How can I tell if my Arduino Pro Mini is 3v3 or 5v?

Development Tools IDE 1.x
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I have this Arduino Pro Mini from years ago.

The only branding on it are the words "Arduino Pro Mini". I'm 99% sure it's genuine because the only clones I can remember buying were from SparkFun.

Either way, the board has markings on the back for 5V or 3.3V yet nothing is checked.

Is there an easy way to tell which one of these I have?

Thanks.

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Plug in and measure Vcc vs. GND. That directly shows 3.3V vs. 5V.

Edit: if you don't have a separate meter, you could do this:
Make a voltage divider with 1x 1k resistor and 1x 10k resistor. One end of 1k to GND, other end to one end of the 10k resistor and the loose end of the 10k resistor to Vcc. Now connect the junction of the 1k and the 10k resistor to one of the analog pins; A0 should do fine. Make a new sketch in which you set the analog reference to 'internal' (which is 1.1V) and perform some measurements by doing analogRead. Average out a couple of measurements to get a good reading. Then calculate back on the basis that 1023 = 1.1V and 0 = 0V. A 3.3V Mini should give a reading of something like 307 whereas a 5V board would read around 466.

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Is there a marking on the top of the crystal? Say 8.0 or 16.0 ?

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That's a good idea. I will try that.

Thanks!

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I looked and there appears to be something. Even with a magnifying glass, my old eyes couldn't make it out. I might dig out the microscope to read it. lol

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