For Testing a low power battery application on a Uno form factor prototype board, I built this Uno clone kit but omitted the voltage regulator and power led : Helvetino Kit v1.01 - Mikrokontroller Arduino Boards Löt Kits - Boxtec Onlineshop. The description is in German but the picture gives enough of an idea. It is very sparse.
The entire project is here: Arduino IR Learning Remote Control
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FYI
A very good friend to the community, Crossroads (miss him very much), made an Atmega1284 version of the UNO.
He had a jumper to enable disable the power option, no need for automatically selecting things.
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Is he not on the forum anymore?
Do transistors use current?
I can't tell if this transistor uses current or just has a current rating.
He pasted 
.
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Your selection will easily pass 2A at logic level inputs.
I quite like the AO3401 for a P channel SOT23 MOSFET.
About $8.00 for 200 on eBay.
I quite like the IRLML2502 for a N channel SOT23 MOSFET.
About $7.00 for 100 on eBay.
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That is what the Uno uses.
I'll use the AO3401 then. Thanks for the advice!
No, it is better as Rds(on) is less than 47milliohms at Vgs = 4.5V
Yours says 60 milliohms, however, 20milliohms difference isn’t much.
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BTW
Off topic a little.
Urge you watch this for some SMPS considerations.
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