Raise Event from Javascript in Web User Control

I finally got a user control working that has a HTML drop down list.

The user control is held in a simple parent test page.

The onchange is set to call a javascript function DropChanged() which is in
my user control. I want to keep the javascript in the user control so that
I can add it to many pages.

But now, how do I signal the parent page that my drop down list changed and
pass back the selectedIndex of the drop down?

I hope I have not travelled too far down the wrong path. Thanks in advance.