Using usercontrols outside a project

Hi,
 
I need to know how to access a usercontrol outside a  project but in a same solution.
 
here's what i had in mind.
 
 CommonSolution
-Project1
-Default.aspx
-Project2
-usercontrol1.ascx
-Default.aspx 
Now how do i access usercontrol1.ascx to be used to Default.aspx in Project1
...