Light, distance and motion

LDR photoresistor with ESP32

An LDR gives you a light level, not lux. Its curve is not linear and differs between cells, so two sensors will disagree. That is fine for the job it is usually bought for: deciding that a room has gone dark and turning the display down.

Wiring takes one resistor. Connect the LDR from 3V3 to an ADC pin, and 10 kΩ from that pin to ground, so the reading rises with light. A vendor figure for the GL5528 is about 1 MΩ in darkness and 8 to 20 kΩ at 10 lux, so 10 kΩ works for room light but loses detail at the extremes. For very dim rooms, try a bigger resistor. Use an ADC1 pin, because ADC2 stops working with WiFi on.

Do not use one for anything fast. CdS cells recover slowly from bright light, and an ESP32 sensor guide notes the slower response compared with phototransistors. Smooth the reading in code, and when you auto-dim a display, add a gap between the on and off thresholds. Otherwise, the screen's own light or a passing shadow makes it flicker. Mount the cell facing away from the display. CdS cells contain cadmium, which is restricted in some products, so a commercial design usually needs another part. If you want real lux values, the BH1750 is cheap and needs no calibration. LCD Ultra Pro can read an analog pin in a rule and compare it to a value, which is enough for a dark or light decision without code.

const int LDR_PIN = 4;                // an ADC1 pin
const int DARK = 800, LIGHT = 1200;   // gap stops flicker; set your own
bool dark = false;
float level = 0;

void setup() {
  Serial.begin(115200);
  level = analogRead(LDR_PIN);
}

void loop() {
  level = level * 0.9 + analogRead(LDR_PIN) * 0.1;   // smooth
  if (!dark && level < DARK) dark = true;
  if (dark && level > LIGHT) dark = false;
  Serial.printf("%.0f %s\n", level, dark ? "dark" : "light");
  delay(100);
}

Use this information at your own risk. It is for general guidance only: the example code compiles, but it has not been tested on every part, and modules differ between sellers. Check the manufacturer's data before you build. Product names belong to their owners, and LCD Ultra Pro is not affiliated with them. See our Terms.