Light Sensors with Arduino: LDR vs BH1750

Measuring light lets a project react to its surroundings: a lamp that turns on at dusk, a display that dims in the dark, a greenhouse logger or a robot that follows a flashlight. Two sensors cover almost every hobby need. The LDR (light-dependent resistor) is cheap and simple, and the BH1750 is a small digital module that reports real light levels in lux. This guide explains both.

The LDR (photoresistor)

An LDR is a resistor whose value drops as light increases. A common part like the GL5528 measures roughly 10 to 20 kΩ in normal indoor light and around 1 MΩ in darkness, falling to a few hundred ohms in bright sunlight.

Wiring it as a voltage divider

The Arduino measures voltage, not resistance, so pair the LDR with a fixed resistor to form a voltage divider, as explained in the Ohm's law guide:

• LDR from 5 V to A0
• 10 kΩ resistor from A0 to GND

More light lowers the LDR's resistance, so the voltage at A0 rises. The 10 kΩ fixed resistor puts the most sensitive part of the range around typical room light. Use a larger value, like 100 kΩ, for dim environments.

Reading it

void loop() {
  int light = analogRead(A0);       // 0 = dark, 1023 = very bright
  Serial.println(light);
  delay(200);
}

A dusk-activated light with hysteresis

A naive "if dark, turn on" flickers at dusk, when the reading hovers around the threshold. Use two thresholds, a technique called hysteresis:

bool lampOn = false;

void loop() {
  int light = analogRead(A0);
  if (!lampOn && light < 300) lampOn = true;    // clearly dark
  if (lampOn && light > 400)  lampOn = false;   // clearly bright
  digitalWrite(7, lampOn);
  delay(200);
}

To switch a real lamp, drive a relay module from pin 7. And keep the sensor out of the lamp's own light, or it will switch itself off.

LDR limitations

• Readings are relative, not calibrated. Two LDRs give different numbers in the same light.
• They respond slowly, taking tens of milliseconds to settle after a change.
• Many older LDRs contain cadmium sulfide, which is restricted in some countries under environmental rules.

For "is it dark or bright?", an LDR is perfect.

The BH1750 digital light sensor

The BH1750 is a small chip, usually sold on a breakout board like the GY-302, that measures illuminance and reports it directly in lux, the standard unit of light. Its range is about 1 to 65 535 lux, and it communicates over I2C, so it needs no analog pin.

Typical lux values for reference:

Situation	Approximate lux
Moonlit night	under 1
Living room	50 to 200
Office desk	300 to 500
Overcast day outdoors	1 000 to 10 000
Direct sunlight	30 000 to 100 000

Direct summer sunlight can exceed the BH1750's range, so for outdoor sun measurements, choose a sensor with a wider range.

Wiring

BH1750 pin	Arduino Uno / Nano
VCC	5 V (the breakout has a regulator)
GND	GND
SDA	A4
SCL	A5
ADDR	Leave unconnected (address 0x23)

Connecting ADDR to VCC changes the address to 0x5C, which lets you use two sensors on the same bus. The OLED display in our digital thermometer project shares the same I2C wires without conflict.

Reading lux

Install the "BH1750" library by Christopher Laws from the Library Manager:

#include <Wire.h>
#include <BH1750.h>

BH1750 meter;

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

void loop() {
  float lux = meter.readLightLevel();
  Serial.print(lux, 1);
  Serial.println(" lx");
  delay(1000);
}

In the default continuous high-resolution mode, the BH1750 takes about 120 ms per measurement with 1 lux resolution.

Which one should you use?

	LDR	BH1750
Output	Relative analog value	Calibrated lux
Interface	1 analog pin	I2C (2 shared pins)
Cost	Very low	Low
Consistency between parts	Poor	Good
Best for	Dark/bright decisions, light-seeking robots	Logging, plant lighting, displays, measurements

Choose an LDR for simple switching and for robots that compare light on the left and right to steer toward a flashlight. Choose a BH1750 when you want real numbers, for example to check whether a plant corner gets enough light for the automatic watering system to be worthwhile.

Placement tips

• Face the sensor toward the light you want to measure, not toward nearby LEDs.
• A small diffuser, such as a piece of white plastic, evens out readings from direct sun.
• Log values for a full day before choosing thresholds; light changes more than your eyes suggest.