#define LED 7  // define the led pin
#define BTN 3  // define the button pin

// run only once
void setup() {
  
  // set the led pin like output
  pinMode(LED, OUTPUT);

  // set the button pin like input
  // pinMode(BTN, INPUT);  // if you have an external PULL-UP or PULL-DOWN move '//' in the line below
  pinMode(BTN, INPUT_PULLUP);
}
// run in loop
void loop() {
  
  if (digitalRead(BTN)) {  // run only if the button is not pressed (with PULL-UP)
    digitalWrite(LED, HIGH);  // turn on the led
    delay(50);  // wait 50 millisecond
    digitalWrite(LED, LOW);  // turn off the led
    delay(50);  // wait 50 millisecond
  }  // END if
  
  delay(1);  // wait 1 millisecond
}