What is a Photoresistor (LDR) in IoT? Working, Applications & Circuit Examples Explained

Introduction

A photoresistor, also known as an LDR (Light Dependent Resistor), is a light-sensitive electronic component whose resistance changes based on the intensity of light falling on it. These components are widely used in various electronics, automation, and sensor-based systems to detect and respond to changes in ambient light.

How Does a Photoresistor Work?

A photoresistor works on the principle of photoconductivity — a phenomenon where the material becomes more conductive (lower resistance) when exposed to light.

• In darkness, the resistance of the LDR is very high (in the mega-ohm range).

• In bright light, the resistance drops significantly (to a few hundred ohms).

This unique behavior makes it an excellent choice for designing light detection and automation systems.

Construction of a Photoresistor

• A semiconducting material such as cadmium sulfide (CdS).

• The material is arranged in a zig-zag pattern on an insulating base.
• Two metal leads are attached for connection in a circuit.
• The entire structure is enclosed in a transparent plastic case to allow light entry.

Types of Photoresistors

There are mainly two types of photoresistors based on their behavior and materials:

1. Intrinsic Photoresistors

• Made of pure semiconductor materials (e.g., pure silicon or germanium).

• Sensitive to high-intensity light.

• Slower response time.

• Used in scientific and research applications.

2. Extrinsic Photoresistors

• Made by doping semiconductor materials (e.g., CdS doped with copper).

• Higher sensitivity to visible light.

• Faster and more stable response.

• Widely used in consumer electronics and automation systems.

Characteristics of Photoresistors

Advantages of Photoresistors

• Cost-effective and readily available

• Simple to use in circuits

• Low power consumption

• Ideal for ambient light detection

• Non-polarized component (can be connected in any direction)

Disadvantages of Photoresistors

• Slow response time (not ideal for fast-changing light)

• Not suitable for precise light measurement

• Affected by temperature

• Toxic materials (CdS) – may not be RoHS compliant

Applications of Photoresistors

• Street lights – automatic ON/OFF based on sunlight.

• Solar garden lights

• Camera light meters

• Light-sensitive alarms

• Brightness control in displays

• Industrial automation systems

• Security and motion detection systems

Practical Example: Photoresistor-Based Light Detection Circuit

Objective:

Components Needed:

• 1 × Photoresistor (LDR)
• 1 × 10kΩ resistor
• 1 × NPN transistor (e.g., BC547)
• 1 × LED
• 1 × Power supply (5V)

Circuit Description:

• The LDR and 10kΩ resistor form a voltage divider circuit.

• The voltage at the divider's junction changes with light intensity.

• This voltage is fed to the base of the transistor.

• When it’s dark, voltage increases, turning ON the transistor, which in turn lights the LED.

Outcome:

• Light ON → LED OFF

• Light OFF → LED ON