How to Test LDR with Multimeter: A Step-by-Step Guide

Light-dependent resistors (LDRs), also known as photoresistors, are crucial components in many electronic circuits. They change their resistance based on the amount of light they are exposed to, making them useful in light-sensing applications.

Testing an LDR with a multimeter is a straightforward process that can help you verify its functionality. In this guide, we’ll walk you through the steps to test an LDR using a multimeter.

Step-by-Step Guide to Testing an LDR

What You Need

Before you start, ensure you have the following tools and components:

  • An LDR (Light Dependent Resistor)
  • A digital multimeter
  • A light source (e.g., a flashlight or a desk lamp)

Step 1: Set Up Your Multimeter

  1. Turn on the Multimeter: Power on your digital multimeter.
  2. Select the Resistance Mode: Set your multimeter to measure resistance. This is usually denoted by the omega (Ω) symbol.

Step 2: Connect the Multimeter to the LDR

  1. Identify the LDR Leads: LDRs typically have two leads. Because resistance is non-polar, it doesn’t matter which lead you connect to which terminal of the multimeter.
  2. Connect the Multimeter Probes: Connect the red probe to one lead of the LDR and the black probe to the other lead.
LDR in circuit

Step 3: Measure the LDR Resistance in Darkness

  1. Cover the LDR: Ensure the LDR is in complete darkness. Cover it with your hand or place it in a dark box.
  2. Read the Resistance: Observe the resistance reading on the multimeter. The resistance should be high in the darkness, typically in the megaohms (MΩ) range.

Step 4: Measure the LDR Resistance in Light

  1. Expose the LDR to Light: Shine a light source directly onto the LDR.
  2. Read the Resistance: Check the resistance reading on the multimeter again. In the presence of light, the resistance should be significantly lower, usually in the kiloohms (kΩ) range or even less, depending on the intensity of the light.

Step 5: Compare and Interpret the Results

  1. High Resistance in Darkness: The LDR should show high resistance when covered or in a dark environment.
  2. Low Resistance in Light: The LDR should show a much lower resistance when exposed to light.

Troubleshooting Tips for Testing LDR with a Multimeter

When testing an LDR with a multimeter, you may encounter issues affecting your measurements. Here are some detailed troubleshooting tips to help you resolve common problems and ensure accurate results.

1. Consistent High Resistance

Issue: The LDR shows consistently high resistance regardless of light conditions.

Possible Causes and Solutions:

  • Defective LDR: The LDR might be damaged or defective. Try testing another LDR to see if the issue persists.
  • Improper Connections: Ensure the multimeter probes are securely connected to the LDR leads. Loose connections can lead to inaccurate readings.
  • Low Light Intensity: The light source might not be bright enough to change the resistance of the LDR significantly. Use a stronger light source or move the light closer to the LDR.
LDR and light bulb

2. No Change in Resistance

Issue: There is no significant change in resistance when transitioning from dark to light.

Possible Causes and Solutions:

  • Incorrect Multimeter Setting: Verify that your multimeter is in the correct resistance range. The multimeter might not display the correct readings if the LDR’s resistance is outside the selected range.
  • Faulty Multimeter: Test the multimeter with a known resistor to ensure it functions correctly. If the multimeter is faulty, consider using another one.
  • LDR Orientation: Since LDRs are non-polar, ensure that both leads are connected properly and that the LDR is free of issues. Swap the multimeter probes to check for anomalies.
  • Ambient Light Interference: Ensure that the testing environment is controlled. Excessive ambient light can affect the LDR readings. Conduct the test in a room with minimal light interference.

3. Fluctuating Resistance Readings

Issue: The resistance readings fluctuate rapidly or are unstable.

Possible Causes and Solutions:

  • Poor Probe Contact: Ensure the multimeter probes make firm and stable contact with the LDR leads. If necessary, use alligator clips to maintain a steady connection.
  • Dirty LDR Surface: Dust or debris on the LDR surface can affect its sensitivity. Clean the LDR gently with a soft cloth to remove any contaminants.
  • Interference from Electronic Devices: Nearby electronic devices can cause electromagnetic interference. Move away from such devices or turn them off during testing.

4. Multimeter Display Issues

Issue: The multimeter display shows an error or does not display a reading.

Possible Causes and Solutions:

  • Low Battery: Check the battery level of your multimeter. A low battery can cause erratic readings. Replace the battery if necessary.
  • Overload Condition: If the resistance of the LDR in darkness exceeds the multimeter’s maximum range, it might display an overload condition. Switch to a higher resistance range on the multimeter.
  • Incorrect Measurement Mode: Ensure the multimeter is set to measure resistance (Ω) and not other modes like voltage or current.

5. Testing Multiple LDRs

Issue: Inconsistent results when testing multiple LDRs.

Possible Causes and Solutions:

  • Variations in LDR Specifications: LDRs can have different specifications based on the manufacturer and model. Check the datasheet for each LDR to understand its characteristics and ensure it meets your requirements.
  • Environmental Factors: Conduct tests in a consistent environment to minimize variations. Ensure all LDRs are exposed to the same light source and intensity during testing.


Testing an LDR with a multimeter is a simple yet effective way to ensure its proper functionality. By following these steps, you can easily determine whether your LDR is working correctly.

This process is essential for troubleshooting and verifying the performance of your light-sensing circuits.

Frequently Asked Questions

Can I use an analog multimeter to test an LDR?

Yes, you can use an analog multimeter, but a digital multimeter provides more precise readings.

What should I do if my LDR is not working properly?

If your LDR shows no change in resistance between light and dark conditions, consider replacing it with a new one.

How does the resistance of an LDR change with light intensity?

The resistance of an LDR decreases with increasing light intensity and increases as the light intensity decreases.

Alex Klein Author


Alex Klein is an electrical engineer with more than 15 years of expertise. He is the host of the Electro University YouTube channel, which has thousands of subscribers.