This is an hexadecimal adder/subtractor, with 4 7-segment display for the numbers, I designed the adder/substractor.
I also want to implement a function to turn it into a counter from 00 to FF and again. The adder is a 4 bits adder, I also made the button selector for the numbers, with at first some torches to get the power down to display the number, before using pistons and slimes block. There is also a little system for the +/-, but not that big.
If you're in calculator mode, you simply have to use the buttons to chose the first and the second numbers, and a lever to choose if you want to add or substract. For the counter(whiwh I'll try to finish today), yhere will be a button to increment by one, one to reset and one lever to enable the clock(will be 1 second between each increase).