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By harrys
I have a number of arduinos pro mini both 5v and 3.3V and today I wanted to use one of them and unfortunately it did not have any markings on the bottom side as to which version it is...

is there any other way to distinguish it?

By esklar81
Off the top of my head, how about:
  1. checking the part number printed on the microcontroller
  2. checking which voltage regulator is installed
  3. applying a raw input voltage and measuring Vcc


SparkFun appears to provide the same schematic for the two versions, with the befuddling note "Vcc = 5V or 3.3V output". It's not at all obvious from that what determines what voltage you get. A bit of investigating into the regulator, though, leads to this, which suggests that the voltage is part of the regulator's part number. I failed in my attempt to read regulator part numbers in the photos on the product pages, but you may have some luck with the parts in hand.

Happy Hunting,
By tecoist
esklar81 wrote:...which suggests that the voltage is part of the regulator's part number.
It is indeed. The microcontroller is the same on the two boards, but the voltage regulator and the crystal are different (the 5V board should have a 16MHz crystal, and the 3.3V board should have an 8MHz crystal).