Upgrading an old PCB to include new demands. So, reversed engineered the current old solution. Part of the solution is to check the presence of the Live and Neutral inputs.

Very simple solution with the indicated nets RA3 and RA1 going to a processor for monitoring.

I can’t get my head around why the Diode pair D15 would be connected to the DC 5V rail.

Anyone got any ideas?

 

They are there to ensure that RA1 and RA3 do not go above 5V which could damage the processor.

 

hi Call,
RA1 and RA2 levels will be clamped to a 5V+ one diode voltage level.
E

 

Thank you.
So obvious when it is pointed out. I feel stupid now.
My expertise lies in digital electronics and software.
Struggling to remember my analogue lectures 40 years ago!
Mind you most of them were valve based.