Lost and confused. Bought on Ebay in 2020 as
"Pro Mini 3.3V 8M 5V 16M Atmega328P Board Replace ATmega128 For Arduino Nano"
Here are photos of both sides, can't tell if I run this on 3.3V or 5V. Any help?
TL;DR: you've got a 5V board.
One approach is to apply about 7V between RAW and GND, and measure the voltage regulator output between Vcc and GND. That won't necessarily reveal the oscillator frequency, though.
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I have this problem all the time, as I have many different ones, and they don't bother to check the box on the silkscreen. The only difference is the voltage regulator, so the VCC pin output is either 5v or 3v. The processor chips are the same, so 5v will not hurt them like other true 3.3v processors. I use to worry if my ftdi was 3v or 5v, but the 5v ones work just fine on a 3.3v board. maybe not any 3.3v only stuff connected to its pins though.
Also, the blue ones are 3.3v, the red ones are 5v...except for the ones that aren't. lol
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