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Built-in project · Arduino Uno R3

Arduino Uno blink

Flashes an LED on pin 13 once a second: the smallest thing an Arduino Uno can do, wired on the breadboard beside it. The whole build, an Arduino Uno and 2 more parts, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.

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Beginner About 10 minutes to build and run. Runs in your browser. Free, and no account needed.

The parts retail for about $32. On Mokxi they're free.

The circuit runs here once the page has loaded.

What you'll learn

  • Blinking an LED with digitalWrite and delay
  • What LED_BUILTIN means on an Uno
  • Setting a pin as an output
  • Why an LED needs a 220 ohm resistor
  • Difficulty: Beginner
  • Time: About 10 minutes
  • Board: Arduino Uno R3
  • Parts: 4

How it works

The Uno has a small LED soldered next to the USB socket on pin 13. You also wire a red LED from pin 13 through a 220 ohm resistor to ground, so you can see it on the breadboard.

The sketch turns pin 13 on for half a second, then off for half a second, forever. That makes one blink a second.

Parts list

4 parts, plus the jumper wires. They retail for about $32; on Mokxi they're free.

Kits that can build it

These 12 starter kits have every part in the circuit. Each kit's page lists what else it can build.

Wiring table

Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.

Part and pinConnects to
LED, red pin CArduino Uno R3 pin GND
Resistor, 220 Ω pin 1Arduino Uno R3 pin 13
LED, red pin AResistor, 220 Ω pin 2

The code

The firmware exactly as the editor opens it. Change a line there and press Run: it compiles in the browser.

sketch.ino · Arduino Uno R3
// Blink: the smallest thing a board can do.
//
// Pin 13 has the LED soldered next to the USB socket, so LED_BUILTIN is 13.
// Wire an LED and a 220 ohm resistor from pin 13 to GND to see it on the
// breadboard too.
//
// delay() puts the chip to sleep between timer ticks rather than spinning, so
// this sketch costs the simulator almost nothing while it waits.

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

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

Code walkthrough

The key functions of the sketch, and what each one does.

setup sketch.ino

Runs once at start. pinMode(LED_BUILTIN, OUTPUT) makes pin 13 an output. LED_BUILTIN is the Uno's name for pin 13.

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

loop sketch.ino

Runs over and over. digitalWrite(LED_BUILTIN, HIGH) puts 5 V on the pin and lights the LED. delay(500) waits half a second. Then LOW turns it off and the second delay(500) waits again.

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

Troubleshooting

The usual mistakes with this circuit, and the fix for each.

  • The breadboard LED does not light This project
    Flip the LED. The longer leg (anode) goes toward the resistor and the shorter leg to GND.
  • The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
    Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure.
  • A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
    Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground.
  • The LED does not light, or it lights once and then dies. LED
    LEDs have polarity. The longer leg (anode) goes toward the positive side. Always add a series resistor, about 220 ohms to 1 k ohm on a 5 V pin.
  • A blue or white LED stays dark on 3.3 V. LED
    Blue and white LEDs drop about 3 V, so 3.3 V leaves almost nothing for the resistor. Use a 5 V supply, or a smaller resistor and a red or green LED.
  • Upload fails or nothing runs after wiring pins 0 and 1 to a module. Arduino Uno R3
    Pins 0 and 1 are the USB serial port. Move the module to other pins, or unplug it while uploading. Also check the right board is selected.
  • Pins misbehave or the board resets when you power motors or servos from it. Arduino Uno R3
    Each I/O pin gives about 20 mA, 40 mA max. Power motors and servos from their own supply and join its ground to the Uno's GND.

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