Arduino Serial Communication Basics: Control LEDs from Your Computer

Serial communication is the simplest way to exchange data between your computer and Arduino. In this lesson, you’ll learn how to receive commands from the computer to control Arduino.

Required Components

• Arduino UNO R3 with USB cable
• Red LED 5mm x3
• Resistor 330 Ohm x3
• Breadboard
• Jumper wires

Circuit Diagram

Connect each LED through a 330 Ohm resistor to digital pins:

Arduino	Component
Pin 3	LED1
Pin 4	LED2
Pin 5	LED3
GND	Cathode (-) of all LEDs (shared)

Basic Serial Commands

Serial.available()

Checks if data has been sent from the computer. Returns a number greater than 0 if data is available.

if (Serial.available() > 0) {
  // Code here runs when computer sends data
}

Serial.read()

Reads the incoming byte from the computer and stores it in a variable.

char key = Serial.read();

Serial.print() and Serial.println()

Sends data back to display on the Serial Monitor.

Serial.print("key : ");
Serial.println(key);  // println adds newline

Code to Control LEDs via Serial

char key;  // Variable to store incoming data

void setup() {
  Serial.begin(9600);  // Start serial at 9600 baud
  
  // Set LED pins as OUTPUT
  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(5, OUTPUT);
}

void loop() {
  if (Serial.available() > 0) {
    // When computer sends data
    key = Serial.read();  // Store value in key variable
    
    Serial.print("key : ");
    Serial.println(key);  // Echo back to Serial Monitor
    
    // Check conditions to control LEDs
    if (key == '1') {
      digitalWrite(3, HIGH);  // LED1 on
    } else if (key == '2') {
      digitalWrite(4, HIGH);  // LED2 on
    } else if (key == '3') {
      digitalWrite(5, HIGH);  // LED3 on
    } else if (key == '0') {
      digitalWrite(3, LOW);   // Turn off LED1
      digitalWrite(4, LOW);   // Turn off LED2
      digitalWrite(5, LOW);   // Turn off LED3
    }
  }
}

How to Use

1. Upload the code above to your Arduino UNO
2. Open Serial Monitor via Tools > Serial Monitor or press Ctrl+Shift+M
3. Set Baud Rate to 9600 (must match the code)
4. Type a number and press Send or Enter

Test Results

Number Typed	Result
1	LED1 turns on
2	LED2 turns on
3	LED3 turns on
0	All 3 LEDs turn off

[image: Results table showing LED states when typing 1, 2, 3, 0 with corresponding LED on/breadboard illustrations]

Summary

The program waits for input from the Serial Monitor. When you type a number, the code stores it in the key variable and checks conditions. If a condition matches, the corresponding LED turns on or off as defined.

Using Serial communication makes debugging easy - you can see values returned via Serial.println() and command Arduino directly from your computer.