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[on:]

My Duemilanove is hardwired in to aquarium controler. How can I upload sketches and not disconect all controller from power . I understand as there is conflict(or  whatever bad thing) between 12VDC and 5v from usb coming from my laptop.

 Thanks

[Reply #1 on:]

As long as your hardware doesn't interfere with Pin 0 and Pin 1 (Serial I/O) you should be able to plug in the USB cable and upload new sketches even while the external power is still selected.  The power jumper only changes the power source and doesn't disable USB when you select external power.

[Reply #2 on:]

I understand as there is conflict(or  whatever bad thing) between 12VDC and 5v from usb coming from my laptop.

Where did you get that misunderstanding?

[Reply #3 on:]

Yes, that is a basic misunderstanding. The Arduino board is designed to still work with both external voltage applied and plugged into USB port. The auto-voltage sensor circuit on the board will default to use external power if it's available.

Lefty

[Reply #4 on:]

Hi . Thanks for input. That all makes sense now.

Where did you get that misunderstanding?

Hehehe. That my own invention

I have hoked relays on 1 and 2 pins(was out of free pins).  Probably that's why my arduino quits whenever I plug USB in to it.