int del = 100; //defining delay value (0.1 second) void setup() { // put your setup code here, to run once: pinMode(2, OUTPUT); //defining pins (2-13) as Outputs pinMode(3, OUTPUT); pinMode(4, OUTPUT); pinMode(5, OUTPUT); pinMode(6, OUTPUT); pinMode(7, OUTPUT); pinMode(8, OUTPUT); pinMode(9, OUTPUT); pinMode(10, OUTPUT); pinMode(11, OUTPUT); pinMode(12, OUTPUT); pinMode(13, OUTPUT); } void loop() { // put your main code here, to run repeatedly: for (int i = 2; i <= 13; i++) { // For loop will count from 2 to 13 to turn ON and OFF every single pin digitalWrite(i, HIGH); // Turning ON 1st output (LED) delay(del); // LED is ON for 20ms before turning OFF digitalWrite(i, LOW); // Turning OFF 1st output (LED) delay(del); // Waiting 20ms before running the 2nd cycle of FOR loop } for (int i = 13; i >= 2; i--) { // For loop will count from 13 to 2 to turn ON and OFF every single pin digitalWrite(i, HIGH); // Turning ON 1st output (LED) delay(del); // LED is ON for 20ms before turning OFF digitalWrite(i, LOW); // Turning OFF 1st output (LED) delay(del); // Waiting 20ms before running the 2nd cycle of FOR loop } }