LDR light sensor with Arduino Nano — automatic night lamp

An LDR module is the classic way to switch a light at dusk: a garden lamp, a porch light or a night lamp for the stairs. It gives both an analog level (AO) and a ready-made on/off output (DO).

On the Nano, AO goes on the analog-only A6 and DO on D3. The sketch uses AO with two thresholds so the on-board LED (standing in for the lamp) switches cleanly. To drive a real lamp, add a relay module and keep any 230 V wiring for an electrician.

Wiring

Every connection
Arduino Nano	LDR light sensor module	Wire	Notes
5V	VCC	red
GND	GND	black
D3	DO	green	Comparator output, threshold set by the trimmer.
A6	AO	yellow	Analog-only pin — ideal for AO.

3.3–5 V part, 5 V board. AO swings up to VCC. On ESP32 and Pico, power the module from 3.3 V so AO never exceeds the ADC's 3.3 V limit; on Uno use 5 V.

Board quirks

On most modules the reading goes up as it gets darker. If yours goes down, swap the comparison in the sketch.
Two thresholds (700 on, 600 off) stop the lamp flickering at dusk. A single threshold makes it chatter on cloudy evenings.
Keep the LDR away from the lamp it controls, or the lamp’s own light switches it off again.
DO is fine for a simple on/off; AO lets you choose the switching level in code without opening the box to turn a trimmer.

Code

ldr-nano.inoArduino C++Open in Playground Flash

// LDR light-sensor module on an Arduino Nano as an automatic night lamp (on-board LED).
// VCC → 5V, GND → GND, AO → A6 (analog-only pin), DO → D3.
// Uses hysteresis so the lamp doesn't flicker at dusk.
const uint8_t PIN_AO = A6;
const uint8_t PIN_DO = 3;
const int DARK_ON = 700;   // higher = darker on most modules (LDR to GND side)
const int DARK_OFF = 600;

bool lampOn = false;

void setup() {
  Serial.begin(115200);
  pinMode(PIN_DO, INPUT);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  int level = analogRead(PIN_AO);
  if (!lampOn && level > DARK_ON) lampOn = true;
  if (lampOn && level < DARK_OFF) lampOn = false;
  digitalWrite(LED_BUILTIN, lampOn ? HIGH : LOW);
  Serial.print(F("AO: "));
  Serial.print(level);
  Serial.print(F("  DO: "));
  Serial.print(digitalRead(PIN_DO) ? F("dark") : F("light"));
  Serial.print(F("  Lamp: "));
  Serial.println(lampOn ? F("ON") : F("OFF"));
  delay(500);
}

// LDR light-sensor module on an Arduino Nano as an automatic night lamp (on-board LED).
// VCC → 5V, GND → GND, AO → A6 (analog-only pin), DO → D3.
// Uses hysteresis so the lamp doesn't flicker at dusk.
const uint8_t PIN_AO = A6;
const uint8_t PIN_DO = 3;
const int DARK_ON = 700;   // higher = darker on most modules (LDR to GND side)
const int DARK_OFF = 600;

bool lampOn = false;

void setup() {
  Serial.begin(115200);
  pinMode(PIN_DO, INPUT);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  int level = analogRead(PIN_AO);
  if (!lampOn && level > DARK_ON) lampOn = true;
  if (lampOn && level < DARK_OFF) lampOn = false;
  digitalWrite(LED_BUILTIN, lampOn ? HIGH : LOW);
  Serial.print(F("AO: "));
  Serial.print(level);
  Serial.print(F("  DO: "));
  Serial.print(digitalRead(PIN_DO) ? F("dark") : F("light"));
  Serial.print(F("  Lamp: "));
  Serial.println(lampOn ? F("ON") : F("OFF"));
  delay(500);
}

Expected serial output

Serial Monitor · 115200Output

AO: 214  DO: light  Lamp: OFF
AO: 712  DO: dark  Lamp: ON

AO: 214  DO: light  Lamp: OFF
AO: 712  DO: dark  Lamp: ON

Common errors

AO stuck at 0 or 1023

AO on a pin that is not analog, or VCC/GND reversed on the module.

Lamp switches on and off at dusk

Thresholds too close together, or the lamp lighting the sensor. Widen the gap and shield the LDR.