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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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
    Source panel: "I = 166.667 μA"; "Vd = 5 V". Upper resistor panel: "R = 20 kΩ"; "Vd = 3.333 V". Lower resistor panel: "R = 10 kΩ"; "Vd = 1.667 V". Midpoint wire panel: "V = 1.667 V".
    Why it matters
    Expected: At 20 kΩ above 10 kΩ, 5 × 10/30 = 1.667 V at the midpoint. Meaning: This resistor stands in for light-dependent resistance; it is not a physical light sensor.

  2. Step 2

    Do
    In the circuit canvas, point at the midpoint wire and read its measurement panel.
    You will see
    Source panel: "I = 166.667 μA"; "Vd = 5 V". Upper resistor panel: "R = 20 kΩ"; "Vd = 3.333 V". Lower resistor panel: "R = 10 kΩ"; "Vd = 1.667 V". Midpoint wire panel: "V = 1.667 V".
    Why it matters
    Expected: 5 × 10/(20+10) = 1.667 V. Meaning: The midpoint is measured relative to the grounded lower end.

  3. Step 3

    Do
    In the circuit canvas, use the resistor labels to calculate 5 × 10000 / 30000.
    You will see
    Source panel: "I = 166.667 μA"; "Vd = 5 V". Upper resistor panel: "R = 20 kΩ"; "Vd = 3.333 V". Lower resistor panel: "R = 10 kΩ"; "Vd = 1.667 V". Midpoint wire panel: "V = 1.667 V".
    Why it matters
    Expected: 20 kΩ + 10 kΩ = 30 kΩ and the lower resistor receives one third of 5 V. Meaning: The measured midpoint verifies the divider calculation.

  4. Step 4

    Do
    In the upper resistor’s right-click menu, choose Edit and set Resistance to 5000 Ω.
    You will see
    Source panel: "I = 333.333 μA"; "Vd = 5 V". Upper resistor panel: "R = 5 kΩ"; "Vd = 1.667 V". Lower resistor panel: "R = 10 kΩ"; "Vd = 3.333 V". Midpoint wire panel: "V = 3.333 V".
    Why it matters
    Expected: With 5 kΩ above 10 kΩ, the midpoint should be 5 × 10/15 = 3.333 V. Meaning: Lower upper resistance leaves a larger share of voltage across the lower resistor.

  5. Step 5

    Do
    In the upper resistor’s right-click menu, choose Edit and set Resistance to 50000 Ω.
    You will see
    Source panel: "I = 83.333 μA"; "Vd = 5 V". Upper resistor panel: "R = 50 kΩ"; "Vd = 4.167 V". Lower resistor panel: "R = 10 kΩ"; "Vd = 833.333 mV". Midpoint wire panel: "V = 833.333 mV".
    Why it matters
    Expected: With 50 kΩ above 10 kΩ, the midpoint should be 5 × 10/60 = 0.833 V. Meaning: Higher upper resistance pulls the divider output toward ground.

  6. Step 6

    Do
    In the upper resistor’s right-click menu, choose Edit and restore Resistance to 20000 Ω.
    You will see
    Source panel: "I = 166.667 μA"; "Vd = 5 V". Upper resistor panel: "R = 20 kΩ"; "Vd = 3.333 V". Lower resistor panel: "R = 10 kΩ"; "Vd = 1.667 V". Midpoint wire panel: "V = 1.667 V".
    Why it matters
    Expected: Restoring 20 kΩ should restore the 1.667 V midpoint. Meaning: The reversible edit isolates resistance as the cause of the voltage change.

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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