Light switch

// Light switch
// Press the button once: the LED turns on. Again: the LED turns off.

const int BUTTON_PIN = 2;   // The push button is on pin 2 (other side to GND)
const int LED_PIN = 13;   // The LED is on pin 13 (through the resistor)

bool ledOn = false;      // Remembers if the LED is on or off now
bool previousButton = HIGH;   // How the button was last time

void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP); // Input with built-in resistor: loose = HIGH, pressed = LOW
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  bool button = digitalRead(BUTTON_PIN);

  // Only react at the moment you press the button
  if (previousButton == HIGH && button == LOW) {
    ledOn = !ledOn;                       // Flip: on becomes off, off becomes on
    digitalWrite(LED_PIN, ledOn ? HIGH : LOW);
    delay(50);                              // Wait a little to stop button "bounce"
  }

  previousButton = button;
}