When I started connecting my Arduino Uno for first project: LED to pin 13, resistor, breadboard, USB connected to Windows Laptop, the orange on-board LED flashes every second and the LED on the breadboard did the same. What program is driving this? I had NOT uploaded any program. Please explain.


It is a preloaded sketch and will be overwritten when you upload your first sketch into the Arduino.

Typically it's the Arduino Blink sketch which can be found here:


Which has the following (functional) content:

// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin LED_BUILTIN as an output.

// the loop function runs over and over again forever
void loop() {
  digitalWrite(LED_BUILTIN, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(1000);                       // wait for a second
  digitalWrite(LED_BUILTIN, LOW);    // turn the LED off by making the voltage LOW
  delay(1000);                       // wait for a second
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  • Thanks all, Should have checked. I had Cura open!!!!! Lessons learned. – Izak Krige Jan 28 at 15:52
  • You're welcome, if it helped you, consider upvoting (the up arrow next to the answer), and if it is the correct answer for you, also accept it (pressing the check sign). – Michel Keijzers Jan 28 at 16:10

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