Reading Light Intensity with the ESP32 LDR Module

Real-time monitoring of the LDR module's raw ADC value and relative light percentage calculated from dark and light reference values in the Serial Monitor. This percentage is not a lux measurement. Teaches the module’s calibration logic. It is ideal to complete this before Project 10, as it is used to determine the threshold value.

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A simple light meter that shows how bright your room is as a percentage. The light sensor measures the surroundings, and the ESP32 turns that into zero for darkness and a hundred for full light, writing it on your computer screen every half second. Before building a night light, you take this measurement to decide at which level the lamp should switch on.

Source Code

Arduino (C++)

1// ─────────────────────────────────────────
2//   BlueGrays · bluegrays.com
3//   Reading Light Intensity with the ESP32 LDR Module
4// ─────────────────────────────────────────
5// Reads the analog output (AO) of the LDR module with the 12-bit ADC and
6// converts it to a 0-100 light percentage using the calibration values.
7// The result is printed to the Serial Monitor (115200 baud) every 0.5 s.
8//
9// Pins:
10//   GPIO 34 (classic ESP32) / GPIO 1 (ESP32-S2/S3) -> LDR module AO
11//   LDR module VCC -> 3V3, GND -> GND
12
13#if defined(CONFIG_IDF_TARGET_ESP32S2) || defined(CONFIG_IDF_TARGET_ESP32S3)
14  const int LDR_PIN = 1;   // ESP32-S2/S3: ADC1_CH0
15#else
16  // Classic ESP32 DevKit V1: ADC1_CH6, input-only. GPIO 1 clashes with Serial TX here
17  const int LDR_PIN = 34;
18#endif
19
20// Calibration: measure in your own room and enter the values here. On the LDR module the AO voltage
21// (and raw) DROPS as the light gets brighter.
22const int RAW_DARK  = 4000;  // Dark raw
23const int RAW_LIGHT = 100;   // Light raw
24
25void setup() {
26  Serial.begin(115200);
27  analogReadResolution(12);  // Force 12-bit
28  Serial.println("LDR Okuma / LDR Reading");
29  Serial.println("-----------------------------");
30}
31
32void loop() {
33  int raw = analogRead(LDR_PIN);
34  // Clip to the calibration range, dark = 0%
35  int clipped = constrain(raw, RAW_LIGHT, RAW_DARK);
36  int percent = map(clipped, RAW_DARK, RAW_LIGHT, 0, 100);
37
38  Serial.print("Raw: ");
39  Serial.print(raw);
40  Serial.print(" | Light: %");
41  Serial.println(percent);
42
43  delay(500);
44}
45

FAQ

What values does the ESP32's ADC return?▾

The ESP32's ADC is 12-bit and returns values ranging from 0 to 4095. Since this differs from the 0–1023 range on the Arduino Uno, you should adjust your thresholds and calculations accordingly.

Analog readings are distorted when Wi-Fi is on. Why?▾

On the ESP32, the ADC2 channel (GPIO 0, 2, 4, 12–15, 25–27) cannot be used while Wi-Fi is enabled. Connecting analog sensors to the ADC1 pins (GPIO 32–39) prevents this issue.

Is the reading the actual lux value?▾

No, this code does not calculate lux; it displays the raw ADC value and a relative light percentage based on the selected dark and light reference values. The LDR's response is not linear and varies from model to model. For precise lux measurement, a calibrated sensor such as the BH1750 is used.

Why does the ESP32 ADC give incorrect readings at the extremes?▾

The ESP32's ADC is not linear near 0 V and 3.3 V. The `analogReadMilliVolts()` function returns calibrated values; for critical measurements, operating within the 0.1–3.1 V range yields the most accurate results.

Reading Light Intensity with the ESP32 LDR Module

Video

Circuit Diagram

Source Code

FAQ