Circuts from Ben Eater's video on making logic gates from transistors:
https://www.youtube.com/watch?v=sTu3LwpF6XI&t=115s
* Top left - a switch
* Top right - an inverter
* Bottom left - an AND gate
* Bottom right - an OR gate
XOR gate is in a different circuit: https://everycircuit.com/circuit/5044443907096576
