Depending on how the µC is used, that would be an "easy" solution...but...it would be tying the low voltage circuit directly to the 120VAC circuit. Very dangerous. No isolation like the transformer or opto-isolator would provide.
The capacitor/opto-isolator circuit is a voltage divider of sorts, as it uses the phase shift in the reactive vs resistive components to reduce the voltage. The series RC circuit dissipates less power (less heat) than the equivalent straight resistive voltage divider.
I've kinda been wondering why you guys have been beating me up about a comment I made. Maybe you guys think I was inadvertently stating that the solution to the problem that was found here was wrong or inferior. This isn't the case! I was just attempting to give an additional solution to the problem.
The capacitor/opto-isolator circuit is a voltage divider of sorts, as it uses the phase shift in the reactive vs resistive components to reduce the voltage. The series RC circuit dissipates less power (less heat) than the equivalent straight resistive voltage divider.
I've kinda been wondering why you guys have been beating me up about a comment I made. Maybe you guys think I was inadvertently stating that the solution to the problem that was found here was wrong or inferior. This isn't the case! I was just attempting to give an additional solution to the problem.
I don't think anyone here is "beating [you] up". I know I'm not trying to. We just try to evaluate different solutions from the view point of what we know and what the OP has told us. For example, my circuit would be useless if the OP wanted to know that an "unloaded" (which was not stated) 120VAC circuit was activated. The SSR, with a triac output, would not work to control a DC circuit, because once a thyristor is turned-on, it stays on. Though it could work if the OP only needed a one-time indication, and never cared if or when the AC circuit was deactivated. Your voltage-divider/rectifier/opto-coupler solution will work, except that it does produce more heat than most people want to deal with.
So, don't take comments on your suggestions as a put-down. Look at this forum as a great place to through out ideas and learn new ones. There are many times I've had to learn of the down-side of what "I" thought was a perfect solution.
If you want to see "beaten up", spend a few days watching the Usenet forums.