Arduino UNO R3 orange led, (pin 13?)

I just installed an UNO and uploaded a script - no problem there...I needed to change a couple of numbers in the script and upload again. The scripts are fairly short and nothing was happening.

My question is why is the orange led now not lit up?

The led is driven by an opamp that is connected to pin 13. Pin 13 is configured as input when your sketch starts and is high impedance. Either environmental noise or the input of the opamp itself pull the voltage on the pin high and the opamp sees a high and as a result the led is on.

To prevent this, you'll have to actively control pin 13. In software (your sketch) you can set the pin as output and drive it low. In hardware, you can use a pull-down resistor (I think that 10k will do).

Note that e.g. clones often don't have the opamp and you don't see this behaviour; the led is directly (obviously via a resistor) connected to pin 13 and a high impedance [u]input[/u] can't deliver enough current to drive the led.

Thanks, as long as I can upload scripts, I’m not worried about it. That’s just the first time in my experience that the led was neither on nor flashing -