But why reduce Zener to 4.7V?


i saw
the 5V model supplied with the unit
and assumed an over 5v alarm was indicated




Out of all the PICs I'm looking at, the one with the lowest max VDD was 6.0V; hence why I chose 5.6V Zener as "fuse".
if you use a 5.6v zenner in my cct then the red alarm led will be lit at 6.5 v

if you want the zenner to be a crowbar and blow a fuse, there are better ways.

https://www.ebay.com.au/itm/203643422252

i use these things , 3-15 v in adj to 3.3 or 5v out or whatever @up to 5w , cheap with no problems