The Arduino Pro min is a cut down version of a full size Arduino. The number of components used have been reduced to a minimum to reduce the size. The most important thing missing is the USB to TTL chip that allows Arduino users (such as the UNO) to upload programs from a PC. You will almost certainly have to solder the header pins to the board. I found the easiest way was to push the pins into a small solderless breadboard then push the Pro Mini onto the header pins and solder. This ensures the pins are straight, but don’t forget the breadboard is plastic so allow each joint to cool before soldering the next pin.
If you are uncertain which version you have you can check the voltage regulator (the middle of the three black components on the right, in the picture above), the 5volt version has a voltage regulator marked K850, although I have seen GL50. The number 5 will be in there somewhere!
Another way is to power the the board with 7 to 12 volts DC at the RAW pin (NOT Vcc) and measure the voltage at the Vcc pin – it will either be 5 volts or 3.3 volts.
Programming the Pro Mini::
The communication port for the Pro Mini is the six pins on the right hand side of the board (in the picture above). An FTDI (USB to TTL) adaptor is required. My Pro Mini is the 5volt 16 mHz and my converter has 5 volt signal levels, but if you have the 3 volt Pro Mini you will either need a FTDI converter with 3volt signal outputs (some can switch from 5 volts to 3.3 volts) or use a bi directional logic level converter. The Tx and Rx connections are NOT swapped between the Pro Mini and the FTDI connector and are shown below (the DTR line on the FTDI connector is used to reset the Pro Mini). Some of the clones of the FTDI adaptor, those using the CH340, do not have CTS or DTR (RTS) broken out and so will work. Note that CTS can be left disconnected.
Once the FTDI is connected to the Pro Mini plug it into the USB port of your computer. If this is the first time you have used the FTDI adaptor the driver should be installed, just follow the on screen instructions. Open up the Arduino IDE, select the Arduino Pro Mini from the list of boards and select the port the FTDI adaptor is connected to. Programs can now be uploaded as normal. Try uploading a simple program like Blink to check all is well, although be aware that some suppliers will load the Blink sketch into the board to test it.