I have many Arduino Mini Pro, every from different seller. Few are 3.3v and most 5v.

I had to clean table for Xmas and now I do not have any idea how to identify 3.3v Arduinos.

They do not have any marks. I bought them on ebay.

I know 3.3v has 8mhz clock but only one my arduino has big crystal with 16.000-30.

  • Look at the central chip. 5V pro mini has 328p printed, 3V3 pro mini has 168.
    – user2497
    Commented Mar 31, 2019 at 11:29

10 Answers 10


The regulator should be marked K850(5.0V) or K833(3.3V).

A 5 volt part has a 16MHz resonator may be marked with "A1" or "A'N"
A 3.3 volt part has a 8MHz resonator may be marked with "80'0"

As others have indicated, you can apply up to 12V at the RAW pin, and measure the output of the regulator.

  • I have some 3.3V with resonator marked "80.0M" and some 5V marked "AE"
    – Daiku
    Commented Apr 15, 2022 at 14:58
  • I have resonator marked as 80'Y, (Atmega 328P AU 1710) And I measured the output ACC-pin (same as VCC) is 3.3V
    – LAamanni
    Commented Feb 5, 2023 at 11:31

Consideration (go down for the actual solution)

If you have a lot of different China-Arduino-clones, the easiest way will be to just test it. The cloned Arduinos use a lot of different voltage regulators and often you won't even be able to find datasheets for them on Google.

Someone here provided an Arduino sketch which helps you identify the frequency with which the Atmel is running and the comments say "it is the safest method". But why? It is complicated (you need to connect the USB-TTL-converter to each of the different Arduinos and sometimes they have different pinouts, so it's error-prone - you could easily fry your Arduino if you connect the wires to the wrong pins)...


I think the safest way is just connecting a voltage in the range of 6V-12V to the RAW-pin and measuring the (regulated) output of the VCC-pin.

There is no reason why this would kill your Arduino (in case your Arduino isn't broken - which would be good to know anyway; and of course you shouldn't short anything).

I've done a short sketch of how you should connect your Arduino Pro Mini: Connecting the Arduino Pro Mini

  • RAW <---> 6V-12V
  • VCC <---> multimeter (5V = 5V Arduino, 3.3V = 3.3V Arduino)
  • One GND pins to the GND of the power source and one GND pins to the GND of the multimeter (you can take any of the Arduino pins labeled with GND)

Don't worry if your Arduino looks a little bit different, just look for the pin labels. For instance, one of my 5V Arduino Pro Minis looks like this, but also has RAW-, VCC- and GND-labels (just one of the GND pins is at a different position, make sure you check your Arduino pin-labels): Different Arduino Pro Mini layout


You don't want to use the method above for some reasons? Then you need sharp eyes and some luck that the following list includes you components (some of these are collected from the other posts, some of them from my Arduinos):

  • 3.3V:
    • Voltage regulators: KB33, S20K, F34V, L0RA, L0RB
    • Oscillators: 80e., 80'0
  • 5V:
    • Voltage regulators: KB50, L05, L0UA, L0UB
    • Oscillators: R160JAC6s, 16.000-30, A1, A'N, A'a

(no limit or warranty for these details...)

More Suggestions

Some of the Arduino Pro Minis come with MHz and V markings on the back. In my case, none of these were checked. Make sure you check them as your new Arduinos arrive, so you won't have any problems identifying them later! Arduino Pro Mini markings

  • 11
    @Paul -> Sure this question is old, but still relevant - and that's the charm of stackoverflow / stackexchange -> you often can choose between multiple answers and most of the time there is still an up-to-date answer. Most people (just like me) will come here via Google, but none of the answers satisfied me completely, so I went to write my own one (and for sure I will come here again in the future when I need this again and forgot how to do it again ;) )
    – mozzbozz
    Commented Aug 16, 2016 at 13:01
  • S2RA is also 3V3.
    – AI0867
    Commented Nov 18, 2021 at 12:56
  • Warning - some Chinese Arduino clones do not accept higher than 11.5V and you can fry them when testing. So, to play it safe, use something lower in RAW. Commented Sep 15, 2022 at 17:22

The safe way (i tested it). First connect to 3.3V USB-TTL output first (also 5V Mini can work with 3.3V). Now prepare this sketch:

void setup() {
  pinMode(13, OUTPUT);

void loop() {
  digitalWrite(13, HIGH);
  digitalWrite(13, LOW);

Choose "Arduino Pro/Mini" with 5v/16MHz board and upload sketch. If you see that the blinking is correct (10 sec ON/10sec OFF) - you have 5V/16MHz Arduino. Otherwise it will blink at 5 sec intervals instead of 10 sec.

