void setup() {
 
 /*NO NEED to write anything.for digital pin 13(or L).(L will glow
 if added just in loop). L is connected to digital pin 13*/
 pinMode(1, OUTPUT); //TX will forever glow if not added anything in loop
 pinMode(0, OUTPUT); //same as TX
//PINMODE 1 and 0 is must to write to operate TX AND RX
pinMode(3, OUTPUT);//not connected to anyone,so doesnt glow anything
pinMode(4, OUTPUT);//same as 3
}

void loop() {
  digitalWrite(13,HIGH);
delay(500);
digitalWrite(13,LOW);
delay(500);

}