LDR Light Sensor Interfacing: Wiring, ADC Math, and Lux Conversion for ESP32

The LDR Light Sensor: Sensing Principle and Output Types

A Light Dependent Resistor (LDR), or cadmium sulfide (CdS) photoresistor, is a passive semiconductor component. When photons strike the CdS material, they excite electrons into the conduction band, which drops the electrical resistance. In total darkness, a standard 5mm LDR like the GL5528 exhibits a "dark resistance" of roughly 1MΩ. Under bright sunlight (approx. 10,000 lux), that resistance plummets to 1kΩ–3kΩ. The response is highly non-linear and logarithmic, meaning a change from 10 to 100 lux causes a massive resistance drop, while a change from 10,000 to 10,100 lux is barely measurable.

Crucially, the raw LDR light sensor does not output a voltage or a digital signal; it is strictly a variable resistor. To interface it with a microcontroller, you must convert that resistance change into a voltage using a voltage divider circuit. Most commercial breakout boards (like the ubiquitous 3-pin or 4-pin blue modules) include a fixed 10kΩ resistor to form this divider, and often an LM393 op-amp comparator to provide a secondary digital threshold output. You must treat the analog and digital outputs as entirely separate signal paths.

Wiring Guide and Pinout Specifications

The standard 4-pin LDR module operates on a supply range of 3.3V to 5V. If you are using an ESP32, power the module from the 3.3V pin to keep the analog output within the ESP32's safe ADC input range (0–3.1V). Feeding a 5V analog signal into an ESP32 GPIO will permanently damage the silicon.

The Math: Converting Raw ADC Readings to Lux

Getting a raw ADC number is easy; converting it to a physical unit (Lux) requires reversing the voltage divider math and applying a logarithmic curve fit. Here is the exact signal chain.

Step 1: Calculate the LDR Resistance
Assuming a 10kΩ fixed resistor ($R_{fixed}$) and a 3.3V supply ($V_{cc}$), the voltage at the analog pin ($V_{out}$) is:

V_out = V_cc * (R_fixed / (R_LDR + R_fixed))

Rearranging to solve for the LDR's resistance:

R_LDR = R_fixed * ((V_cc / V_out) - 1)

Step 2: Convert Resistance to Lux
The GL5528 datasheet specifies a resistance of ~10kΩ at 10 Lux, with a log-log slope (gamma) of roughly -0.7. The empirical formula to approximate Lux from resistance (in kΩ) is:

Lux = 10 ^ (1.7 - 0.7 * log10(R_LDR_kOhm))

Here is the complete, compilable C++ implementation for the ESP32, utilizing the modern ESP-IDF ADC calibration API to fix the ESP32's notorious hardware non-linearity near 0V and 3.1V.

#include <Arduino.h>
#include <esp_adc/adc_oneshot.h>
#include <esp_adc/adc_cali.h>
#include <math.h>

#define LDR_PIN ADC_CHANNEL_6 // GPIO 34
#define R_FIXED 10000.0       // 10k Ohm fixed resistor
#define V_CC 3300.0           // 3.3V in millivolts

adc_oneshot_unit_handle_t adc_handle;
adc_cali_handle_t cali_handle;

void setup() {
  Serial.begin(115200);
  adc_oneshot_unit_init_cfg_t init_config = {.unit_id = ADC_UNIT_1};
  adc_oneshot_new_unit(&init_config, &adc_handle);
  
  adc_oneshot_chan_cfg_t config = {.bitwidth = ADC_BITWIDTH_DEFAULT, .atten = ADC_ATTEN_DB_11};
  adc_oneshot_config_channel(adc_handle, LDR_PIN, &config);
  
  adc_cali_curve_fitting_config_t cali_config = {
    .unit_id = ADC_UNIT_1, .chan = LDR_PIN, .atten = ADC_ATTEN_DB_11, .bitwidth = ADC_BITWIDTH_DEFAULT
  };
  adc_cali_create_scheme_curve_fitting(&cali_config, &cali_handle);
}

void loop() {
  int raw_adc;
  int voltage_mv;
  adc_oneshot_read(adc_handle, LDR_PIN, &raw_adc);
  adc_cali_raw_to_voltage(cali_handle, raw_adc, &voltage_mv);
  
  float v_out = voltage_mv;
  float r_ldr = R_FIXED * ((V_CC / v_out) - 1.0);
  float r_ldr_k = r_ldr / 1000.0;
  
  // Clamp to prevent math domain errors in total darkness
  if (r_ldr_k < 0.1) r_ldr_k = 0.1; 
  if (r_ldr_k > 2000) r_ldr_k = 2000;
  
  float lux = pow(10.0, (1.7 - 0.7 * log10(r_ldr_k)));
  
  Serial.printf('Voltage: %d mV | R_LDR: %.1f Ohm | Lux: %.1f\n', voltage_mv, r_ldr, lux);
  delay(500);
}

Calibration, Interference, and Real-World Gotchas

If your Lux readings look wrong on the serial monitor, you are likely hitting one of three physical interference sources inherent to CdS cells.

• Spectral Mismatch (The IR Problem): CdS photoresistors peak in sensitivity around 540nm (green light), but they remain highly sensitive into the near-infrared (IR) spectrum up to 900nm. If you calibrate your LDR under a cool-white LED desk lamp, and then test it in direct sunlight or under an incandescent bulb, the reading will spike artificially high because the LDR is "seeing" the IR heat radiation that human eyes cannot. Fix: Place a visible-pass IR-blocking filter (like a piece of exposed photographic film or specialized optical gel) over the sensor if you need human-eye-matched readings.
• Mains Flicker (100/120Hz Ripple): AC-powered room lighting turns on and off 100 or 120 times a second. An LDR is fast enough to catch this ripple, causing your ADC readings to jitter wildly. Fix: Solder a 100nF ceramic capacitor directly across the LDR legs on the module to create a low-pass hardware filter, or average 20 sequential ADC reads in software.
• Temperature Drift: CdS resistance shifts with ambient temperature. A sensor calibrated at 20°C will read roughly 10% darker at 40°C. For outdoor enclosures, this means your "sunset trigger" might fire an hour early on a hot summer day.

Decision Matrix: LDR vs. BH1750 vs. Photodiode

Do not default to an LDR just because it is cheap. Choose your sensor based on the physical requirement of your project. Use this decision path to lock in your bill of materials.

For deeper technical specifications on ESP32 ADC hardware calibration, refer to the Espressif ADC Calibration API documentation. For I2C digital alternatives, review the SparkFun BH1750 Hookup Guide to compare wiring complexity.