#include <SoftwareSerial.h>

SoftwareSerial mySerial(2, 1); // RX, TX

int a, b, c;
int wait = 15; 

void setup()
{
  mySerial.begin(9600);
  pinMode(4, OUTPUT);
}

void loop() {

  digitalWrite(4, HIGH);   //Turning ON LED
  delayMicroseconds(500);  //wait
  a = analogRead(A3);      //take reading from photodiode(pin A3) :noise+signal
  digitalWrite(4, LOW);    //turn Off LED
  delayMicroseconds(500);  //wait
  b = analogRead(A3);      //again take reading from photodiode :noise
  c = b - a;               //taking differnce:[ (noise) - (noise+signal)] just signal

//  mySerial.println(c);     //denoised signal   OUTPUT JUST FOR DEBUGGING

  if( c <= 200 ){
    mySerial.print("10");
    delay(wait);
    mySerial.print("20");

  } if ( c >= 205 && c <= 450 ){
    mySerial.print("11");
    delay(wait);
    mySerial.print("20");

  } if ( c >= 455 && c <= 850 ){
    mySerial.print("11");
    delay(wait);
    mySerial.print("21");

  } if ( c >= 855 ){
    mySerial.print("10");
    delay(wait);
    mySerial.print("20");
    delay(wait);
    mySerial.print("11");
    delay(wait);
    mySerial.print("21");

  }
  delay(wait);
}