Photoresistor voltage divider

Explain how light-dependent resistance becomes a voltage.

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Predict the reading, then compare it with the measurement.

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Explain how light-dependent resistance becomes a voltage.

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Version tested: Browser tool opened 2026-09-06 (Europe/London); no version number exposed. Date: 2026-09-06.

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Steps

Screens captured against Falstad CircuitJS Browser tool opened 2026-09-06 (Europe/London); no version number exposed on 2026-09-06; the tool may have moved on — the text steps are the contract.

  1. Step 1

    Do
    In the circuit canvas, inspect the upper 20 kΩ resistor representing the light-dependent resistor.
    You will see
    The upper resistor is 20 kΩ, the lower resistor is 10 kΩ, and the supply is 5 V.
    Why it matters
    The chapter turns resistance changes from a light sensor into measurable voltage. Locating the sensor resistor tells you which part of the divider will change.

  2. Step 2

    Do
    In the circuit canvas, point at the midpoint wire and read its measurement panel.
    You will see
    The midpoint measurement panel shows 1.667 V.
    Why it matters
    This node carries the signal toward a microcontroller input. Following the voltage path helps separate sensor behaviour from changes introduced by the circuit.

  3. Step 3

    Do
    In the circuit canvas, use the resistor labels to calculate 5 × 10000 / 30000.
    You will see
    The labels show 20 kΩ above 10 kΩ, giving a calculated 1.67 V that agrees with the measurement.
    Why it matters
    The output depends on the resistance ratio. Checking that ratio gives a prediction before the light-sensor model changes.

  4. Step 4

    Do
    In the upper resistor’s right-click menu, choose Edit and set Resistance to 5000 Ω.
    You will see
    The upper resistor changes to 5 kΩ, and the midpoint measurement rises to 3.333 V.
    Why it matters
    Brighter light is represented by a lower sensor resistance here. With the sensor above the fixed resistor, that change raises the divider output.

  5. Step 5

    Do
    In the upper resistor’s right-click menu, choose Edit and set Resistance to 50000 Ω.
    You will see
    The upper resistor changes to 50 kΩ, and the midpoint measurement falls to 833.333 mV.
    Why it matters
    The dimmer-light model produces the opposite voltage change. Comparing both directions tests the circuit response rather than trusting one plausible reading.

  6. Step 6

    Do
    In the upper resistor’s right-click menu, choose Edit and restore Resistance to 20000 Ω.
    You will see
    The upper resistor returns to 20 kΩ, and the midpoint measurement returns to 1.667 V.
    Why it matters
    A return to the original input checks the starting result. The chapter also tests supply and connection changes before trusting the complete signal path.

Chapter checks

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

  1. A thermistor (10kohm at 25C) is placed as R1 in a voltage divider with a 10kohm fixed resistor R2 and 3.3V supply. As temperature increases and thermistor resistance drops to 5kohm, what happens to the output voltage at the junction?

    Return to the chapter’s knowledge check

  2. You need to filter 60Hz mains noise from a temperature sensor that updates every 2 seconds. What RC filter cutoff frequency should you choose?

    Return to the chapter’s knowledge check

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