Tricky Circuit

MUX: a controlled crossroads

Choose which signal gets through

MUX is short for Multiplexer, a circuit that selects one input for its output. The select bit is a control: here 0 chooses input A and 1 chooses input B.

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EXPERIMENT BENCH

Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.

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Why it works

Ideal binary logic: 0 means low and 1 means high, not a measured voltage. Supply behavior, current, output loading and response delay are omitted; diagrams show functions, not chip pinouts.

A and B are data inputs, S the select bit and Y the output. S = 0 gives Y = A; S = 1 gives Y = B. Selection chooses a route, not a sum or a comparison of inputs.

Difficulty

  1. Build · Build it

    Power on with A = 1, B = 0 and S = 0. Route A to output Y = 1.

    Need a hint?

    A = 1; B = 0; S = 0 → Y = 1

  2. Explore · Change the inputs

    Keep A = 1 and B = 0; set S = 1. Route B to output Y = 0.

    Need a hint?

    A = 1; B = 0; S = 1 → Y = 0

  3. Solve it · Solve the challenge

    Set A = 0, B = 1 and S = 1. Route B to output Y = 1.

    Need a hint?

    A = 0; B = 1; S = 1 → Y = 1

Extra reading and circuit recipes

Choose which signal gets through

Think of choosing one of two camera feeds. The select control chooses the source, while the chosen source decides what the output contains.

S = 0 → Y = A; S = 1 → Y = B

The two data values and the control have different jobs. To test routing, use unequal data inputs so the output makes the choice visible.

A useful pattern

Changing the unselected input leaves the output unchanged. Changing the selected input changes the output.

Beyond the model

When A and B are equal, the output looks the same for either selection. An unchanged output does not prove that the route stayed the same.

A useful trap to avoid

The select bit is not a third data value added to the output. A digital multiplexer also does not guarantee it can route any analog voltage.

A = 1, B = 0 and S = 0. Only B changes to 1. Output Y…

  1. stays 1
  2. becomes 0
  3. adds up to 2
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S still selects A, so the unselected B does not determine the output.

Build it

Set A = 1, B = 0, S = 0. A is selected and Y = 1.

Y = 1

Change the inputs

Keep the data inputs; set S = 1. B is selected and Y = 0.

Y = 0

Try another path

Choose A, then toggle B several times. Choose B and repeat. Observe when the same input starts affecting the output.

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