Light Sensor Circuit: Build an Automatic LED with a Photoresistor

A light sensor circuit uses a photoresistor (LDR) to detect ambient light and automatically turn on an LED when it gets dark. This is the same principle behind automatic night lights, street lights, and garden solar lamps. You can build the basic version with just a few components and no programming, or add an Arduino for precise control and adjustable thresholds.

What you need

Basic version (no Arduino)

Arduino version (adds precision)

Same components as above, but the transistor is optional — the Arduino can drive the LED directly from a digital pin and read the LDR on an analog pin.

How the photoresistor works

A photoresistor (light-dependent resistor, LDR) is made of a semiconductor material whose resistance changes with light exposure:

The LDR is not polarized — it works in either direction, just like a regular resistor.

The voltage divider principle

The LDR and a fixed resistor form a voltage divider. The midpoint voltage changes with light level:

V_mid = V_supply × R_fixed / (R_LDR + R_fixed)

• Bright light: R_LDR is low (~1 kΩ). V_mid ≈ 5V × 10k / (1k + 10k) ≈ 4.5V (high).
• Darkness: R_LDR is high (~1 MΩ). V_mid ≈ 5V × 10k / (1M + 10k) ≈ 0.05V (low).

To make the LED turn on in the dark, we swap the positions — put the LDR on top (connected to positive) and the fixed resistor on the bottom (connected to ground). Now V_mid is high when it's dark and low when it's bright:

V_mid = V_supply × R_LDR / (R_LDR + R_fixed)

• Darkness: R_LDR is high → V_mid is high → transistor turns on → LED lights.
• Bright: R_LDR is low → V_mid is low → transistor off → LED off.

Building the basic circuit (no Arduino)

1. Set up power: connect your 5V supply to the breadboard power rails (positive and ground).
2. Voltage divider: Connect the LDR between the positive rail and a row (call it row A). Connect the 10 kΩ resistor between row A and the ground rail. Row A is now the midpoint — its voltage varies with light.
3. Transistor: Place the 2N2222 on the breadboard. Connect the base to row A through a 10 kΩ resistor (to limit base current). Connect the emitter to ground.
4. LED: Connect the 220Ω resistor from the positive rail to the LED's anode (long leg). Connect the LED's cathode to the transistor's collector.
5. Cover the LDR with your hand — the LED should light up. Uncover it — the LED should turn off.

Adjusting sensitivity

If the LED turns on/off at the wrong light level, change the 10 kΩ fixed resistor:

• Higher value (47 kΩ): LED turns on in dimmer conditions (less sensitive).
• Lower value (4.7 kΩ): LED turns on earlier, in brighter conditions (more sensitive).

For fine-tuning, replace the fixed resistor with a 100 kΩ potentiometer. Turn the pot to set the exact light level where the LED triggers.

Arduino version

Using an Arduino adds precision — you can read exact light levels, set thresholds in code, and add features like hysteresis (to prevent flickering at the threshold).

Wiring

1. Connect the LDR between 5V and analog pin A0.
2. Connect a 10 kΩ resistor between A0 and GND (completing the voltage divider).
3. Connect an LED with a 220Ω resistor from digital pin 8 to GND.

Code

const int LDR_PIN = A0;
const int LED_PIN = 8;
const int DARK_THRESHOLD = 700;  // Adjust this value (0-1023)

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

void loop() {
  int lightLevel = analogRead(LDR_PIN);
  
  Serial.print("Light level: ");
  Serial.println(lightLevel);
  
  if (lightLevel > DARK_THRESHOLD) {
    digitalWrite(LED_PIN, HIGH);  // Dark - turn LED on
  } else {
    digitalWrite(LED_PIN, LOW);   // Bright - turn LED off
  }
  
  delay(200);
}

How to calibrate

1. Upload the code and open the Serial Monitor (Tools → Serial Monitor).
2. Observe the light level value in bright conditions (might be ~200–400).
3. Cover the LDR and note the dark value (might be ~800–950).
4. Set DARK_THRESHOLD to a value between the two (e.g., 700).
5. Re-upload and test — the LED should turn on when you cover the LDR.

Adding hysteresis (preventing flicker)

Without hysteresis, the LED flickers when the light level is right at the threshold. Fix this by using two thresholds — one to turn on and a lower one to turn off:

const int THRESHOLD_ON = 700;   // Turn on when above this
const int THRESHOLD_OFF = 500;  // Turn off when below this
bool ledState = false;

void loop() {
  int lightLevel = analogRead(LDR_PIN);
  
  if (lightLevel > THRESHOLD_ON) {
    ledState = true;
  } else if (lightLevel < THRESHOLD_OFF) {
    ledState = false;
  }
  // Between thresholds: keep current state (hysteresis)
  
  digitalWrite(LED_PIN, ledState ? HIGH : LOW);
  delay(100);
}

Real-world applications

Troubleshooting

Summary

A light sensor circuit uses an LDR (photoresistor) whose resistance changes with light level. Paired with a fixed resistor in a voltage divider, it produces a variable voltage that can drive a transistor switch (basic version) or be read by an Arduino analog input (programmable version). The circuit turns an LED on in darkness and off in light. Adjust sensitivity by changing the fixed resistor value; add hysteresis to prevent flickering at the switching threshold.