Overview

The KY-018 is a classic photoresistor (Light Dependent Resistor / LDR) module from the Keyes 37-in-1 sensor kit series. Built around a 5mm Cadmium Sulfide (CdS) photoconductive cell, its internal electrical resistance decreases continuously as ambient light exposure increases.

The module pairs the LDR in series with an onboard $10\ \text{k}\Omega$ fixed pull-down resistor to form a simple voltage divider circuit. It outputs a continuous analog voltage (AO) that rises under bright illumination and drops in dark environments, making it ideal for automatic nightlights, solar panel trackers, and light level monitors.

Quick reference

Pinout

Standard 3-pin 0.1" (2.54 mm) module header:

        ┌───────────────────┐
        │  [CdS LDR Sensor] │
        └─┬───────┬───────┬─┘
          S      VCC     GND
          1       2       3

Specifications

Voltage Divider Circuit Math

$V_{AO} = V_{CC} \times \left( \frac{10\text{k}\Omega}{R_{LDR} + 10\text{k}\Omega} \right)$

• Bright Room ($R_{LDR} \approx 2\ \text{k}\Omega$):

$V_{AO} = 5.0\text{V} \times \left( \frac{10\text{k}}{2\text{k} + 10\text{k}} \right) \approx 4.16\text{V} \quad (ADC \approx 850)$

• Pitch Darkness ($R_{LDR} \approx 2\ \text{M}\Omega$):

$V_{AO} = 5.0\text{V} \times \left( \frac{10\text{k}}{2000\text{k} + 10\text{k}} \right) \approx 0.02\text{V} \quad (ADC \approx 5)$

Wiring

Example

const int ldrPin = A0;

void setup() {
  Serial.begin(9600);
}

void loop() {
  int rawADC = analogRead(ldrPin);
  float voltage = (rawADC / 1023.0) * 5.0;

  Serial.print("Raw ADC: "); Serial.print(rawADC);
  Serial.print(" | Voltage: "); Serial.print(voltage); Serial.print(" V | Status: ");

  if (rawADC > 700) {
    Serial.println("Bright Daylight");
  } else if (rawADC > 300) {
    Serial.println("Indoor Lighting");
  } else {
    Serial.println("Dark Environment");
  }

  delay(300);
}

Common mistakes

• Expecting calibrated Lux values: CdS photoresistors have high batch tolerances ($\pm 40\%$) and non-linear logarithmic response curves. They are qualitative ambient light indicators rather than precision Lux meters (use BH1750 or TSL2591 for calibrated Lux).
• Inverting power and ground pins: Connecting VCC to Pin 3 (-) short-circuits the supply rail through the $10\ \text{k}\Omega$ resistor when the LDR becomes bright.

Notes

• KY-018 vs TEMT6000 vs BH1750: KY-018 uses a simple CdS photoresistor; TEMT6000 uses an NPN phototransistor; BH1750 uses a calibrated digital $I^2C$ sensor.