Qsilverrdc:
I have revised the diagram for clarity.I understand that I can't protect everything, but without a second power supply, I don't know how to do better.
If you have any thoughts on the matter, they are appreciated.Thanks,
Rich
Let me put it this way, by using a single voltage source and single ground system you are not adding ANY additional static discharge protection by using the opto-isolator. AVR chips already offer some limited built in static discharge protection using input clamping diodes on all the I/O pins. You would have to better quanitify the additional protection level you are seeking, in a amplitude Vs time format, to see which additonal protection methods might be useful and practical.