create a car is controlled from Bluetooth via Android SmartPhone

main.c

#include <SoftwareSerial.h> // Import Soft Uart library
#include <Servo.h>  // Import Servo library

// Bluetooth communication pins
#define TX 2 
#define RX 4

// L298N communication pins
#define IN1 3
#define IN2 5
#define IN3 6
#define IN4 7

// Servo communication pins
#define Servo_Pin 9

// Speed value
#define MAX_SPEED 255 
#define MIN_SPEED 155

// Servo angle
#define pos 85

// Init
SoftwareSerial blue(RX, TX);
Servo servo;

// Init bluetooth speed
void Blue_Init(){
  blue.begin(9600);  // Bluetooth use baudrate 9600
}

// Init L298N pins
void Motor_Init(){
  pinMode(IN1, OUTPUT);
	pinMode(IN2, OUTPUT);
	pinMode(IN3, OUTPUT);
	pinMode(IN4, OUTPUT);
}

// Init servo
void Servo_Init(){
  servo.attach(Servo_Pin);

  // Set servo at "pos" angle (straight forward - default)
  servo.write(pos);
}

// Stop function
void Stop(){
  digitalWrite(IN1, LOW);
	digitalWrite(IN2, LOW);

  digitalWrite(IN3, LOW);
	digitalWrite(IN4, LOW);

  servo.write(pos);
}

// Back function
void Back(int speed){
  // Speed
  speed = constrain(speed, MIN_SPEED, MAX_SPEED);
  digitalWrite(IN2, HIGH);
  analogWrite(IN1, 255 - speed);

  analogWrite(IN3, speed);
	digitalWrite(IN4, LOW);
}

// Forward function
void Forward(int speed){
  speed = constrain(speed, MIN_SPEED, MAX_SPEED);
  digitalWrite(IN2, LOW);
  analogWrite(IN1, speed);

  analogWrite(IN3, 255 - speed);
	digitalWrite(IN4, HIGH);
}

// Turn servo left
void Turn_Left(){
  servo.write(pos - 35);
}

// Turn servo right
void Turn_Right(){
  servo.write(pos + 35);
}


void setup() {
  Serial.begin(9600);
  Blue_Init();
  Motor_Init();
  Servo_Init();
}

void loop() {
  char _read;
  int speed = MIN_SPEED;
  
  //Serial.println(servo.read());
  
  if (blue.available()){ // If receive signal from Bluetooth
    _read = blue.read(); // Get the data and assign to "_read" variable

    // Speed level from 0 -> 9, respectively
    if (_read >= '0' && _read <= '9'){
      if (_read == '0') // If press '0' -> MIN SPEED
        speed = MIN_SPEED;
      else 
        speed += (int(_read) - 48)*10;
    }

     // If press 'q' -> MAX SPEED 
    if (_read == 'q')
      speed = MAX_SPEED;
      
    Serial.println(_read);

    // If system receive one of data below 
    // System will invoke corresponding functions above
    switch (_read){
      case 'S':{
        Stop();
        Serial.println("STOP");
        break;
      }
      case 'F':{
        Forward(speed);
        Serial.println("Forward");
        break;
      }
      case 'B':{
        Back(speed);
        Serial.println("BACK");
        break;
      }
      case 'L':{
        Turn_Left();
        Serial.println("LEFT");
        break;
      }
      case 'R':{
        Turn_Right();
        Serial.println("RIGHT");
        break;
      }
      case 'G':{
        Turn_Left();
        delay(10);
        Forward(speed);
        Serial.println("FL");
        break;
      }
      case 'I':{
        Turn_Right();
        delay(10);
        Forward(speed);
        Serial.println("FR");
        break;
      }
      case 'H':{
        Turn_Left();
        delay(10);
        Back(speed);
        Serial.println("BL");
        break;
      }
      case 'J':{
        Turn_Right();
        delay(10);
        Back(speed);
        Serial.println("BR");
        break;
      }
    }
  }
  delay(100);
}