In this lesson students explore photoresistors, also called light-dependent resistors (LDRs). These special resistors change their resistance based on the amount of light in the environment. Students learn that a photoresistor is still a passive component, but unlike a regular resistor with a fixed value, its resistance varies as light increases or decreases. Students build a simple monitored circuit on a breadboard, use a multimeter to track voltage changes, and test light levels around the room. The goal is to help students understand how environmental sensors work by giving them a calm, hands-on experience with a simple, friendly component.

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Student Learning Goals
By the end of this lesson students will be able to:
• Explain what a photoresistor is and how it responds to light
• Build a simple circuit using a photoresistor and fixed resistor
• Use a multimeter to measure voltage changes caused by changing light levels
• Record and compare light readings from different parts of the classroom
• Identify real-world devices that use photoresistors

Materials Needed
Breadboard
Photoresistor (LDR)
Fixed resistor (10K recommended)
Battery pack with batteries
Jumper wires
Multimeter with jumper clamps
Journals and pencils

Teacher Preparation Notes
Test a sample circuit before class so you know what voltage ranges to expect. Make sure your photoresistor and resistor fit comfortably across the terminal strips. Prepare a few examples of real devices that use photoresistors (night lights, garden lights, screen brightness sensors, etc.). Review the PPT so you can guide students smoothly and slowly.

Safety Notes
We disconnect the battery pack before adjusting breadboard wires.
We keep jumper wires organized and untangled.
We handle the photoresistor legs gently—they can bend easily.
There is virtually zero risk of electrical shock when using a small battery pack.

Warm Up Activity
Hold up a photoresistor and ask:
Why might a circuit want to know how bright it is in the room?
Let students guess freely. Introduce the idea that some resistors respond to the world around them.

Lesson Flow

Step One: What Is a Photoresistor?

Use the PPT slide explaining that a photoresistor changes its resistance depending on how much light shines on it.
Explain gently:
• More light = lower resistance
• Less light = higher resistance

Have students copy the diagram symbol into their journals.

Step Two: Build the Base Circuit

Guide students to power their breadboards:
• Red wire from battery pack to positive bus
• Black wire to negative bus

Students draw this step in their journals.

Step Three: Insert the Photoresistor

Students place the photoresistor so each leg sits in two separate terminal strips.

Explain that, like a resistor, it does not have polarity.

Step Four: Add the Fixed Resistor

Students place the resistor so that:
• One leg shares the same row as one leg of the photoresistor
• The other resistor leg goes into an empty row

Then run a black jumper from the resistor’s free row to the negative bus.

Step Five: Complete the Circuit

Students add a red jumper from the lone side of the photoresistor to the positive bus.

Tell them the circuit is now complete and ready to monitor.

Step Six: Connect the Multimeter

Guide students to attach their multimeter using jumper clamps:
• Positive (red) clamp goes to the resistor’s row (the third hole on that strip works well)
• Negative (black) clamp goes to the negative bus

Turn the multimeter to DC voltage.

Journal prompt:
What voltage do you see when the sensor is in normal classroom light?

Step Seven: Test Light Levels

Have students explore:
• Cover the photoresistor with their hand
• Cast a shadow
• Shine a flashlight
• Move near a window

Ask what happens to the multimeter reading each time.

Let them record measurements from three or four places around the classroom.

Step Eight: Real-World Connections

Ask students to brainstorm:
Where have you seen a device that reacts to light?

Guide them toward ideas like:
• Night lights
• Garden pathway lights
• Street lamps
• Phone brightness sensors
• Solar toys

Step Nine: Diagram Practice

Show the PPT’s circuit diagram and help students trace the loop slowly:
positive → photoresistor → resistor → negative.

Have students draw the same diagram in their journals.

Teacher Notes for Each Slide
Slide 1 Title: Set an open, encouraging tone.
Slides 2–5 What is a photoresistor: Move slowly and make the idea friendly.
Slides 6–10 Building the circuit: Model every step.
Slides 11–14 Multimeter setup: Pause to help students position probes correctly.
Slides 15–end Testing light: Encourage gentle exploration.

Independent or Group Activity
Partners choose three areas of the classroom with different lighting and record all three readings. Then they switch circuits and check each other’s results.

Vocabulary and Concepts
Photoresistor A resistor that changes resistance with light
LDR Light-dependent resistor
Resistance A measure of how much a component slows electrical flow
Sensor A component that reacts to the environment

Wrap Up
Ask:
What surprised you about how your circuit reacted to light?
Why might a photoresistor be useful in a real project?

Exit Ticket
Draw the circuit and label the photoresistor and resistor.

Quiz

  1. What does a photoresistor do?
  2. Does a photoresistor have polarity?
  3. What tool did we use to measure voltage?
  4. What happens to resistance when light increases?
  5. Name one device that uses a photoresistor.

Teacher Reflection
Did students understand how light affects resistance?
Was the multimeter step comfortable for most learners?
Would adding a flashlight or lamps make the exploration easier?