potentiometre widget

Hello,
I want to make a "potentiometer widget" which can be adjusted by mouse press events. Does anybody have an idea about how can i rotate the potentiometer by using mouse events(not wheel event). How will i use the mouse coordinates to rotate the widget correctly?

Thanks in advance..

Why to invent circle ones more time ?
Look at Qwt widgets

I want to make a draggable widget. I mean ,"keep the widget by left mouse click; set its positon to the desired value; leave left mouse.. " like something...