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    Arduino Two LEDs Blinking Alternately

    easyDigital outputArduino (C++)

    Controlling two outputs at once — the step between a single blink and a traffic light.

    The question

    Two LEDs are on pins 11 and 3. Make them blink alternately: when one is ON the other is OFF. Switch every 300 ms.

    The sketch

    sketch.ino

    void setup() {
  pinMode(11, OUTPUT);
  pinMode(3, OUTPUT);
}

void loop() {
  digitalWrite(11, HIGH);
  digitalWrite(3, LOW);
  delay(300);
  digitalWrite(11, LOW);
  digitalWrite(3, HIGH);
  delay(300);
}
    Open the Arduino simulator »

    Wiring

    LED 1 → 220Ω → pin 11; LED 2 → 220Ω → pin 3; both cathodes → GND.

    How it works

    1. 1Both pins are set as OUTPUT in setup().
    2. 2In the first half of loop() one LED is HIGH and the other LOW.
    3. 3After the delay the states swap, so the LEDs take turns.

    Common mistakes

    • Setting only one pin as OUTPUT — the other LED glows dimly or not at all.
    • Forgetting a delay after the second half, so one LED looks permanently on.

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