User control change event

I want a user control contains only one dropdownlist and has some properties like ID.
I want to invoke this control from several aspx pages and passing those property Id from aspx page.

<UC1:Mydropdown id="Ratingdropdown" runat="server" AddedDTKey="" ScreenName="Background"
Description="Rating" TemplateID="1" ID="BK"></UC1:Mydropdown >

When I change value of this dropdown from invoked aspx page, I want to open a modal popup window via javascript/jquery and selected value of dropdown should be displayed in a textbox.

how can I do that? Can you please provide the sample?