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
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
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
Arduino Code
Open the Arduino IDE and upload the following sketch to your Arduino board.
/*
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
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).
Matériaux
- •SparkFun Inventor's Kit - V3.2 - 1 kitNOK 999.20
- •Arduino Uno R3 - 1 pieceEspace réservé
- •Breadboard - 1 pieceEspace réservé
- •Photoresistor - 1 pieceEspace réservé
- •5mm LED - 1 pieceEspace réservé
- •330 Ohm Resistor - 1 pieceNOK 24.00
- •10K Ohm Resistor - 1 pieceNOK 24.00
- •Jumper Wires - 6 piecessNOK 39.20
Outils requis
- Computer with Arduino IDE
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