2-to-1 Multiplexer. A multiplexer (or mux) is a common digital circuit used to mix a lot of signals into just one. ... In a 2-to-1 multiplexer, there's just one select line. More inputs means more select lines: a 4-to-1 multiplexer would have 2 select lines, an 8-to-1 has 3, and so on (2n inputs requires n select lines).