4051 – 8-Channel Analog Multiplexer/Demultiplexer


What is a Multiplexer?

A multiplexer is a device that has multiple inputs and one output. It also has some sort of selection system that decides which of those inputs should connect to the output. The 4051 chip can be used as a multiplexer.

When used as a multiplexer, pin 3 is the output, and the 8 channels are different inputs.

What is a Demultiplexer?

A demultiplexer on the other hand is a device that has multiple outputs and only one input. The selector decides what output channel the input should connect to. The 4051 chip can also be used as a demultiplexer. Just wrap your brain to think of the inputs and outputs the opposite way.

When used as a demultiplexer, pin 3 is the input, and the 8 channels are outputs.

Selector Pins –Truth Table

The 4051 has three binary control inputs (A, B ,and C). The signals decide which channel is connected to the input/output pin. Note that the inhibit should be connected to GND (LOW) in normal use. If you make that pin HIGH, it will turn off all of the channels.

There are multiple ways to feed the control signals to the multiplexer, but take a close look at the table. Where have we seen this before?

Hint: Check out the page for the 4040 IC.

The 4040 is a binary counter, if we send the outputs Q0, Q1 and Q2 from the 4040 to the 4051, we are able to count through the eight steps.