You should be able to use either one of them. However, if you use the one without bootloader then the board will no longer be Arduino-compatible and you won't be able to use the Arduino IDE to recompile and upload the program. Here is yet another detail to complicate things: you must set the AVR's fuse bits properly as well. For use with bootloader they should be: Extended fuse: 0xcb High fuse: 0xd8 Low fuse: 0xff avrdude command to do this: avrdude -p atmega32u4 -P /dev/ttyACM0 -U lfuse:w:0xff:m -U hfuse:w:0xd8:m -U efuse:w:0xcb:m -U lock:w:0xff:m Without bootloader the high fuse bits value changes: Extended fuse: 0xcb High fuse: 0xdf Low fuse: 0xff avrdude command to do this: avrdude -p atmega32u4 -P /dev/ttyACM0 -U lfuse:w:0xff:m -U hfuse:w:0xdf:m -U efuse:w:0xcb:m -U lock:w:0xff:m Hope this helps.

I read somewhere - maybe a misunderstanding - that it should be : the hexfile WITHOUT bootloader ?!?!?

There seems to be some misunderstandings here. When we said the Arduino IDE does avrdude for you, it was supposed that you compiled the _source_ code inside the Arduino IDE and then let it upload it to the board, not that you would use the Arduino IDE to upload a HEX file. Compile & upload (by clicking the Arduino IDE button with the arrow pointing to the right) is the standard and easy Arduino procedure of programming Arduino-compatible boards. Uploading precompiled HEX files is a bit more complicated. The Arduino source code for this project is available for download on this page, allowing you to go the easy Arduino way. Where the article said that a serial console has been included in the firmware, it meant that the solder station firmware outputs data on a serial port. You can visualise this data in a Serial Terminal program. This is _not_ the same thing as a Linux terminal window. The Arduino IDE integrates a suitable Serial Terminal that you can use here. You open it by pressing ctrl-shift-M (at least on Windows) or by clicking the little button at the top right of the IDE, with the tiny looking glass. So, now that we understand better what you are trying to do and on which OS we can maybe improve our example avrdude command a bit (still untested):  avrdude -p atmega32u4 -P /dev/ttyACM0 -carduino -D -U flash:w:Soldering-Station.ino.with_bootloader.leonardo.hex:i Hope this helps you a bit further.  

Los firmware. Donde los consigo? 

Los archivos de este proyecto no se encuentran aquí? O como los consigo?