The Arduino Pro Mini is a great alternative to a full sized Uno or Mega when you have a small space you want to fit an Arduino into or for when you’re ready to make that project permanent.
It’s got all the same features of an Uno at a fraction of the size(18x33mm) and cost, unfortunately that means that due to the lack of space there is no USB programmer on board and you have to use a separate USB to Serial adapter to program it.
On the bright side if you own another Arduino which chances are you do then you can program a Pro Mini with that very easily. For this post I’m using an Arduino Uno but you can also use a Mega, Mini or Leonardo to upload code to it.
Here is a diagram of how to connect your boards with a 5v version pro mini
If you have a 3.3v version of the Pro Mini you HAVE to use the 3.3v line on your Arduino or you will fry your board.
Now plug in your Arduino and open your IDE and select Arduino Pro Mini from the board drop down menu.
If all has gone well you were able to upload code and are good to go.
If you would like to buy a clone compatible with the pro mini you can do so here for under 3 dollars.