LDR Sensors - Elegets Learn

LDR Sensor Overview

LDRs (Light Dependent Resistors) are passive electronic components whose resistance decreases with increasing light intensity.

Variable Resistance: Resistance changes with light intensity (typically 1kΩ in light to 1MΩ in dark)
Spectral Sensitivity: Most sensitive to visible light (400-700nm)
Non-Polarized: Can be connected in either direction
Slow Response: Typically 100ms response time
Low Cost: Extremely inexpensive light sensing solution

Simple to Use: No complex circuitry required
Analog Output: Provides continuous light measurement
Wide Resistance Range: Good dynamic range
Robust: No fragile components
No Power Required: Passive component

Light Measurement: Ambient light sensing
Automatic Lighting: Street lights, night lights
Security Systems: Light-based triggers
Camera Exposure Control: Simple light meters
Educational Projects: Basic light sensing experiments

Technical Specifications

Resistance Characteristics

Dark Resistance: Typically 1MΩ-10MΩ (no light)
Light Resistance: 1kΩ-10kΩ (bright light)
Gamma Value: ~0.7 (non-linear response)
Response Time: ~100ms (rise and fall)
Spectral Peak: ~540nm (green light)

Standard LDR Parameters

Maximum Voltage	150V (typical)
Power Dissipation	100mW (max)
Operating Temperature	-30°C to +70°C
Spectral Range	400nm-700nm
Resistance Tolerance	±20%

GL5528 Common Type

Light Intensity vs Resistance

Circuit Design

Basic Voltage Divider

The standard way to use an LDR is in a voltage divider configuration with a fixed resistor:

Connect LDR from VCC to analog input
Connect fixed resistor from analog input to GND
Output voltage = VCC × (R_fixed / (R_fixed + R_LDR))

Typical resistor values: 10kΩ works well for most applications

Component Selection

LDR Selection: GL5528 (common), GL5537 (more sensitive), GL5516 (less sensitive)
Fixed Resistor: Choose based on desired range (usually 1kΩ-100kΩ)
Capacitor: Add 0.1μF capacitor across LDR for noise reduction
Voltage: Works with 3.3V or 5V systems

Resistor Selection Guide

Bright environments: 1kΩ-4.7kΩ
General purpose: 10kΩ
Dark environments: 47kΩ-100kΩ

Typical Wiring Diagram

Code Examples

Basic Arduino Example

// Basic LDR Sensor Example
#define LDR_PIN A0

void setup() {
    Serial.begin(9600);
}

void loop() {
    int ldrValue = analogRead(LDR_PIN);
    Serial.print("LDR Value: ");
    Serial.println(ldrValue);
    
    // Simple threshold detection
    if(ldrValue < 500) {
        Serial.println("Dark environment");
    } else {
        Serial.println("Bright environment");
    }
    
    delay(1000);
}

Advanced Example with Calibration

// LDR with auto-calibration
#define LDR_PIN A0
#define CALIBRATION_TIME 5000 // 5 seconds

int minLDR = 1023;
int maxLDR = 0;

void setup() {
    Serial.begin(9600);
    calibrateSensor();
}

void calibrateSensor() {
    Serial.println("Calibrating - expose sensor to min and max light");
    unsigned long startTime = millis();
    
    while(millis() - startTime < CALIBRATION_TIME) {
        int val = analogRead(LDR_PIN);
        if(val < minLDR) minLDR = val;
        if(val > maxLDR) maxLDR = val;
        delay(100);
    }
    
    Serial.print("Calibration complete. Min: ");
    Serial.print(minLDR);
    Serial.print(" Max: ");
    Serial.println(maxLDR);
}

void loop() {
    int ldrValue = analogRead(LDR_PIN);
    int normalized = map(ldrValue, minLDR, maxLDR, 0, 100);
    
    Serial.print("Light Level: ");
    Serial.print(normalized);
    Serial.println("%");
    
    delay(500);
}

Automatic Light Control Example

// Automatic light control with LDR and relay
#define LDR_PIN A0
#define RELAY_PIN 3
#define THRESHOLD 300 // Adjust based on your LDR

void setup() {
    pinMode(RELAY_PIN, OUTPUT);
}

void loop() {
    int ldrValue = analogRead(LDR_PIN);
    
    if(ldrValue < THRESHOLD) {
        digitalWrite(RELAY_PIN, HIGH); // Turn lights on
    } else {
        digitalWrite(RELAY_PIN, LOW); // Turn lights off
    }
    
    delay(10000); // Check every 10 seconds
}

Project Ideas

Automatic Night Light

LED light that turns on automatically when dark.

LDR LED Transistor

Light Intensity Meter

Measure and display light levels on LCD or OLED.

LDR OLED Arduino

Smart Curtain Controller

Automatically open/close curtains based on sunlight.

LDR Servo Motor

Camera Shutter Trigger

Trigger camera when light changes (lightning, etc.).

LDR Optocoupler Camera

Plant Light Monitor

Monitor and log light exposure for plants.

LDR SD Card Data Log

Security Light Alarm

Trigger alarm if light is detected when it should be dark.

LDR Buzzer ESP8266

Advanced Project: Smart Daylight Harvesting System

// Components needed:
- Multiple LDR sensors (different locations)
- Arduino/ESP32
- Relay module
- LED lighting system
- OLED display
- Real-time clock

// Features:
1. Monitor ambient light at multiple points
2. Automatically adjust artificial lighting
3. Maintain consistent light levels
4. Energy saving algorithm
5. Data logging
6. Time-based control profiles

// Implementation Tips:
- Use multiple LDRs for better coverage
- Implement PID control for smooth adjustments
- Add manual override capability
- Create energy usage reports
- Add wireless control option
- Consider solar input for outdoor applications

Best Practices & Tips

Installation

Point sensor at light source/area of interest
Use diffuser to prevent directional sensitivity
Protect from dust and moisture
Avoid shadows that could affect readings
Consider multiple sensors for large areas

Calibration

Calibrate in actual operating environment
Measure min/max expected light levels
Use map() function to scale readings
Document calibration settings
Re-calibrate if environment changes

Signal Processing

Use moving average for stable readings
Add hysteresis to prevent rapid toggling
Implement debouncing for threshold detection
Log raw data for performance analysis
Consider temperature compensation

Common Mistakes to Avoid

Mistake	Consequence	Solution
Wrong resistor value	Limited measurement range	Choose resistor based on expected light levels
No calibration	Inconsistent performance	Always calibrate in actual environment
Direct sunlight exposure	Saturation or damage	Use diffuser or limit exposure
Ignoring response time	Missed rapid light changes	Account for 100ms response time
Single sensor for large area	Inaccurate light measurement	Use multiple sensors for better coverage

LDR (Light Dependent Resistor) Sensors