Pull resistor and floating logic node

Study Floating Input and Pull Resistor Contracts from the Electronics module guide by comparing the node voltage and resistor current with open and closed switches.

Trace a DC path from the node to a rail before interpreting any numeric voltage., your practice guide

Trace a DC path from the node to a rail before interpreting any numeric voltage.
Predict the reading, then compare it with the measurement.

Falstad CircuitJS

Third party Tool

Study Floating Input and Pull Resistor Contracts from the Electronics module guide by comparing the node voltage and resistor current with open and closed switches.

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Version tested: CircuitJS live in Playwright Chromium on 2026-10-09; no simulator version exposed. Date: 2026-10-09.

Open the prepared circuit.

Open this circuit in Falstad (new tab)

Steps

Screens captured against Falstad CircuitJS CircuitJS live in Playwright Chromium on 2026-10-09; no simulator version exposed on 2026-10-09; the tool may have moved on — the text steps are the contract.

  1. Step 1

    Do
    Open the circuit with no pull resistor on the Falstad canvas and inspect the undriven node wire.
    You will see
    Input: synthetic fixed-seed circuit set, seed 27. Supply: 5 V DC, disconnected from the node. Switch: open. Wire readout: I = 0 A; V = 0 V. Voltmeter readout: Vd = 0 V.
    Why it matters
    The node has no DC path to either rail. This numerical zero is the simulator solution, not a defined digital LOW or a GPIO noise measurement.

  2. Step 2

    Do
    Open the 10 kΩ pull-up circuit on the canvas and inspect the node wire with the switch open.
    You will see
    Supply: 5 V DC. Pull-up resistor: R = 10 kΩ. Switch: open. Node wire: V = 5 V. Pull-up resistor: I = 0 A.
    Why it matters
    The resistor creates a defined idle HIGH path while the open switch leaves no ground current path.

  3. Step 3

    Do
    Open the 10 kΩ pull-up circuit with the switch closed and point to the resistor on the canvas.
    You will see
    Supply: 5 V DC. Node wire: V = 0 V. Pull-up resistor: R = 10 kΩ. Pull-up resistor: I = 500 μA. Pull-up resistor: Vd = 5 V.
    Why it matters
    Closing the switch pulls the node LOW and makes 5 V / 10 kΩ = 500 μA flow through the pull path.

  4. Step 4

    Do
    Open the 100 kΩ pull-up circuit on the canvas and inspect its resistor with the switch closed.
    You will see
    Supply: 5 V DC. Node wire: V = 0 V. Pull-up resistor: R = 100 kΩ. Pull-up resistor: I = 50 μA. Pull-up resistor: Vd = 5 V.
    Why it matters
    Increasing pull resistance tenfold reduces steady closed-switch current tenfold in this idealized model.

  5. Step 5

    Do
    Remove the pull resistor by reopening the no-pull circuit on the canvas and inspect the node wire again.
    You will see
    Pull resistor: absent. Switch: open. Wire: I = 0 A. Wire: V = 0 V in CircuitJS. There is still no physical DC pull path.
    Why it matters
    The tool displays a numerical voltage for its floating node, but that value cannot specify a real CMOS input state.

  6. Step 6

    Do
    Restore the 10 kΩ pull-up circuit on the canvas and inspect the open-switch node readout.
    You will see
    Supply: 5 V DC. Pull-up resistor: R = 10 kΩ. Switch: open. Node wire: V = 5 V. Resistor current: I = 0 A.
    Why it matters
    The real simulator comparison shows which topology defines idle HIGH; it does not establish actual GPIO leakage, noise, or thresholds.

Chapter checks

These questions refer to the chapter’s examples. Use the return links to review their answers.

  1. Why does an unused or button-driven CMOS logic input usually need a pull-up or pull-down resistor?

    Return to the chapter’s knowledge check

  2. Choosing a pull-up resistor, what is the core trade-off between a strong (small) and a weak (large) value?

    Return to the chapter’s knowledge check

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