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I made this circuit yesterday using normal logic gates and chips:

\$(AB)+(A\overline BC)+(\overline AC)\$

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Now I have to remake it using only a MUX but I'm having some trouble. From my understanding I should use a 3:1 MUX, because I have 3 different combinations that light up that bulb.

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How could I replace all these circuit logic gates with a MUX?

EDIT: Truth Table

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  • \$\begingroup\$ your mux is controlled by a binary input, so a 3:1 mux isn't possible. Try to build the thing from 2:1 muxes, where the selecting input is also one of your inputs (it's easy if you write down the truth table for one 2:1 mux outpu with a binary selector once). \$\endgroup\$ Commented Dec 30, 2019 at 11:52
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    \$\begingroup\$ The Boolean expression simplifies considerably. \$\endgroup\$
    – Chu
    Commented Dec 30, 2019 at 11:55
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    \$\begingroup\$ Simplified expression is AB+C \$\endgroup\$
    – lordvader
    Commented Dec 30, 2019 at 12:38
  • \$\begingroup\$ So you need an 8:1 mux. How to connect it is your homework. \$\endgroup\$
    – Chu
    Commented Dec 30, 2019 at 12:43

2 Answers 2

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Recall the definition of a multiplexer: it's a device with several inputs and one output. Specifically, it has n binary selection inputs (usually denoted as \$S_0\$ through \$S_{n-1}\$) to select between 2n data inputs (usually denoted \$D_i\$). The selected data input is passed through to a single output.

Your multiplexer circuit symbols are missing the selection inputs. For example, a three-input, eight-output multiplexer is usually drawn like this:

8-to-1 multiplexer circuit symbol.

The simplest way to use a multiplexer to implement a logic circuit is to map, one-to-one, the truth table's values to each of the mux data inputs. In other words, the data inputs will either be 0 or 1 (or electrically, HIGH or LOW) and the selection lines will be controlled by your logic inputs.

For extra credit, you can think about how you could use some logic signals as data inputs, which may allow you to reduce the size of your multiplexer. For example, I notice that when \$A\$ is zero, the output equals \$C\$. With a bit of thought you might be able to see how to extend this concept, and produce your same truth table with just a 4-to-1 mux.

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  • \$\begingroup\$ I need to start checking the dates on posts before I answer them... \$\endgroup\$
    – Matt S
    Commented Feb 11, 2023 at 18:35
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Write down the truth table for all ABC combinations what the output is, and youl'll realize you are not looking fo a 3:1 mux, and you can use the truth table directly to know how to connect the mux.

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  • \$\begingroup\$ Added it in the main post. So now how can I know how to connect the mux by using it? I assume I only need the "T" Combinations. \$\endgroup\$
    – lordvader
    Commented Dec 30, 2019 at 12:38
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    \$\begingroup\$ You use the mux as a lookup table to select correct result based on input. \$\endgroup\$
    – Justme
    Commented Dec 30, 2019 at 14:05

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