focalist:
right, but if optically isolated, that becomes a non-concern as there is no shared ground.. correct? The optoisolator's LED doesn't share a ground with the transistor (wouldn't be isolating then, would it?).. the only thing to pass between the led and phototransistor sides is light...
Correct, the use of an optical isolator means that you will not have a direct electrical connections from the AC mains and your arduino ground pin. You will however still be controlling a pretty high voltage loop. It should work, as I said, it would however not be as I would personally construct it. Be sure your switching transistors/opto device is/are rated well above the peak DC voltage that the rectifier will produce. Perhaps a complete schematic drawing of your proposal will help in looking for possible hazards or risks?
Lefty