Make an LED blink

Give the board power and a way to receive your code.

1. Start with the board by itself. Disconnect any breadboard circuits or shields before this first test.
2. Connect the Arduino to your computer using the data USB cable, then open Arduino IDE.
3. Find the LED marked L on the UNO. This is the light your sketch will control; the ON power light is different.

Tell the IDE what you are programming and where it is connected.

1. In IDE 2, open the board selector near the top of the window. Select the connected board and its port. If it says Unknown, choose Select other board and port and select the model yourself.
2. For UNO R3, select Arduino Uno from the Arduino AVR Boards package. Install the package if the IDE asks. An UNO R4 needs its own UNO R4 package and model selection.
3. You can also make these choices through Tools → Board and Tools → Port. Select the port that appears when you connect your Arduino.

Start from a working example instead of a blank editor.

1. Choose File → Examples → 01.Basics → Blink. A sketch is simply the program you upload to the Arduino.
2. Save a copy with File → Save As so you can experiment with your own version. If you prefer, create a new sketch and replace its contents with the code in the next step.
3. Keep the LED on the board for this project. External LEDs need a correctly wired circuit and a current-limiting resistor; they are not part of this first exercise.

Follow one repeating instruction: light on, wait, light off, wait.

1. setup() runs once at startup. pinMode(LED_BUILTIN, OUTPUT) prepares the board's LED pin to be controlled by your program.
2. loop() repeats while the board runs. For the UNO used here, digitalWrite(..., HIGH) turns the L LED on and digitalWrite(..., LOW) turns it off.
3. delay(1000) pauses for 1,000 milliseconds, or one second. With one wait after on and another after off, a complete cycle lasts about two seconds.
4. LED_BUILTIN uses the LED pin defined for the board selected in the IDE. On UNO R3, this is digital pin 13. Keep the constant rather than guessing another board's pin number.

// Based on Arduino's public-domain Blink example.
void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
}

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

Send the sketch to your Arduino and see a physical result.

1. Click Verify (the checkmark) to check that the sketch compiles. If it reports an error, fix it before continuing.
2. Click Upload (the right arrow). Leave the board connected and wait for the upload to finish.
3. Watch the L LED: it should stay on for about one second, then off for about one second, and repeat. The ON power light stays lit.

Change the pattern and prove that you control what happens.

1. Change both delay(1000) lines to delay(500), then upload again. The LED now spends half a second on and half a second off.
2. Try a short flash: use delay(100) after HIGH and delay(900) after LOW. Upload and compare the pattern.
3. Choose your own pair of values. Predict how the LED will behave, upload the change, then check your prediction.
4. When you are done, restore both delays to 1000 if you want the original pattern. Save your sketch so you can return to it.