> Well, based on what you and Roman have input here the following would
> work for 240VAC (assuming shitty Australia and New Zealand power);
>
> * Two resistors in series - say 3~4M each
> * up to say 100nF Metalised cap (rated at 400V) strapped across Zener

This will not work for a very different reason that those brought up so
far.  Two "3~4M" resistors in series gets you at least 6M ohms.  6M ohms
and 100nF make quite a low pass filter.  If my quick and dirty
calculations are right, you end up with about +-1.8V from 240V 50Hz mains.
With 6Mohm small leakages that could normally be ignored become
significant.

> * 5.1V Zener from Pin to GND - only small wattage, I assume is OK

The zener leakage won't let the voltage get anywhere near 5V with 6Mohm in
series.


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