#include <SoftwareSerial.h> //Using the Arduino library Software Serial
in order to avoid using the Hardware Serial on the Arduino Uno / microcontroller

char junk; //Defining an empty character variable named junk
String inputString=""; 			   //Defining an empty string variable named inputString

SoftwareSerial mySerial(6, 7);     // RX, TX Initializing the Software Serial

void setup() {
  Serial.begin(9600);              // Initializing the Hardware Serial
  mySerial.begin(38400);           // Initializing the Software Serial
  pinMode(4, OUTPUT);              // Defining the pin for an I/O device (vibrating motors, in our case)
  pinMode(5, OUTPUT);              // Defining the pin for an I/O device (vibrating motors, in our case)
}

void loop()
{
  if(mySerial.available()){       //Checks the Serial connection
  while(mySerial.available())
    {
      char inChar = (char)Serial.read(); //read the input
      inputString += inChar;      //make a string of the characters coming on serial
    }
    Serial.println(inputString);
    while (mySerial.available() > 0)  
    { junk = mySerial.read() ; }  // clear the serial buffer
    if(inputString == "a"){       //in case of 'a' turn the left motor on and the right motor off
      digitalWrite(l, HIGH);  
      digitalWrite(r, LOW); 
    }else if(inputString == "b"){ //incase of 'b' turn the left motor off and the right motor on
      digitalWrite(l, LOW);
      digitalWrite(r, HIGH); 
    }
    inputString = "";
  }
}