2-to-1 multiplexer

Two signals walk into a circuit. A selector decides which one walks out. A ready-made routing circuit you can open in the TorchAnvil simulator.

Picking one signal out of two

A multiplexer — "mux" for short — is a traffic director. You feed it several data inputs, plus a selector that says "this one, please," and it forwards exactly that one to the output. This is the smallest version: two inputs, one selector bit, one output.

How it works

The whole thing is one formula:

Out = (A AND NOT Sel) OR (B AND Sel)

Read it as two gated paths meeting at an OR:

• The A path is alive only when Sel = 0 (NOT Sel is true).
• The B path is alive only when Sel = 1.
• Exactly one path carries signal at a time, so the OR at the end just forwards whichever one's awake.

You could build the same thing with a single Multiplexer component in one node, but wiring it from AND/OR/NOT shows you what's really happening inside.

A	B	Sel	Out
0	0	0	0
0	0	1	0
0	1	0	0
0	1	1	1
1	0	0	1
1	0	1	0
1	1	0	1
1	1	1	1

Why it matters

Multiplexers are everywhere. Inside a CPU they pick which register feeds the ALU. In memory they route a read through the right bank. In your graphics card they gate billions of pixel decisions per frame. The 2-to-1 is the atom — wider muxes are just trees of them.

Try this

• Set A on and B off. Flip Sel. Watch the lamp turn off as Sel goes high — the circuit stopped listening to A and switched to B, which is currently silent.
• Set both A and B on. Flipping Sel now changes nothing — both inputs agree, so it doesn't matter which one is selected.