Hi?
I have a lot of regards about replacing the 7805 by a switching module. If the 7805 is damaged, little or no output voltage would be present, but if the active switch a switching module is damaged, then the 12V would be connected straight to the 5V rail through the inductor, potentially killing the raduino, and even a computer connected to its USB port. Also, a step-down switching module generates a lot of noise on the 12V rails in the RF range.
Remember: a step-down is constantly "shorting" the input, and a step-up is constantly "shorting" the output.?Unless you use a coupled inductor???uk?design, there is no way to have no RF being generated by a SMPS in one of its rails.
That said, on the other hand, the 7805 needs some kind of protection, as pointed out by Gerard, and I believe it can be as simple as a schottky diode.
Rafael - PU1OWL
Rafael Pinto [PU1OWL]