So far so good, but what about the VIN? Would it be right to power the arduino through that, now when I got a regulated power supply for it? And what about the step from 27V to 5V?
Going from +27vdc through a 7805 type regulator is a large voltage drop and depending on how much +5vdc current you will be drawing on the Arduino board, the external 7805 might run very hot. If you are only powering the board then the current draw will be very low and it would work fine. However if you are using the Arduino +5vdc pin to power lots of LED's, etc then the current usage will be much higher.
To keep it simple, I would just try using your external +12vdc regulator's output and wire it to the Arduino's Vin pin. Again the current draw of the Arduino board and external components become the limiting factor as to how hot the +12vdc regulator will run at.
Be sure you wire backward connected diodes right across the relay terminals to keep transient spikes from propagating into your power circuits.
Thought that when everyone did it like that, it must be the way it was done with it. But also easy for me to add one over each coil, just wonder why everyone does it the other way.
Good engineering practice and/or common sense should never be substituted by "following the crowd"
Wasn't it the leming who said "I'm jumping off this cliff because everyone in front is doing so" Good engineering practice and/or common sense should never be substituted by "following the crowd"jack
and then how it was done with the ULN2803, I figured that must be the way. But apparently that was 'wrong' to search in an attempt to solve the problem too.