I think you refer to this one:

http://www.avrfreaks.net/modules/F...oc2508.pdf

You will find the following passage in it:


By these lines the authors admit, that their circuit cannot withstand "surges"! And they don't want to disclose how to enhance the design to be able to withstand surges.

The last sentence is sheer cynicism, especially as this article addresses
hobbyists, who usually don't know how to properly handle mains voltage.

Let me remember, that short overvoltages on mains up to 6kV can be found quite often. Only very special resistors, like the VR25 from Vishay, can withstand such high voltages, best two of them in series.

And, of course, these spikes shouldn't be allowed to flow directly into the protection diodes of microcontroller, but must be shunted into separate protection diodes. Not because the built-in diodes couldn't withstand the currents, but because the involved components and the layout provide enough stray capacitance to inject unsane spikes into the micro directly from mains! The consequences are latch-up, lock-up or even destruction. Never wondered why these thingies are damaged after a few months??

So, connecting mains directly to the microcontroller via unsuited low cost resistors is worst design practise and results in a time bomb. And, the best, the authors themselves explicitely say, that you shouldn't do that...

Kai Klaas