Today I've been messing around with transistors and I thought I' d make a "calculator" entirely out of transistors. It can add numbers up to 15. I'm still working on it and I think I might make an Arduino version, because this one is much less advanced than a real calculator. The circuit that I built for it is called a full adder, and then I'm going to improve it to a 4 bit ripple carry adder. What most people do is use logic chips, but I didn't have the right logic gate in my parts bin, so I had to build the circuit entirely from transistors.