This is the groundwork for the half ... full adder logic in terms of AND and XOR. That’s really all there is to the logic of the adder itself. The next step is to chain the adders together ...

This 2-bit adder was a lot of work to build. It uses a total of thirty-six 555 timers and it does have the option of adding or subtracting numbers. It’s a rather unorthodox use of the part ...