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Automatic Nightlight (Photoresistor + Arduino)

Turn an LED on automatically when it gets dark, using a photoresistor (LDR) voltage divider and analogRead.

Beginner20 min

What you need

Wiring

Arduino Uno
Photoresistor
LED

Hover a pin or a wire to trace the connection.

Code

nightlight.ino
// Turn an LED on when it gets dark (photoresistor voltage divider on A0).
const int ldrPin = A0;
const int ledPin = 13;
const int threshold = 400;  // tune for your room (watch the Serial Monitor)

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

void loop() {
  int light = analogRead(ldrPin); // higher in light, lower in the dark
  Serial.println(light);
  digitalWrite(ledPin, light < threshold ? HIGH : LOW);
  delay(200);
}

Steps

  1. Build the divider: LDR from 5V to A0, and a 10kΩ resistor from A0 to GND.
  2. Wire the LED on D13 through a 220Ω resistor, cathode to GND.
  3. Open the Serial Monitor, note the values in light vs. dark, set `threshold` between them.
  4. Cover the LDR — the LED should switch on.