LDR light sensor explained — how a photoresistor works with Arduino

#What it is

An LDR (light-dependent resistor, also called a photoresistor) is a resistor whose value changes with light: bright light lowers its resistance, darkness raises it. It has two legs and, like any resistor, works either way round. See it in 3D.

#How it works inside

The wavy orange line on top is a track of cadmium sulphide, a material that hardly conducts in the dark. Light knocks electrons loose inside it, and the more light, the more free electrons to carry current — so the resistance falls.

These are the values the 3D lab uses (a typical GL5528-type sensor):

Lux measures how much light falls on a surface. Ten times more light gives only about six times less resistance, so the LDR is great for "brighter or darker" but not a precise light meter.

#Why it needs a 10 kΩ resistor

An Arduino cannot measure resistance — it measures voltage. So the LDR is paired with a fixed 10 kΩ resistor to make a voltage divider: two resistors in a line between 5V and GND, with the analog pin connected where they meet.

The 5 V is shared between them in proportion to their resistance. With the LDR on top (5V side) and 10 kΩ below:

Brighter = higher reading. Swap the two parts and it flips round: brighter = lower. Why 10 kΩ? It matches the LDR in a dim room, so the reading sits in the middle of the range where it changes the most.

#The pins

#How to wire it to an Arduino

This is the wiring of the night light project:

1. Push the LDR into the breadboard, each leg in its own row.
2. 5V → one LDR leg.
3. The other LDR leg → A0, and in the same row one leg of a 10 kΩ resistor.
4. The resistor's other leg → GND.
5. For the light: pin 9 → a white LED's long leg, its short leg → 220 Ω → GND.

#Example code

The exact code of the night light — it gets brighter as the room gets darker:

const int SENSOR = A0;
const int LED = 9;
const int DARK = 400;   // below this it counts as dark

void setup() {
  pinMode(LED, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int light = analogRead(SENSOR);   // brighter = higher
  Serial.print("light ");
  Serial.println(light);
  if (light < DARK) {
    // the darker it is, the brighter the night light
    analogWrite(LED, map(light, 0, DARK, 255, 30));
  } else {
    analogWrite(LED, 0);
  }
  delay(200);
}

• analogRead(SENSOR) gives the divider's voltage as 0–1023.
• DARK = 400 is the threshold. A reading of 400 means the LDR is at about 15.6 kΩ — roughly 17 lux, a dim room.
• map(light, 0, DARK, 255, 30) turns "darker" into "brighter LED".

Play in 3DRun the code · press the parts · turn it around

#Common mistakes

• No 10 kΩ resistor. A0 reads close to 1023 all the time, whatever the light. The divider needs both parts.
• Wired to a digital pin. analogRead() only works on A0–A5.
• The LED shines on the sensor. The night light switches itself off, then on, then off… Point them away from each other.
• The threshold does not suit your room. Watch the serial monitor in the light and in the dark, and pick a number in between.

#Check yourself

#Questions people ask

#What is an LDR used for?

Anything that should react to light: night lights and street lamps that switch on at dusk, light-following robots, alarms that notice a shadow, and screens that dim in the dark.

#Does an LDR have a positive and negative leg?

No. It is a resistor, so it works either way round.

#What value resistor do I use with an LDR?

10 kΩ is the usual choice because it matches the LDR in normal indoor light. Use a bigger one (say 47 kΩ) to see more detail in the dark, or a smaller one for bright light.

#Why are my LDR readings different from yours?

Every LDR, room and lamp is a little different. The numbers are for comparing — "brighter than before" — so measure your own light and dark values first.

#Where it is used

Build the night light. The same divider idea reads a knob in the potentiometer project and a squeeze in the force sensor guide.

Last updated 2026-10-29