Arduino Pro Mini mysteries

Anyone have a clue what those four unlabelled pins in the middle are?


A4, A5 for the two next to A2, A3, and A6, A7 for the other two.


Awesome, cheers. The official page was no help

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Hi Kimmo
Straight from the Sparkfun page

Which is linked from The Core product page

Cheers Bob
PS: On looking up the Arduino site this seems to be a retired product. A pity as it is a great little board especially where real estate is ar a premium. I have one driving an array of XMAS lights.


Hey Bob, I meant this page.

You can still find the Pro Mini on eBay, if you want to stock up.

I’m still getting confused… I successfully loaded the servo/knob example sketch; I have a little servo happily pointing where I tell it to via a pot, when it’s powered off my phone via the FTDI cable, but when I hook up a 2s LiPo instead to the raw power input, it doesn’t work - even though the power light is on and I can find 5V on VCC like that, so obviously the voltage regulator is working.

Can’t figure that out…

Hi Kimmo
I agree the page you linked (Arduino page) does not help much.
For what it is worth I programmed mine using the Arduino IDE with from a computer using the Sparkfun FTDI breakout which works well. In its final resting place I have a 5V 14A MeanWell supply and I power the Pro Mini from that via the 5V pin which can be used as a 5V output or input pin. Works fine.

Will have a look at getting a couple more if they are going to be retired. I already have 1 spare but they are not that expensive and are a handy little unit.
Cheers Bob

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Yeah, in this instance I’ll be able to power it the same way, since the brushless motor controller I’ll be using is for RC use and has a regulated 5V output, but it refusing to work on raw power while successfully getting 5V from it is super weird, wish I knew what was going on here :face_with_diagonal_mouth:

Just tried the fade LED example on this same board and that works fine off the battery. The plot thickens…

So I did my own servo sketch, with a little start-up tone for a delay (a RC ESC will replace the servo, and requires zero throttle for a bit), and a serial print for the servo degree value, so I can see what that is when I find the optimum motor speed for my project with the pot, which I’ll ditch when I know that value.

It works fine off the FTDI cable, but when I hook it up to the battery via the raw power input, it’s looping the setup section of the code (which contains the tone) without moving on to the loop section. Weird, huh?

Sounds a bit strange.
Do you have Raw power and FTDI cable connected at the same time? I don’t know what would happen if you did. Might be worth checking all the ground connections. The 7.4V from the LIPOs should be OK but I wouldn’t go any lower as the 5V regulator needs a bit of headroom.

It might be worth trying if you can go up to about 8V or a bit more on the Raw pin or try applying 5V to the 5V pin in case there is some conflict. Don’t apply raw volts and 5V at the same time though.The Pro Mini is a stripped down Arduino and might not have much protection with conflicting voltage inputs. Make sure you only have one at any one time.
Cheers Bob


Yeah, only using one power method at a time. And the LiPo is putting out a hair over 8V fully charged. I’ll dig up a USB A wire and throw some connectors on it for 5V without using the FTDI cable to investigate further…

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This article includes the complete pin diagram, specs and other details about the Arduino Pro Mini. You can keep it handy.

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That one has the 4 holes in question in a different position.
Cheers Bob

Hey Kimmo,

Unsure if you have tested how much current is being supplied here, the Arduino Micro can consume up to one full Amp with peripherals attached. If you are running servos on top of that, its possible it just doesn’t have the current to efficiently run everything, though I would assume with a 2s or dual 18650 setup, it should be sufficient.

You just never know sometimes and its well worth checking to see if that’s the issue.


The weird thing is, I had this same 1Ah RC battery pack powering one of these Pro Minis running two of the same servos like ten years ago. I should probably look at how much the voltage sags when I hook it up, but it’s hard to imagine there isn’t still a decent amount of headroom.