It's easy... to make. Thinking of it is not so easy. It's really hard to explain. You may have to do what i did, and look at others, in other calculators... e.g. mine, and just think about where sparks pass and don't pass. If we all tell you exactly how to do it, or give you one, you will have a creation made by other people, and you won't have any self fulfillment.
everything in my calc was made from scratch, but i think the original idea for the bin-dec may have come from Massey101. I'm not sure