How to start a sketch already uploaded to Arduino

Hi, I am just starting with Arduino and need help. Can anyone tell me how to run a sketch already uploaded to Arduino , via program(python , java, or any) without having to re-upload the sketch every time or pressing the reset button in the board.

The last program uploaded is ‘always’ running unless you specifically stop it, or remove power.

The trick you’ll learn is to provide a trigger (a switch or other event), that will let it continue running until it’s ‘paused’ again.

Consider loop() is running continuously, and testing those ‘triggers’ to see if you need to run code or not.

If NO, then just loop around again continuously until YES - then perform whatever tasks you need and stop when it’s done… until your looping test says ‘YES, do it again ’.

Please post that code, autoformatted in rhe IDE using code tags, </>, here.

Thank you , I will try to add a logic to listen to and run.

The uploader (avrdude) can reset the Arduino through USB to prepare it for an upload. On an UNO/Nano/Mega it uses one of the serial interface signals (Request To Send?). On the Leonardo/Micro there is something about setting the baud rate to 1200 to force a reset. Your Python/Java/C program should be able to do the same if it knows what model of Arduino it is connecting to.

Thank you all for the answers , I resolved by adding serial listener and passing in a serial via python and adding a condition to run it.

  if (Serial.available() > 0) {
    // read the incoming byte:
    incomingByte =;
    if (incomingByte == '1')
      for(int i=0; i<=4; i++)