Kai:

This is a very clever concept. I've gone off and implemented a different type of circuit that uses a resistive divider but then shuts off the opto coupler when the peak voltage exceeds a certain threshold. This scheme has the attribute of dramatically lowering the power consumption on the resistors that feed the opto. In this regard it is very much like your circuit but with on/off control in a forward manner instead of your feed back scheme. My scheme takes a couple of high voltage transistors but they are lower cost than having to use two opto couplers per AC line detect.

Thanks for taking the time to ponder the problem and make a great suggestion. I really miss this type of good solid technical discussion on this web site instead of all the petty crap that typically goes on here now.

Michael Karas