//first sketch-01

int ledPin = D3;    

void setup(){
    // declare how the pins will be used
    pinMode(ledPin, OUTPUT);     // pinMode is a function that let’s us say what the pins will do
}

void loop() {
  // on
  digitalWrite(ledPin, HIGH);   // digitalWrite is like a "pump" that can be turn on and off
  delay(3000);  // 3000 ms

  // off
  digitalWrite(ledPin, LOW);
  delay(3000);
}