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Built-in project · Arduino Uno R3

Blink an LED on pin 8, on an Arduino Uno

An LED and a 220 ohm resistor on pin 8, switched on for a second and off for a second: the first sketch, on a pin you wire yourself. 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

  • Setting a pin as an output with pinMode
  • Switching a pin with digitalWrite
  • Timing with delay
  • Why an LED needs a resistor
  • Difficulty: Beginner
  • Time: About 10 minutes
  • Board: Arduino Uno R3
  • Parts: 4

How it works

The LED's long leg (anode) connects through a 220 Ω resistor to pin 8, and its short leg (cathode) goes to GND. When pin 8 is HIGH it sits at 5 V and current flows through the LED. When it is LOW the pin is at 0 V and the LED goes dark.

The sketch switches the pin HIGH for a second, then LOW for a second, forever. It also prints "on" and "off" to the serial monitor so you can see each change. The resistor holds the current to about 14 mA.

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 8
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 8, one second on and one second off.
//
//   pin 8 -> 220 ohm resistor -> LED long leg (anode), short leg -> GND
//
// pinMode() makes pin 8 an output, then digitalWrite() switches it between
// 5 V (HIGH) and 0 V (LOW). The resistor keeps the current to about 14 mA,
// which the LED and the pin are both happy with.
//
// Things to try: change the two delays and make it blink faster, or give
// the LED a short flash and a long pause, like a smoke alarm.

const int LED_PIN = 8;

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: blink on pin 8");
}

void loop() {
  digitalWrite(LED_PIN, HIGH);
  Serial.println("on");
  delay(1000);
  digitalWrite(LED_PIN, LOW);
  Serial.println("off");
  delay(1000);
}

Code walkthrough

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

setup sketch.ino

Runs once. pinMode(LED_PIN, OUTPUT) turns pin 8 into an output, and Serial.begin(9600) opens the serial monitor at 9600 baud. Then it prints a hello line.

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: blink on pin 8");
}

loop sketch.ino

Runs forever. It sets LED_PIN HIGH, prints "on", and waits 1000 ms. Then it sets the pin LOW, prints "off", and waits another 1000 ms. Change those two delays to change the blink speed.

void loop() {
  digitalWrite(LED_PIN, HIGH);
  Serial.println("on");
  delay(1000);
  digitalWrite(LED_PIN, LOW);
  Serial.println("off");
  delay(1000);
}

Troubleshooting

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

  • The LED never lights This project
    Check the LED direction. The anode (A) goes to the resistor and the cathode (C) goes to GND.
  • The LED blinks on a different pin This project
    The wire and LED_PIN must agree. Both use pin 8 here.
  • 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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