Arduino UNO

Learn

What you’ll learn

• What a microcontroller is and what’s on an Arduino UNO
• What each group of pins does
• How to install the Arduino IDE and upload the Blink example

What is an Arduino?

Arduino is a family of microcontroller boards plus free software to program them. A microcontroller is a tiny computer on one chip that runs one program, forever, from the moment it gets power. It reads sensors through its input pins and controls LEDs, motors and buzzers through its output pins.

The UNO is the classic beginner board. You’ll see a few versions:

Board	Notes
UNO R3 (and most clones)	The classic. Huge number of tutorials.
UNO R4 Minima / R4 WiFi	Current official boards, same shape and pin layout. R4 WiFi adds Wi-Fi, Bluetooth and an LED matrix.
Nano	Same idea, much smaller, plugs into a breadboard.

Everything in this course works on any of them.

A tour of the board

Part	What it’s for
USB port	Power from your computer and uploading programs
Barrel jack / VIN	Power from a battery, 7–12 V recommended
Digital pins 0–13	On/off inputs and outputs. Pins marked ~ (3, 5, 6, 9, 10, 11) can also do PWM, a fake “in-between” output for dimming and motor speed
Analog pins A0–A5	Read voltages between 0 and 5 V, for sensors like knobs and light sensors
5V and 3.3V pins	Power small sensors and modules (not motors!)
GND pins	Ground, the 0 V reference. Every circuit connects back here
Built-in LED “L”	Connected to pin 13; great for testing
Reset button	Restarts your program from the beginning

Pins are delicate

Each pin can only supply a small current: around 20 mA on an UNO R3, and only about 8 mA on an UNO R4. That’s enough for an LED with a resistor, but never connect a motor directly to a pin. Never connect a pin straight to 5V or GND while it’s set as an output.

Install the Arduino IDE

The IDE (Integrated Development Environment) is the free app where you write code and send it to the board.

1. Download Arduino IDE 2 from arduino.cc/en/software and install it.
2. Plug the UNO into your computer with a USB cable that carries data. Some cables are charge-only and won’t work. The green ON LED should light.
3. At the top of the IDE, open the board/port menu and pick your board and port. The port is named like COM3 on Windows or /dev/cu.usbmodem… / /dev/cu.usbserial… on Mac.
4. UNO R4 only: if asked, install the Arduino UNO R4 Boards package (Tools → Board → Boards Manager).
5. Open File → Examples → 01.Basics → Blink.
6. Click Upload (the → arrow). Wait for “Done uploading”.

What you should see

The orange L LED on the board blinks: 1 second on, 1 second off, forever. Unplug and replug the board: it starts blinking again by itself, because the program is stored on the chip.

If the upload fails

Problem	Fix
No port appears	Try another USB cable (data, not charge-only) or another USB socket
Clone board not detected	Many clones use a CH340 USB chip; install its driver if your computer doesn’t recognise it
“Wrong board” or upload errors	Check the board selected in the IDE matches your actual board
Port is busy	Close other programs using the board (like a second IDE window)

Examples

Example: Blink, slower

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

void loop() {
  digitalWrite(LED_BUILTIN, HIGH);
  delay(2000);  // on for 2 seconds
  digitalWrite(LED_BUILTIN, LOW);
  delay(500);   // off for half a second
}

Example: a heartbeat

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

void loop() {
  for (int i = 0; i < 2; i++) {   // two quick beats
    digitalWrite(LED_BUILTIN, HIGH);
    delay(100);
    digitalWrite(LED_BUILTIN, LOW);
    delay(150);
  }
  delay(800);                     // then a pause
}

Practice

No board yet? Use the virtual Arduino

The simulator below is a virtual UNO. Start it, replace the code in sketch.ino with the Blink code below, and press the green ▶ button. The little LED marked L on the virtual board will blink.

sketch.ino

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

void loop() {
  digitalWrite(LED_BUILTIN, HIGH);
  delay(1000);
  digitalWrite(LED_BUILTIN, LOW);
  delay(1000);
}

Virtual Arduino UNOOpen full screen ↗

Start simulator

Runs on Wokwi, free, no account needed.

Faster blinkEasy

Make the built-in LED blink 4 times per second. Each blink is on + off, so use delay(125) for both: 125 + 125 = 250 ms per blink.

SOSMedium

Blink · · · — — — · · · (short = 200 ms, long = 600 ms, 200 ms gap) and then pause 2 s. Write a small function void flash(int ms) that turns the LED on for ms, then off for 200 ms.

Show an answer

void flash(int ms) {
  digitalWrite(LED_BUILTIN, HIGH); delay(ms);
  digitalWrite(LED_BUILTIN, LOW);  delay(200);
}

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

void loop() {
  for (int i = 0; i < 3; i++) flash(200);
  for (int i = 0; i < 3; i++) flash(600);
  for (int i = 0; i < 3; i++) flash(200);
  delay(2000);
}

Two LEDs taking turnsHard

In the simulator, add a second LED with a resistor on pin 9 and make the two LEDs blink alternately.

Show an answer

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

void loop() {
  digitalWrite(LED_BUILTIN, HIGH); digitalWrite(9, LOW);  delay(400);
  digitalWrite(LED_BUILTIN, LOW);  digitalWrite(9, HIGH); delay(400);
}

Quick check

1. What does a microcontroller do when it gets power?

Waits for you to type commandsRuns the program stored on it, over and overDownloads appsNothing until you press reset

2. Which pins would you use to read a knob that gives any voltage between 0 and 5 V?

Digital pins 0–13Analog pins A0–A5GNDVIN

3. The built-in LED is connected to which pin?

Pin 0Pin 2Pin 13A0

4. Why can’t you drive a motor straight from a digital pin?

Motors need far more current than a pin can safely supplyPins only do analogMotors need Wi-FiYou can, it is fine

Check my answers