

  • 1. Complete the control panel project
  • 2. Create a binary counter with 4 LEDs
  • 3. Design a simple alarm system
  • 4. Research different types of switches
  • 5. Practice circuit diagrams and schematics

💡 Pro Tips for Success:

  • • Always check your wiring before powering on
  • • Use the Serial Monitor to debug your programs
  • • Start with simple circuits and add complexity gradually
  • • Keep a notebook of working circuit configurations
Take Quiz 📝