Reading a Photoresistor — SIK Circuit 6

Use a photoresistor (light sensor) to control LED brightness. Learn about voltage dividers, analog input, and the map() function for sensor-to-output conversion.

Instructions

Parts & Introduction

A photoresistor (or LDR — Light Dependent Resistor) changes resistance based on light levels. Combined with a fixed resistor, it forms a voltage divider that the Arduino can read as an analog value. You'll use this to control LED brightness automatically.

Parts Needed

1x Arduino Uno + USB cable
1x Breadboard
1x Photoresistor
1x LED (any color)
1x 330Ω Resistor
1x 10KΩ Resistor (for voltage divider)
6x Jumper Wires

Hardware Hookup

Wiring Instructions

Connect one side of the photoresistor to 5V.
Connect the other side to Analog Pin A0.
Connect a 10K resistor from Analog Pin A0 to GND (this completes the voltage divider).
Connect the LED positive leg to Digital Pin 9 (PWM-capable).
Connect the LED negative leg through a 330Ω resistor to GND.

The voltage divider produces a voltage proportional to light level, which the Arduino reads as 0-1023.

Arduino Code

Open the Arduino IDE and upload the following sketch to your Arduino board.

photoresistor.inoarduino

/*
SparkFun Inventor's Kit
Example sketch 06 — PHOTORESISTOR

Use a photoresistor (light sensor) to control LED brightness.

Hardware connections:
  Photoresistor: one side to 5V, other side to analog pin 0
  10K resistor between analog pin 0 and GND
  LED: positive to digital pin 9 (PWM), negative through 330 ohm to GND

This code is completely free for any use.
*/

const int sensorPin = 0;
const int ledPin = 9;

int lightLevel, high = 0, low = 1023;

void setup()
{
  pinMode(ledPin, OUTPUT);
}

void loop()
{
  lightLevel = analogRead(sensorPin);

  manualTune();

  //autoTune();

  analogWrite(ledPin, lightLevel);
}

void manualTune()
{
  lightLevel = map(lightLevel, 0, 1023, 0, 255);
  lightLevel = constrain(lightLevel, 0, 255);
}

void autoTune()
{
  if (lightLevel < low)
  {
    low = lightLevel;
  }
  if (lightLevel > high)
  {
    high = lightLevel;
  }

  lightLevel = map(lightLevel, low+30, high-30, 0, 255);
  lightLevel = constrain(lightLevel, 0, 255);
}

Test & Experiment

What You Should See

The LED brightness changes based on ambient light. Cover the photoresistor to dim or brighten the LED (depending on orientation).

Troubleshooting

LED stays dark: Check LED polarity. Also verify the photoresistor is in the circuit correctly.
Not responding to light: The photoresistor spacing is non-standard — make sure both legs are making good contact.
Subtle changes: Try using a flashlight or covering the sensor completely for more dramatic results.

Experiments to Try

Uncomment autoTune() to let the Arduino automatically calibrate to your lighting conditions.
Use the sensor to trigger actions at specific light thresholds (e.g., turn on a "night light" when dark).

Materials

•SparkFun Inventor's Kit - V3.2 - 1 kitNOK 999.20
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•Arduino Uno R3 - 1 piecePlaceholder
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•Breadboard - 1 piecePlaceholder
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•Photoresistor - 1 piecePlaceholder
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•5mm LED - 1 piecePlaceholder
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•330 Ohm Resistor - 1 pieceNOK 24.00
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•10K Ohm Resistor - 1 pieceNOK 24.00
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•Jumper Wires - 6 piecessNOK 39.20
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Tools Required

Computer with Arduino IDE

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