Visible light communication

Mar 29, 2016

Background & Concept

One day, I was at my friend’s dorm room and we noticed that one could see my dorm from his. We were thinking about communicating with lasers from his dormitory to mine and the other way round. Of course, I took this seriously and started building it. So far I have one arduino that is able to communicate with itself via visible light. Theoretically, the only thing we would have to do is replace the LEDs used right now with lasers that the photoresistors can detect over such a large distance. But there is another problem: the signals won’t be exactly synced, unless we power both arduinos on at the exact right time. For example, a problem that can occur now if we use two distinct arduinos is that one arduino starts sending, and the other starts receiving when the fourth bit already has been sent. That’s why I will need to implement a preamble or something alike. This remains to be done. I already ordered the lasers, so I’ll keep you guys up to date if I ever find the time to work on this again.

TODOs

A lot is still wrong with this prototype. Problems/TODOs that I know of right now include:

  • Encoding should be according to ASCII
  • Code should be cleaned and documented for understandability, simplicity and compactness
  • Problem of syncing between two arduinos should be solved
  • Error correction code can be added

Components

  • 1x photoresistor
  • 1x LED
  • 1x Resistance (of about same value as photoresistor)
  • 1x Arduino USB Cable
  • 1x Arduino Uno R3
  • Some wires and a breadboard

Video

The video is available on youtube.

Circuitry

Actually it’s very simple here. What we are doing is making a voltage divider from the arduino’s 5V pin to GND, using a fixed resistor and a photoresistor. The voltage between these two resistors is then read by an analog pin.
When light shines on the photoresistor, it’s value drops and a bigger part of the 5V from the arduino drops over the fixed value resistor.

On popular demand, I also made a circuit diagram:
vlc_circuit_diagram

Arduino Code

static int TRESHOLD = 500;
static unsigned int standardDelay = 1350; //1350 is limit

void setup() {
  Serial.begin(9600); // 9600 bits per second
  pinMode(3,OUTPUT); //digital PWM 3 on output
  int sensorValue = 0;
}

void loop() {
  //read every 100ms
  //high A0 value is one, low A0 value is zero
  WriteChar('a');
  WriteChar('b');
  WriteChar('c');
  WriteChar('d');
  WriteChar('e');
  WriteChar('f');
  WriteChar('g');
  WriteChar('h');
  WriteChar('i');
  WriteChar('j');
  WriteChar('k');
  WriteChar('l');
  WriteChar('m');
  WriteChar('n');
  WriteChar('o');
  WriteChar('p');
  WriteChar('q');
  WriteChar('r');
  WriteChar('s');
  WriteChar('t');
  WriteChar('u');
  WriteChar('v');
  WriteChar('w');
  WriteChar('x');
  WriteChar('y');
  WriteChar('z');
}

void WriteChar(char str){
  switch (str - '0'){
    case 49:
      //code for a is 011 0001
      LightFlash(false, true, true, false, false, false, true);
      break;
    case 50:
      //code for b is 011 0010
      LightFlash(false, true, true, false, false, true, false);
      break;
    case 51:
      //code for c is 011 0011
      LightFlash(false, true, true, false, false, true, true);
      break;
    case 52:
      //code for d is 011 0100
      LightFlash(false, true, true, false, true, false, false);
      break;
    case 53:
      //code for e is 011 0101
      LightFlash(false, true, true, false, true, false, true);
      break;
    case 54:
      //code for f is 011 0110
      LightFlash(false, true, true, false, true, true, false);
      break;
    case 55:
      //code for g is 011 0111
      LightFlash(false, true, true, false, true, true, true);
      break;
    case 56:
      //code for h is 011 1000
      LightFlash(false, true, true, true, false, false, false);
      break;
    case 57:
      //code for i is 011 1001
      LightFlash(false, true, true, true, false, false, true);
      break;
    case 58:
      //code for j is 011 1010
      LightFlash(false, true, true, true, false, true, false);
      break;
    case 59:
      //code for k is 011 1011
      LightFlash(false, true, true, true, false, true, true);
      break;
    case 60:
      //code for l is 011 1100
      LightFlash(false, true, true, true, true, false, false);
      break;
    case 61:
      //code for m is 011 1101
      LightFlash(false, true, true, true, true, false, true);
      break;
    case 62:
      //code for n is 011 1110
      LightFlash(false, true, true, true, true, true, false);
      break;
    case 63:
      //code for o is 011 1111
      LightFlash(false, true, true, true, true, true, true);
      break;
    case 64:
      //code for p is 100 0000
      LightFlash(true, false, false, false, false, false, false);
      break;
    case 65:
      //code for q is 100 0001
      LightFlash(true, false, false, false, false, false, true);
      break;
    case 66:
      //code for r is 100 0010
      LightFlash(true, false, false, false, false, true, false);
      break;
    case 67:
      //code for s is 100 0011
      LightFlash(true, false, false, false, false, true, true);
      break;
    case 68:
      //code for t is 100 0100
      LightFlash(true, false, false, false, true, false, false);
      break;
    case 69:
      //code for u is 100 0101
      LightFlash(true, false, false, false, true, false, true);
      break;
    case 70:
      //code for v is 100 0110
      LightFlash(true, false, false, false, true, true, false);
      break;
    case 71:
      //code for w is 100 0111
      LightFlash(true, false, false, false, true, true, true);
      break;
    case 72:
      //code for x is 100 1000
      LightFlash(true, false, false, true, false, false, false);
      break;
    case 73:
      //code for y is 100 1001
      LightFlash(true, false, false, true, false, false, true);
      break;
    case 74:
      //code for z is 100 1010
      LightFlash(true, false, false, true, false, true, false);
      break;
    default:
      Serial.println(str - '0');
      Serial.println("CAME IN DEFAULT IN WRITECHAR");
      break;
  }
}

