Right now, I am in a lot of trouble. I've made the following diagrams (in Eagle) and before routing them in Eagle, I've tested everything on breadboard EXCEPT I didn't add the eight diodes to the output lines of the L298, as this was a request from my electronics teacher after I pulled the breadboard apart. The fact is, that everything was working and playing nice without the eight diodes when setting it all up on breadboard.
Here is my Eagle schematics for you to see: (would you prefer another link, than my Dropbox?) https://www.dropbox.com/sh/ytt9y21tavdpgsp/i_TjfbBq2J
I have connected everything on the pcb I produced in Eagle, and checking every connection twice against my Eagle schematic, all connections seems fine (equal to my schematic). But here comes my real problem. When supplying 12v for the L298 through the connector named "12v" I can measure about 11.5v in the connector called "GND_5V" - which is the connector providing a common ground and 5v for my two buttons and the hall sensor.
I really don't understand why I am able to measure 11.5v in the connector going to the Arduinos 5v and gnd? - those 11.5v obviously killed my Arduino... If I leave my unipolar 6 leaded stepper unconnected from the "STEPPER"-molex, I can't measure any voltage traveling from the 12v line to the Arduino's 5v line, but when I connect the stepper (I have checked that it is wired up correctly) I can measure the 12v traveling into the 5v line, which has, as i said, killed my Arduino.
My own guess is that it has something to do with the eight diodes, as everything worked great without them when trying everything on my breadboard. But I really don't know if the diodes is my real problem.
Can someone please help me point out, what the failure(s) in my schematic are? as I can't figure them out myself. That would be wonderful, if you need any more details please ask for them :-)
Thank you best regards Johannes T. Nielsen