[Geoff45 (Geoff)]

Originally Posted by joe_smith

http://www.astropix.com/HTML/J_DIGIT/COMP2.HTM

Just the thing you want to do and it dose it with M42 as well

This is an old article by Jerry Lodriguss. He does something similar with more bells and whistles in his ebook, which is well worth getting.
see http://www.astropix.com/GADC/INTRO.HTM