void LightFlash(boolean a, boolean b, boolean c, boolean d, boolean e, boolean f, boolean g){


  digitalWrite(3,LOW);
  if (a == true){
    digitalWrite(3,HIGH);
  }
  delayMicroseconds(standardDelay/2);
  boolean sensorValue1 = (analogRead(A0)>TRESHOLD);
  delayMicroseconds(standardDelay/2);

  digitalWrite(3,LOW);
  if (b == true){
    digitalWrite(3,HIGH);
  }
  delayMicroseconds(standardDelay/2);
  boolean sensorValue2 = (analogRead(A0)>TRESHOLD);
  delayMicroseconds(standardDelay/2);

  digitalWrite(3,LOW);
  if (c == true){
    digitalWrite(3,HIGH);
  }
  delayMicroseconds(standardDelay/2);
  boolean sensorValue3 = (analogRead(A0)>TRESHOLD);
  delayMicroseconds(standardDelay/2);

  digitalWrite(3,LOW);
  if (d == true){
    digitalWrite(3,HIGH);
  }
  delayMicroseconds(standardDelay/2);
  boolean sensorValue4 = (analogRead(A0)>TRESHOLD);
  delayMicroseconds(standardDelay/2);

  digitalWrite(3,LOW);
  if (e == true){
    digitalWrite(3,HIGH);
  }
  delayMicroseconds(standardDelay/2);
  boolean sensorValue5 = (analogRead(A0)>TRESHOLD);
  delayMicroseconds(standardDelay/2);

  digitalWrite(3,LOW);
  if (f == true){
    digitalWrite(3,HIGH);
  }
  delayMicroseconds(standardDelay/2);
  boolean sensorValue6 = (analogRead(A0)>TRESHOLD);
  delayMicroseconds(standardDelay/2);

  digitalWrite(3,LOW);
  if (g == true){
    digitalWrite(3,HIGH);
  }
  delayMicroseconds(standardDelay/2);
  boolean sensorValue7 = (analogRead(A0)>TRESHOLD);
  delayMicroseconds(standardDelay/2);
  delayMicroseconds(3*standardDelay); //1 second passed

  long result = 0;
  result = 1000000*sensorValue1 + 100000*sensorValue2 + 10000*sensorValue3
  + 1000*sensorValue4 + 100*sensorValue5 + 10*sensorValue6 + 1*sensorValue7;
  //Serial.println(result);
  PrintChar(result);
}

void PrintChar(long binary){
  switch (binary){
    case 110001:
      //code for a is 011 0001
      Serial.println("a");
      break;
    case 110010:
      //code for b is 011 0010
      Serial.println("b");
      break;
    case 110011:
      //code for c is 011 0011
      Serial.println("c");
      break;
    case 110100:
      //code for d is 011 0100
      Serial.println("d");
      break;
    case 110101:
      //code for e is 011 0101
      Serial.println("e");
      break;
    case 110110:
      //code for f is 011 0110
      Serial.println("f");
      break;
    case 110111:
      //code for g is 011 0111
      Serial.println("g");
      break;
    case 111000:
      //code for h is 011 1000
      Serial.println("h");
      break;
    case 111001:
      //code for i is 011 1001
      Serial.println("i");
      break;
    case 111010:
      //code for j is 011 1010
      Serial.println("j");
      break;
    case 111011:
      //code for k is 011 1011
      Serial.println("k");
      break;
    case 111100:
      //code for l is 011 1100
      Serial.println("l");
      break;
    case 111101:
      //code for m is 011 1101
      Serial.println("m");
      break;
    case 111110:
      //code for n is 011 1110
      Serial.println("n");
      break;
    case 111111:
      //code for o is 011 1111
      Serial.println("o");
      break;
    case 1000000:
      //code for p is 100 0000
      Serial.println("p");
      break;
    case 1000001:
      //code for q is 100 0001
      Serial.println("q");
      break;
    case 1000010:
      //code for r is 100 0010
      Serial.println("r");
      break;
    case 1000011:
      //code for s is 100 0011
      Serial.println("s");
      break;
    case 1000100:
      //code for t is 100 0100
      Serial.println("t");
      break;
    case 1000101:
      //code for u is 100 0101
      Serial.println("u");
      break;
    case 1000110:
      //code for v is 100 0110
      Serial.println("v");
      break;
    case 1000111:
      //code for w is 100 0111
      Serial.println("w");
      break;
    case 1001000:
      //code for x is 100 1000
      Serial.println("x");
      break;
    case 1001001:
      //code for y is 100 1001
      Serial.println("y");
      break;
    case 1001010:
      //code for z is 100 1010
      Serial.println("z");
      break;
    case 11000:
      Serial.println("Debug 1: character not recognised");
      break;
    case 1011001:
      Serial.println("Debug 2: character not recognised");
      break;
    case 100001:
      Serial.println("Debug 3: character not recognised");
      break;
    case 1011000:
      Serial.println("Debug 4: character not recognised");
      break;
    default:
      Serial.println(binary);
      Serial.println("CAME IN DEFAULT ON PRINTCHAR");
      break;
  }
}