Arduino Automatic Night Light with LDR

This project uses a light-dependent resistor (LDR) — a component whose resistance decreases as light increases — to sense ambient brightness. The Arduino reads the LDR’s value via an analogue pin and switches an LED (or a relay to control a mains lamp) on when it gets dark. It’s a simple but practical introduction to analogue sensors.

What You’ll Need

• Arduino Uno or Nano
• LDR (light-dependent resistor / photoresistor)
• 10 kΩ resistor (forms a voltage divider with the LDR)
• White or warm LED + 220 Ω resistor
• Breadboard and jumper wires
• USB cable or 9 V battery with barrel connector

Step-by-Step Instructions

1. Build the voltage divider: Connect one leg of the LDR to 5 V. Connect the other LDR leg to both Arduino A0 and one end of the 10 kΩ resistor. Connect the other end of the 10 kΩ resistor to GND. This forms a voltage divider whose midpoint voltage changes with light.
2. Wire the LED: Connect Arduino pin 9 through a 220 Ω resistor to the LED anode. LED cathode to GND.
3. Write the sketch:

   int ldrPin = A0;
   int ledPin = 9;
   int threshold = 400; // adjust based on your lighting
   
   void setup() { pinMode(ledPin, OUTPUT); }
   void loop() {
     int ldrVal = analogRead(ldrPin);
     if (ldrVal < threshold) digitalWrite(ledPin, HIGH);
     else digitalWrite(ledPin, LOW);
     delay(100);
   }

4. Calibrate: Upload, open Serial Monitor, and add Serial.println(ldrVal); to see real values. Note the value in normal light and in darkness. Set threshold between them.
5. Test: Cover the LDR with your hand — the LED should turn on. Uncover — it should turn off.

Safety Notes

• If extending this to control a mains lamp via a relay module, use a relay rated for your mains voltage and treat mains wiring with full safety precautions — if unsure, ask an electrician.

Tips & Troubleshooting

• If the LED flickers near the threshold, add a small hysteresis band in code: turn ON below threshold-20, turn OFF above threshold+20.
• Use a brighter LED or add more LEDs in parallel (each with own resistor) for a useful bedside night light.
• A 5 V relay module can replace the LED to switch a real lamp — keep the Arduino circuit separate from mains voltage.