Using LDRs with Arduino for Light Detection

Making a light detection circuit is possible using Light Dependent Resistors (LDRs) with a microcontroller like Arduino. This lesson shows how to do so using the simple concept of voltage dividers.

List of Content

---

1. Needed Components

• Arduino Uno
• LDR (Light Dependent Resistor)
• 10kΩ Resistor
• 330Ω Resistor
• LED
• Breadboard
• Jumper Wires

---

2. Connections and Schematics

1. LDR Voltage Divider Circuit:
   • Connect LDR to 5V and analog pin A0.
   • Connect 10kΩ resistor between A0 and GND.
2. LED:
   • Connect anode to digital pin 9 (with 330Ω resistor).
   • Cathode to GND.

---

3. How LDRs Work & Voltage Divider

• LDR Principle: A Light dependent Resistor has a resistance that decreases or increases depending on the amount of light it is exposed to. For the LDR in this example, resistance decreases with increasing light intensity.
• Since the resistance is what changes, we cannot get a measurement directly to Arduino, because Arduino can only measure and get electrical input (voltage), not resistance. Solution? A voltage divider circuit.
  

---

4. Voltage Dividers

Voltage Divider: Convert changes in resistance into voltage changes.

The output voltage (V_out) in a voltage divider circuit is calculated using:

V_out = (R₂ / (R₁ + R₂)) × V_cc

Where:

• V_out = Output voltage (measured at the junction between R₁ and R₂)
• R₁ = Resistance of the first component (LDR in this case)
• R₂ = Fixed resistor value (10kΩ in our circuit)
• V_cc = Supply voltage (5V from Arduino)

How it works with an LDR:

As light increases → LDR resistance (R₁) decreases → V_out increases

As light decreases → LDR resistance (R₁) increases → V_out decreases

Here, LDR acts as R1 (variable resistance).

---

5. Code

const int ldrPin = A0;
const int ledPin = 9;
int threshold = 500; // Adjust based on ambient light

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

void loop() {
  int ldrValue = analogRead(ldrPin);
  Serial.print("LDR Value: ");
  Serial.println(ldrValue);
  
  if (ldrValue < threshold) {
    digitalWrite(ledPin, HIGH); // Turn LED on in darkness
  } else {
    digitalWrite(ledPin, LOW);
  }
  delay(100);
}

---

6. Notes and Advanced Tips

• While experimenting, you may want to make sure the LED light doesn’t reach the LDR (otherwise it may give a false light signal and close up the LED)
• Try to add potentiometer to adjust threshold dynamically. You should use analogRead() with a large resistor in series to the potentiometer.
• Create sunrise/sunset simulator with PWM, using analogWrite().

Now that you learned how to use LDR, you can expand even further by integrating more sensors or adding better functionality.