Tricky Circuit

LDR + resistor

Light becomes a signal.

Change the light and watch a sensor turn it into voltage.

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LDR · Light-dependent resistor
Light-dependent resistor. A light-sensitive resistor. In this model, more light means less resistance; the second resistor turns that change into a voltage.
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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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Light changes resistance.

Teaching model: LDR resistance falls from 100 kΩ to 1 kΩ. The light scale is relative, not lux.

LDR above, resistor below: Vout = 5 × R / (RLDR + R). Light lowers RLDR, so Vout rises. A real sensor needs calibration.

Difficulty

  1. Build · Detect daylight

    Power on. Set light to at least 80% and output above 3 V.

    Need a hint?

    Try 100% light and 10 kΩ.

  2. Explore · Find darkness

    Set light to at most 20% and output below 1 V.

    Need a hint?

    Try 0% light and 10 kΩ.

  3. Solve it · Calibrate the midpoint

    At exactly 50% light, reach 2.50 V ±0.05 V.

    Need a hint?

    At 50%, RLDR is 10 kΩ. Match it with the lower resistor.

Extra reading and circuit recipes

Turn a shadow into a signal

A light-dependent resistor changes resistance with illumination. In our circuit it sits above a fixed resistor. More light lowers its resistance, leaving a larger share of the supply across the fixed resistor. That output can become a brightness measurement.

Vout = 5 × Rfixed / (RLDR + Rfixed)

At 50% on our slider, the model LDR is 10 kΩ. With a 10 kΩ fixed resistor, the output is 2.5 V. At full light the LDR becomes 1 kΩ and the output rises to about 4.55 V.

Position changes the meaning

Swap the sensor and fixed resistor and the response reverses: brighter light then makes the midpoint voltage fall.

Halfway is a useful clue

The output is half the supply when sensor and fixed resistor have equal resistance. That gives you a simple way to choose a useful measurement range.

Percent is not lux

This lab uses a relative light slider and an illustrative sensor curve. A real sensor needs a datasheet and calibration before voltage can be converted to physical illumination.

A useful trap to avoid

A light sensor divider cannot power a lamp directly. Read its output with a suitable high-impedance input, then use a switching stage for the load. A real night-light also benefits from hysteresis so it does not flicker near its threshold.

In this circuit, covering the LDR makes the output…

  1. rise
  2. fall
  3. stay constant
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Darkness raises the upper resistance. More voltage falls across it, leaving less across the lower fixed resistor.

Shadow detector

Start in darkness, then slide the light upward.

The starting output is about 0.68 V. Watch it increase as you reveal the sensor.

Find the balance point

Match the fixed resistor to the sensor at mid-light.

Both resistances are 10 kΩ, so the midpoint sits at 2.50 V.