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Built-in project · Arduino Uno R3

First blink on an Arduino Uno

The first circuit on an Arduino Uno: an LED and a 220 ohm resistor on pin 13, blinking once a second, with a sketch that explains every line. 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.

Build it as the Starter Kit: the parts in numbered bags and a booklet that checks every step. Retails for about $34, free on Mokxi.

The circuit runs here once the page has loaded.

What you'll learn

  • Your first circuit: LED, resistor and a pin
  • Setting a pin as an output
  • Timing with delay
  • Which way around an LED goes
  • Difficulty: Beginner
  • Time: About 10 minutes
  • Board: Arduino Uno R3
  • Parts: 4

How it works

Pin 13 of the Uno is wired through a 220 ohm resistor to a red LED, and the LED's other leg goes to ground. The resistor limits the current so the LED is not damaged.

The sketch turns pin 13 on for half a second, then off for half a second, forever. That is one blink per second. Change both delays to 100 and it blinks five times as fast.

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
// Your first circuit: an Arduino Uno
// blinking the red LED on pin 13.
//
// Press Run to watch it blink. Then change
// both delay(500) lines to delay(100) and
// press Run again: it blinks five times
// as fast.

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 when the board starts. pinMode(LED_BUILTIN, OUTPUT) tells the Uno that pin 13 sends signals out. 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 the LED lights. delay(500) waits 500 milliseconds. 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 LED does not light This project
    Check the LED direction. The longer leg (anode) goes toward the resistor, the shorter leg (cathode) 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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