Tricky Circuit

Pull-up resistor and digital input

Give a button a definite answer

A pull-up resistor connects an input weakly to the positive supply. A digital input reads high or low voltage. Pressing the button connects the input to ground, the circuit’s 0 V reference.

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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 input takes no current; ideal switch has zero resistance. Leakage, contact bounce and undefined logic thresholds are omitted.

Released: the input equals the supply and resistor current is zero. Pressed: the input is 0 V and I = V/R flows through the resistor and switch.

Difficulty

  1. Build · Build it

    Power on at 5 V with 10 kΩ and the button released. Read an input of 5 V.

    Need a hint?

    5 V; 10 kΩ → 5 V

  2. Explore · Change one thing

    Keep 5 V and 10 kΩ; press the button. Read 0 V and 0.50 mA.

    Need a hint?

    5 V; 10 kΩ → 0 V; 0.50 mA

  3. Solve it · Solve the challenge

    With 5 V and the button pressed, reduce current to 0.100–0.125 mA.

    Need a hint?

    5 V; 40 kΩ → 0.125 mA

Extra reading and circuit recipes

Give a button a definite answer

A useful pattern

A larger pull-up saves current while the button is held.

Beyond this model

Without a pull-up or pull-down, an unconnected input can float: its voltage is not reliably defined.

A useful trap to avoid

High voltage here means released, not pressed. Software can invert that meaning.

You change 10 kΩ to 20 kΩ while pressed at 5 V. Current…

  1. halves
  2. doubles
  3. stays unchanged
Read more

The switch still holds the input at 0 V; doubling R halves V/R.

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