Hi, I want to suggest a tiny change to the PWR_SEL jumper.
It's easy to modify an existing board, even easier on a kit.
While it doesn't matter much in the bare configuration, once you stack shields onto the Arduino, it becomes a pain to change from USB power to external and vice versa. Simple solution: replace the jumper with a micro switch. This adds minimally to the cost, but improves usability by so much.
You can see the modification here:
Hope this may be useful to some.
Hi, I want to suggest a tiny change to the PWR_SEL jumper.
It's easy to modify an existing board, even easier on a kit.While it doesn't matter much in the bare configuration, once you stack shields onto the Arduino, it becomes a pain to change from USB power to external and vice versa. Simple solution: replace the jumper with a micro switch. This adds minimally to the cost, but improves usability by so much.
You can see the modification here:
Hope this may be useful to some.
Possibly Corax,
How often do you want to change power source?
The Geek.
Possibly Corax,
How often do you want to change power source?
The Geek.
Apparently often enough to warrant spending five minutes replacing a jumper with a switch 
While developing a transportable application, it's convenient to be able to go from battery to USB power and back without having to either remove the shield or fiddle with a pair of pincers to replace an inaccessible jumper.
I think it would be a sensible thing to change on a fresh batch of Diecimilas as it doesn't call for a board redesign and only minimally increases parts cost, but it's also easy to retrofit a ready made Arduino.
I don't think I get it; you don't have to set the board to USB power just to connect it to USB; if you're developing a battery-powered application, why don't you want to run off the battery (or other external power) ALL the time?
Anyway, this will be better in the next revision of the board.
I don't think I get it; you don't have to set the board to USB power just to connect it to USB; if you're developing a battery-powered application, why don't you want to run off the battery (or other external power) ALL the time?
In my specific case it's because of power constraints (battery, not wallwart). Whenever I have to take the Arduino from the development environment to the project's final place (a bicycle), I unplug the USB, place the Arduino in its cradle and plug in the battery. When after testing there's the need for any changes, I unplug the battery, remove the Arduino from its cradle, and plug it into the USB port. The Arduino is wearing a shield, so the jumper is not readily accessible. A power switch makes all the difference to me
With mellis' post, I hope there's an even easier solution coming up (automatic power source detection?), but for the time being, the modification I made is simple and quick to do. Even if "you don't get it", maybe it's of some use to someone else 
No; your explanation helps a lot. I hadn't considered the case where the battery pack was a physically separate piece from the arduino; I can see how that would be a pain to move back and forth...
You could consider powering the arduino through the USB jack instead of the power jack; that would mean making your battery pack include a 5V regulation circuit, and a USB plug for "output"; using USB jacks/cables for power-only seems to be gaining popularity...
(In fact, I've been wondering if the regulator circuit on arduinos hasn't become nearly obsolete. It seems these days that I can find 5V 500mA regulated power supplies (mostly from/for cell phones) more easily and cheaply than I can find 9Vdc 500mA supplies...)
Using a regulated power pack with a USB connector is another possibility indeed, but as both ways involve power regulation circuitry (and the accompanying efficiency losses), I went for the direct path and saved myself some redundancy - and to be honest, I didn't think of it to begin with :).
While a 5V USB compatible power pack would be the more versatile solution (power your cellphone or whatnot from it), in my eyes my approach was the simplest solution to the problem at hand
Not that it matters anymore with the new duemilanove board, but for those still using the Diecimila, here's a more recent VIP version by Make Magazine's Mr. Cunningham