Digital Output: Controlling LEDs

int ledPin = 9;

void setup() {
    pinMode(ledPin, OUTPUT);
}

void loop() {
    digitalWrite(ledPin, HIGH);  // LED ON
    delay(500);
    digitalWrite(ledPin, LOW);   // LED OFF
    delay(500);
}

Digital Input: Reading Buttons

int buttonPin = 2;
int ledPin = 13;

void setup() {
    pinMode(buttonPin, INPUT_PULLUP);  // Built-in pull-up resistor
    pinMode(ledPin, OUTPUT);
}

void loop() {
    int buttonState = digitalRead(buttonPin);

    if (buttonState == LOW) {      // Button pressed (LOW with pull-up)
        digitalWrite(ledPin, HIGH); // LED ON
    } else {
        digitalWrite(ledPin, LOW);  // LED OFF
    }
}

Multiple LEDs: Traffic Light

int red = 11, yellow = 10, green = 9;

void setup() {
    pinMode(red, OUTPUT);
    pinMode(yellow, OUTPUT);
    pinMode(green, OUTPUT);
}

void loop() {
    digitalWrite(green, HIGH);  delay(3000);  // Green 3s
    digitalWrite(green, LOW);
    digitalWrite(yellow, HIGH); delay(1000);  // Yellow 1s
    digitalWrite(yellow, LOW);
    digitalWrite(red, HIGH);    delay(3000);  // Red 3s
    digitalWrite(red, LOW);
}