The topic has come up several times already here on the board, but has never been answered, so I will describe it again:
1. I upload a simple program via USB while powering the Arduino board with an external 9V power supply.
2. The program runs (LED connected to Pin 13 blinks).
3. I unplug USB. The program still runs, LED continues to blink. Everything fine up to here.
4. I unplug Power, wait a few seconds.
5. I plug in the external power supply, the onboard PWR LED lights up, the LED connected to Pin 13 flickrs for maybe a quarter of a second.
6. Nothing happens. The program does not load, does not start to run.
When I go through the same steps without the external power supply, using USB power instead, the stored program always starts automatically (as would be expected) a few seconds after plugging in USB again.
To rephrase the problem more succinctly: Uploaded programs do not start running when I plug in an external power source.
They do start running when using USB as power supply.
Previous topics concerning, but not answering the exact same question:
- Always have to do a Reset: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1160632445
- Power supply: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1156568369
- Standalone/External Power Supply with USB-Arduino: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1137169795
- Reply #7: DC power for USB Arduino: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1138892708/7#7
- Sometimes Arduino board does not start up...: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1157473262
I have so far tested 3 of our Arduino Boards, will test the rest, but am not very optimistic as it does not seem to be a fluke, but a common problem... and a very inconvenient one, as it makes using the boards away from a computer pretty much impossible.