Sensing Light

In this lesson, you will learn how to measure light intensity using an Analog Input. You will build on lesson 8 and use the level of light to control the number of LEDs to be lit.

• (8) 5mm red LED
• (8) 270 Ω Resistors (red, purple, brown stripes)
• (1) kΩ Resistor (brown, black, red stripes)
• (1) 74HC595 Shift Register
• (1) Photocell (Light Dependent Resistor)
• (1) Half-size breadboard
• (1) Arduino Uno R3
• Jumper wires

int lightPin = 0;

int latchPin = 5;

int clockPin = 6;

int dataPin = 4;

int leds = 0;

void setup() 

{

  pinMode(latchPin, OUTPUT);

  pinMode(dataPin, OUTPUT);  

  pinMode(clockPin, OUTPUT);

}

void loop() 

{

  int reading  = analogRead(lightPin);

  int numLEDSLit = reading / 57;  //1023 / 9 / 2

  if (numLEDSLit > 8) numLEDSLit = 8;

  leds = 0;   // no LEDs lit to start

  for (int i = 0; i < numLEDSLit; i++)

  {

    leds = leds + (1 << i);  // sets the i'th bit

  }

  updateShiftRegister();

}

void updateShiftRegister()

{

   digitalWrite(latchPin, LOW);

   shiftOut(dataPin, clockPin, LSBFIRST, leds);

   digitalWrite(latchPin, HIGH);

}