Light sensors (LDR)

What you’ll learn

How an LDR (light-dependent resistor) works
Why it needs a voltage divider
How to calibrate a light sensor and use a threshold

The LDR

An LDR’s resistance changes with light: high in the dark (hundreds of kΩ or more) and low in bright light (around 1 kΩ or less). The Arduino can’t read resistance directly, only voltage.

The voltage divider

Put the LDR in series with a fixed resistor (often 10 kΩ) and read the voltage in the middle:

 5V ── LDR ──┬── 10 kΩ ── GND
             │
             A0

With the LDR on top, more light → lower LDR resistance → higher voltage on A0 → bigger number. Swap the two parts and it works the other way round. Many LDR modules contain this divider already and give an analog output (AO), sometimes plus a digital output (DO) with a threshold you set with a small screw.

Calibrate, then threshold

Light levels differ in every room, so don’t guess numbers:

Print the reading in the dark (cover the sensor) and in normal light.
Pick a threshold halfway between them.
Use it: if (light < threshold) { ... }.

Example: an automatic night light

const int THRESHOLD = 400;   // set from your own readings

void setup() { pinMode(LED_BUILTIN, OUTPUT); }

void loop() {
  int light = analogRead(A0);
  digitalWrite(LED_BUILTIN, light < THRESHOLD ? HIGH : LOW);
  delay(50);
}

(If your wiring gives bigger numbers in the dark, flip the < to >.)

Example: a light-seeking robot

Two LDRs, one on each front corner. Steer towards the brighter side:

int left = analogRead(A0), right = analogRead(A1);
if (left > right + 30) {
  // turn left
} else if (right > left + 30) {
  // turn right
} else {
  // go straight
}

The + 30 is a dead band: it stops the robot twitching when both sides are nearly equal.