  • Thanks, I thing this is the safest method ;) (and you don't need a magnifying glass!) I just tested it and the 5V mini can work fine with 3.3V Commented Mar 6, 2016 at 1:00
  • Thank you also. For me this is the simplest solution and works with arduino clones also.
    – cristi
    Commented Jul 7, 2016 at 20:15
  • 1
    Otherwise it will blink at 5 sec intervals instead of 10 sec. should be 20 seconds since, when running at 8MHz, things take twice as long versus 16MHz. Tested experimentally.
    – Aloha
    Commented Sep 3, 2017 at 14:19
  • I hope you are all wrong otherwise it would mean the specifications of delay() function are not acurate since they would depend on the crystal frequency, I believe the function takes the clock frequency into account...so this test cannot be relevant...10000 ms would run the same delay on any arduino model (hopefully)
    – Phil
    Commented May 2, 2018 at 16:34
  • No, they are not wrong. When you program your sketch into the Pro Mini you have to select whether you are using the 3.3v 8Mhz part or the 5v 16Mhz part. The timing delays are adjusted accordingly. You can, in fact, program Blink into a 5v part using the 3.3v settings and it will blink twice as fast as it should and talk to the Serial monitor at 19200 instead of 9600.
    – jksemple
    Commented Jun 30, 2018 at 23:04

Plug it in and measure the voltage on pin 4 of the MCU.

  • Is it safe to plug it in 5v FTDI ?
    – Max
    Commented Feb 8, 2015 at 22:06
  • 1
    The way you ask that, it sounds like what you have isn't a Pro Mini at all, or even a clone. Commented Feb 8, 2015 at 23:08
  • 1
    I have this boards arduino.cc/en/Main/ArduinoBoardProMini Some have a bit different pins.
    – Max
    Commented Feb 9, 2015 at 9:19

Check the label of the voltage regulator, KB50 for 5V and KB33 for 3.3

Voltage Regulator


The only difference would be the regulator and crystal (8MHz on 3.3V). As @IgnacioVazquez-Abrams suggest supply power to the "raw" and measure Vcc.

  • Supply 5v to RAW and measure VCC ?
    – Max
    Commented Feb 10, 2015 at 11:34

On my board the regulator is marked L0UA which is the code for a 5V regulator type LP2985, L0UB is also 5V.

A 3.3V regulator would be marked L0RA or L0RB.


Some clones on ebay labeled "the simple" come with 4B2X. They are 3.3v but the maximum voltage is 10v instead of 12v so beware to not burn it out.


AVR isn't specified to work at 16MHz at a 3.3V supply; it only can work at this high frequency at 5V supply. So you'll be able to tell them apart from reading the crystal. (It's possible, but unlikely, that you have a 5V version working with an 8MHz crystal. In any case 16MHz is safe for 5V.)

  • I wish to know if I have 3.3v 8Mhz or 5v 16Mhz. (The correct answer right is now is The regulator should be marked K850(5.0V) or K833(3.3V). But regulator is too small I can't see K850(5.0V) neither K833(3.3V).
    – Max
    Commented Feb 10, 2015 at 11:33
  • Please post a photograph of it. Also, try shining a light on it at a 45-ish degree angle and attempt to read the reflection, rather than the ink itself.
    – Jon
    Commented Feb 12, 2015 at 18:27

On my board, the voltage regulator is marked F34V. On the back is listed "The Simple" and Arduino-Pro-Mini" I purchased it as a 3.3 v board off e-Bay, and I assume it is. I have not yet applied vcc to it and measured the output.

Not the answer you're looking for? Browse other questions tagged or ask your own question.