ESP32 Reading a Photosensitive Resistor (LDR) - Lesson 5

In this lesson you will learn how to use a photosensitive resistor module (LDR module) with your ESP32. You will read analog values to measure light levels and also monitor the built-in digital output LED on the module. We’ll be using the LM393 LDR version.

This lesson is part of our ESP32 tutorials series, where each lesson builds on the previous one.

What you will learn

• How a photosensitive resistor module works
• How to connect the module to the ESP32
• How to read analog values using analogRead()
• How to read the digital output from the module
• How to display readings on the Serial Monitor

Components needed

• ESP32
• Photosensitive resistor module
• Breadboard
• Jumper wires
• USB cable

Step 1 - Understanding the Photosensitive Resistor Module

The module has a built-in LDR and a voltage divider circuit. It provides:

• AO (analog output): a voltage that changes continuously depending on light levels. On most modules with AO connected through a pull-down resistor, AO decreases as light increases.
• DO (digital output): switches HIGH or LOW when the light level passes a set threshold. The module has a built-in LED that lights up when DO is HIGH.

Step 2 - Wiring the LDR

1. Place the module on the breadboard
2. Connect VCC to 3.3 V on the ESP32
3. Connect GND to GND on the ESP32
4. Connect AO to GPIO 34 on the ESP32
5. Connect DO to GPIO 32 on the ESP32

Step 3 - Writing the Code to Read the LDR

Open the Arduino IDE and enter the following code. This reads the light level and prints it to the Serial Monitor. It also shows the digital output from the module.

int ldrAnalogPin = 34; // AO pin connected to ESP32
int ldrDigitalPin = 32; // DO pin connected to ESP32
int ldrValue = 0;
int ldrDigital = 0;
void setup() {
  Serial.begin(115200);
  pinMode(ldrDigitalPin, INPUT); // Set DO as input
}
void loop() {
  ldrValue = analogRead(ldrAnalogPin); // Read analog light level
  ldrDigital = digitalRead(ldrDigitalPin); // Read digital threshold
  Serial.print("Analog light level (AO): ");
  Serial.println(ldrValue);
  Serial.print("Digital output (DO): ");
  Serial.println(ldrDigital);
  delay(500); // Update every half second
}

How the Code Works

This program reads light levels using a photosensitive resistor (LDR) module and shows the results in the Serial Monitor. It reads both the analog light level and the digital output threshold from the module.