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LDR Light Sensor Lab

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About This MicroSim

A light-dependent resistor (LDR) has a resistance that falls as the light gets brighter: about 1 MΩ in the dark and as little as 500 Ω in direct sunlight. The Pico's ADC measures voltage, not resistance, so the LDR is wired into a voltage divider with a fixed resistor:

\[ V_{out} = 3.3 \times \frac{R_{fixed}}{R_{LDR} + R_{fixed}} \]

The ADC turns that voltage into a number from 0 to 65,535:

\[ \text{ADC} = \frac{V_{out}}{3.3} \times 65535 \]

The MicroSim shows four panels: the schematic (3.3 V, the LDR, the GP26 ADC pin, the fixed resistor and ground), a room whose light follows the brightness slider, the formula worked out with the current numbers, and a gauge of the ADC reading. On narrow screens the gauge is hidden and the reading appears in the formula panel.

How to Use

  1. Move the Brightness slider from Dark room to Direct sunlight. Watch \( R_{LDR} \) fall and the ADC reading rise.
  2. Move the Fixed resistor slider through standard resistor values from 1 kΩ to 100 kΩ. See how it changes the reading for the same light.
  3. Check Hide Vout and the ADC reading to hide the answers. Calculate \( V_{out} \) and the ADC value by hand, then uncheck the box to check your work.

Iframe Embed Code

You can add this MicroSim to any web page by adding this to your HTML:

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<iframe src="https://dmccreary.github.io/clocks-and-watches/sims/ldr-light-sensor-lab/main.html"
        height="537px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Grade Level

9-12 (Clocks and Watches with AI)

Duration

15-20 minutes

Prerequisites

  • Voltage dividers and Ohm's law (Chapter 2)
  • ADC readings from 0 to 65,535 (Chapter 18)

Learning Objectives

Students will be able to:

  • Apply the voltage divider formula to calculate \( V_{out} \) for a given LDR resistance
  • Convert \( V_{out} \) into the 16-bit ADC reading that read_u16() returns
  • Explain how the choice of fixed resistor changes the sensor's useful range

Activities

  1. Check the worked example (3 min): Set Bright room so that \( R_{LDR} \) is 2.0 kΩ with a 10 kΩ fixed resistor. Confirm the chapter's answer of 2.75 V.
  2. Predict first (7 min): Check Hide Vout and the ADC reading. For three brightness settings of your choice, write down \( R_{LDR} \), calculate \( V_{out} \) and the ADC value, then reveal the answers. Within how many counts were you?
  3. Choose the resistor (7 min): For a bedroom clock that must tell Dark room from Dim room, which fixed resistor gives the biggest change in the ADC reading between those two settings? Try 1 kΩ, 10 kΩ and 100 kΩ, and explain the result.

Assessment

  • The student calculates \( V_{out} \) and the ADC reading for a given \( R_{LDR} \) and \( R_{fixed} \).
  • The student explains why brighter light gives a bigger reading with this wiring.
  • The student justifies a fixed resistor value for a dark-room or a sunny-room application.

References

  1. Photoresistor - Wikipedia article on light-dependent resistors and how their resistance changes with light.
  2. Voltage divider - Wikipedia article deriving the divider formula used here.
  3. MicroPython ADC class - Official documentation for read_u16() and its 0 to 65,535 range.
  4. E series of preferred numbers - Why resistors come in values like 4.7 kΩ and 6.8 kΩ.