switch small DC voltage with 90v. 30hz AC signal
I see from the comments so far that this circuit requirement is for a telephone ring detector circuit. There is a plethora of information on the web for telephone ring detector circuits. I suggest that you use a circuit that incorporates an opto-coupler to isolate the telephone line from what ever circuit that is used to drive the down wind ringers. The reason for this is that if you directly couple a phone line to circuits that are locally powered you can end up getting a lot of line frequency feedback into the phone line which is not a good thing. Here is one circuit that looks like it could be a good starting place for you.