Pro Mini flashing red LED, seemingly died

I am quite new to arduino, so I wouldn't be suprised if this is caused by a mistake I made.

I very recently recieved my first Pro Mini's (a 5v and a 3.3v pair). After soldering on the provided headers, I wasnt sure which was which (perhaps I missed a label or some sort of mark?). So to figure that out, I tried to power one up through its Raw pin, and measure the voltage on the Vcc pin.

I connected it to my power supply, set it to 9v and enabled the output. The green LED lit up as expected, but I also noticed a red LED flashing as well (at pin 10). I thought nothing of it, and went on to measure the voltahe, but as soon as a probe touched the GND pin, both LEDs went out. Not even pressing the Reset button helped. Needless to say, I did not try this on the other one.

I read a few posts about the flashing red LED, but noone had their Pro Mini die on them.

Can someone tell me what I did wrong? Or was it a faulty unit? Any help would be much apprechiated. Thanks in advance.

There are often two or more LED's (depending on source of the board)

Almost always one for power and almost always one connected to pin 13 that should flash if the board was built by a reputable supplier (even clones)

Almost all come with the BLINK example pre-loaded so it is not uncommon to see a blinking LED.

Unless your probe touched a wrong pin against another causing a SHORT then it should not have dimmed the LED's or stopped them working.

In cases like this double and triple check connections before moving on.


I'm fairly certain that I didn't short anything with the probe, but explanation would make perfect sense though.

I tried connecting 3.3v directly to the Vcc pin just now, and it works, so somehow I killed the voltage regulator. Is there anywhere I can buy one and replace it, or do I just have to use an external buck converter from now on?

If its a clone you may still be able to read the printing on the device and use that as the component reference. If its a TRUE Arduino then the schematics from the products section of this site will tell you what you need.

Either way there are plenty of sources on the internet and maybe even closer to you if you are in a larger metropolitan area.

The pro mini was retired BTW so I suspect a clone.

Just make sure it also matches the footprint of the original.

A useful snippet of information here