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Built-in project · Arduino Uno R3

Blink an LED on pin 13, once a second, on an Arduino Uno

An LED and a 220 ohm resistor on pin 13, beside the board’s own LED on the same pin: both blink together, a second on and a second off. 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 $30. 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
  • A series resistor protecting an LED
  • Naming a pin with a constant
  • The board's own L lamp shares pin 13
  • Difficulty: Beginner
  • Time: About 10 minutes
  • Board: Arduino Uno R3
  • Parts: 4

How it works

Pin 13 goes through a 220 ohm resistor to a red LED, and the LED's short leg goes to ground. Pin 13 also drives the small L lamp on the Uno, so both LEDs blink together.

The sketch turns the pin on for one second, then off for one second, forever. It also prints a line to the Serial Monitor at start.

Parts list

4 parts, plus the jumper wires. They retail for about $30; 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 an LED on pin 13.
//
//   pin 13 -> 220 ohm resistor -> the LED's long leg; the short leg -> GND
//
// Pin 13 also drives the little L lamp on the board, so the two LEDs blink
// together. digitalWrite(13, HIGH) puts 5 V on the pin; LOW takes it away;
// delay(1000) waits a second in between.

const int LED_PIN = 13;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: blinking the LED on pin 13");
}

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

Code walkthrough

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

setup sketch.ino

Runs once. pinMode(LED_PIN, OUTPUT) makes pin 13 an output. Serial.begin(9600) opens the serial connection, and Serial.println prints a hello line to the Serial Monitor.

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: blinking the LED on pin 13");
}

loop sketch.ino

Runs forever. digitalWrite(LED_PIN, HIGH) puts 5 V on pin 13, delay(1000) waits a second, then LOW takes it away and another delay(1000) waits. One full blink takes two seconds.

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

Troubleshooting

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

  • The board lamp blinks but the breadboard LED does not This project
    Flip the LED. The long leg (anode) goes toward the resistor and the short 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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