After reading several books and testing few basic things with my first Arduino kit I decided to try to realize a simple project: a bar graph made of LEDs controlled by a light sensor. Nothing fancy or very original, but I needed something to start!

This is what I used for this project:

• 1x Arduino Uno
• 1x large 700 tie-points breadboard
• 5x 3mm red LEDs
• 5x 220Ω resistors
• 1x LDR light sensor
• 1x 10kΩ resistor
• 9x jumper wires (male-male)

Pic1 – Arduino project assembled and ready to go!

I’m using 2 levels of brightness (full and 1/4) for every LED, so the bar graph can represents 10 levels of light intensity (f.e.: light level 5 ==> LEDs 1,2 ON full brightness, LED 3 ON 1/4 brightness, LEDs 4,5 OFF).

When starting the device in a dark room no LED is ON, as showed in Pic2.

Pic2 – low luminosity -> no LEDs ON

And obviously exposing the sensor to a close lamp turns more LEDs ON, as showed in Pic3.

Pic3 – more light more LEDs are ON

The sketch I wrote to control this device is pretty simple and the (many) comments should make it clear to everybody.

// === CONSTANTS ===
// -- INPUT --
const int PIN_SENSOR = 0;

// -- OUTPUT --
const int NUM_LEDS = 5;
const int PIN_LEDS[NUM_LEDS] = { 3, 5, 6, 9, 10 };
// === CONSTANTS END ===

// === INITIALIZATION ===
void setup()
{
  // initialize serial communication at 9600 bits per second
  Serial.begin(9600);
}
// === INITIALIZATION END ===

// === MAIN LOOP ===
void loop()
{
  // read the value from the LDR and convert it into a [0-10] range
  int light_val = analogRead(PIN_SENSOR);
  int light_lvl = light_val / 102;
  // number of full-LEDs ON
  int leds_on = light_lvl / 2;

  // DEBUG
  Serial.print("light_val: ");
  Serial.print(light_val);
  Serial.print(" -> light_lvl: ");
  Serial.print(light_lvl);
  Serial.print(" -> leds_on: ");
  Serial.println(leds_on);

  int i;

  // turn LEDs ON - full brightness
  for(i = 0; i < leds_on; i++)
    analogWrite(PIN_LEDS[i], 255);

  // light level is odd -> turn one more LED ON - 1/4 brightness 
  if(light_lvl % 2)
  {
    analogWrite(PIN_LEDS[i], 64);
    i++;
  }

  // turn remaining LEDs OFF
  for(; i < NUM_LEDS; i++)
    analogWrite(PIN_LEDS[i], 0);

  // 10 FPS
  delay(100);
}
// === MAIN LOOP END ===

This sketch is also using the serial port to print some debug output, but that code (lines 14 and 28-33) is totally optional and could be deleted without affecting the functionality of the device.

Finally, I made a short video to show you the device in action:

I perfectly know this is not going to win the “project of the year” award… but if you have any questions just leave a comment and I’ll be happy to answer.