LDR + ESP32: Pinout, Wiring Diagram and Working Code

What is the LDR pinout?

A photoresistor is a two-terminal resistor whose value falls as light increases, as the GL55-series manual explains. It has no polarity, so either lead can go to either side of the circuit. The ESP32 cannot measure resistance directly, which is why the LDR goes into a voltage divider.

5 mm LDR terminals

Lead	Function	Source
Lead 1	Resistor terminal (no polarity)	GL55-series manual + photo
Lead 2	Resistor terminal (no polarity)	GL55-series manual + photo

How do you wire the LDR to an ESP32?

LDR divider to ESP32 DevKit (30-pin) wiring

LDR pin	ESP32 DevKit pin	Why this pin
LDR lead 1	3V3	Top of the divider
LDR lead 2	GPIO34	Input-only ADC1 pin: works with Wi-Fi on, cannot be driven by mistake
10 kΩ resistor	GPIO34 to GND	Bottom of the divider; sets the useful range

Swap the LDR and the resistor if you want the reading to rise in darkness instead. The listing pairs this LDR with a 10 kΩ resistor, which suits indoor light levels.

Which ESP32 pins should you avoid?

Every GPIO choice on this page follows the same rules for the 30-pin ESP32-WROOM-32 DevKit (the board Compoden stocks as the ESP32 30-pin CP2102 development board). Espressif's ESP32 GPIO reference is the source:

• Never use GPIO6–11. They connect to the module's SPI flash.
• Avoid the strapping pins GPIO0, 2, 5, 12 and 15 for sensors. Their level at reset decides boot mode and flash voltage, so a sensor holding one of them can stop the board booting. GPIO0 is not even on the 30-pin header; it only goes to the BOOT button.
• GPIO34, 35, 36 and 39 are input-only and have no internal pull-up or pull-down. They are ideal for analog inputs and useless for outputs or clocks.
• Use ADC1 (GPIO32–39) for analog readings. ADC2 pins (GPIO0, 2, 4, 12–15, 25–27) cannot be read while Wi-Fi is running.
• Leave GPIO1 and GPIO3 alone. They carry the USB serial link used for uploads and the Serial Monitor.
• ESP32 GPIOs are 3.3 V only. They are not 5 V tolerant.

What voltage and resistance should you expect?

The GL55-series manual rates the 5 mm parts for up to 150 V and 100 mW (90 mW for the GL5516), far above anything a 3.3 V divider applies. It lists the GL5528 at 10–20 kΩ at 10 lux, at least 1 MΩ in darkness and about 20 ms rise and 30 ms decay time. Treat those as typical figures and measure your own part with a multimeter if the exact value matters.

The ESP32 reads 0–4095 by default with 12-bit resolution. Its ADC is least accurate near both ends of the range, so choose a resistor that puts your normal light level near the middle. analogReadMilliVolts() applies the chip's calibration and is more useful than the raw count when you compare boards.

Working Arduino code for the LDR on ESP32

No library is needed. The sketch prints the raw value, the calibrated millivolts and a bright/dark label with a threshold you can tune.

// 5 mm LDR (photoresistor) + ESP32 DevKit (ESP32-WROOM-32)
// Wiring: 3V3 -> LDR -> GPIO34 -> 10 kOhm resistor -> GND (brighter light = higher reading)
// GPIO34 is an input-only ADC1 pin, so it keeps working when Wi-Fi is on.
const int LDR_PIN = 34;
const int BRIGHT_THRESHOLD = 2000;   // tune for your room

void setup() {
  Serial.begin(115200);
  analogReadResolution(12);          // 0-4095
}

void loop() {
  int raw = analogRead(LDR_PIN);
  int mv = analogReadMilliVolts(LDR_PIN);
  Serial.printf("LDR raw=%d  %d mV  %s\n", raw, mv, raw > BRIGHT_THRESHOLD ? "bright" : "dark");
  delay(500);
}

Compile check: this exact sketch compiled for the esp32:esp32:esp32 board (ESP32 Dev Module, ESP32 Arduino core 3.3.10) on 5 October 2026. A clean compile proves the code and libraries agree. It does not prove your wiring, so test it on the bench with the Serial Monitor at 115200 baud.

Common mistakes with the LDR and ESP32

• Using an ADC2 pin (GPIO0, 2, 4, 12–15, 25–27). Its readings stop when Wi-Fi starts.
• No fixed resistor. Without the divider the pin floats and the readings are noise.
• Wiring the top of the divider to 5 V. The pin could then see more than 3.3 V in bright light.
• Expecting lux. An LDR gives relative light levels. For calibrated lux, use a digital light sensor.
• Too fast a response expected. The manual lists tens of milliseconds to respond, so it is not a data receiver.

Where can you get the parts?

Parts used in this guide (Compoden catalogue)

Item	Role	Link
5 mm LDR photoresistor	Light-dependent resistor	View LDR
ESP32 30-pin CP2102 development board	Controller with ADC, Wi-Fi and Bluetooth	View ESP32 board

Want this sensor in a bigger build? Describe the whole device in Soldr, for example “I want to build an automatic night lamp with an ESP32 and an LDR”. Soldr returns the exact parts, a wiring map and code that compiles, all from one parts list, so the pins in the code match the pins in the wiring. Compoden stocks the parts and delivers across India.

Sources

Photoresistor behaviour, ratings and GL5528 figures: Senba Optical & Electronic GL55-series CdS photoresistor manual (retailer-hosted copy). ESP32 ADC and pin rules: Espressif ESP32 GPIO reference. Model and divider value: Compoden LDR listing.

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Frequently asked questions

Which ESP32 pin should I use for an LDR?

Use an ADC1 pin, GPIO32 to GPIO39. GPIO34 is a good choice because it is input-only and keeps working while Wi-Fi is on.

What resistor goes with an LDR on an ESP32?

A 10 kΩ resistor from the analog pin to GND, with the LDR from 3V3 to the pin, works for typical indoor light. Pick a value close to the LDR's resistance at your normal light level.

Does an LDR have polarity?

No. It is a resistor, so either lead can go either way.