Bluetooth RC car with an HC-05 on an Arduino Uno

// Bluetooth RC car: an HC-05, an L298N and a 2WD chassis, driven by the
// letters the phone apps send.
//
//   HC-05    TXD -> pin 12 (SoftwareSerial RX)
//            RXD <- pin 11 through 1k, with 2k from RXD to GND (5 V down to 3.3 V)
//            VCC 5V, GND
//   L298N    ENA pin 5, IN1 pin 6, IN2 pin 7    left motor (ENA jumper off)
//            ENB pin 10, IN3 pin 8, IN4 pin 9   right motor (ENB jumper off)
//            12V from the 7.4 V pack, GND to the Uno's GND
//
// The "Arduino Bluetooth RC Car" style apps send one letter while a button
// is held, and the same letters are what most car sketches expect:
//
//   F forward   B back   L spin left   R spin right   S stop
//   G forward left   I forward right   H back left   J back right
//   0 to 9 and q   the speed slider: 0 is stopped, q is full speed
//
// In Mokxi the far end of the HC-05 is the module's own box in the serial
// monitor, so type a letter there (F, then Enter) and the car drives across
// the floor drawn in the chassis. There is no radio here: what the board
// sees is the UART, which is all it ever sees of a paired phone.
//
// The HC-05's RXD is a 3.3 V input. The Uno's pin 11 is 5 V, so the 1k and 2k
// divider brings it down to 3.3 V. Its TXD is 3.3 V, which the Uno reads as
// HIGH without help.
//
// A real app keeps sending the letter while the button is held and sends S
// when it is let go. If the link drops mid-drive the car keeps going, so the
// STATE pin or a timeout is worth adding before it goes near the stairs.

#include <SoftwareSerial.h>

SoftwareSerial bluetooth(12, 11);  // RX, TX

const int ENA = 5;
const int IN1 = 6;
const int IN2 = 7;
const int ENB = 10;
const int IN3 = 8;
const int IN4 = 9;

int speed = 200;  // 0 to 255, set by the app's slider
char moving = 'S';

// One side of the car, as a signed speed from -255 to 255. Zero brakes.
void side(int en, int a, int b, int s) {
  digitalWrite(a, s > 0 ? HIGH : LOW);
  digitalWrite(b, s < 0 ? HIGH : LOW);
  if (s < 0) s = -s;
  analogWrite(en, s == 0 ? 255 : (s > 255 ? 255 : s));
}

void drive(int left, int right) {
  side(ENA, IN1, IN2, left);
  side(ENB, IN3, IN4, right);
}

// The inner wheel of a curve turns at a third of the speed.
void move(char c) {
  int s = speed;
  switch (c) {
    case 'F': drive(s, s); break;
    case 'B': drive(-s, -s); break;
    case 'L': drive(-s, s); break;
    case 'R': drive(s, -s); break;
    case 'G': drive(s / 3, s); break;
    case 'I': drive(s, s / 3); break;
    case 'H': drive(-s / 3, -s); break;
    case 'J': drive(-s, -s / 3); break;
    default: drive(0, 0); c = 'S'; break;
  }
  moving = c;
}

void setup() {
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
  pinMode(IN3, OUTPUT);
  pinMode(IN4, OUTPUT);
  drive(0, 0);
  Serial.begin(9600);
  bluetooth.begin(9600);  // the HC-05's factory baud
  Serial.println("Bluetooth car ready: F B L R G I H J S, 0-9 and q for speed");
}

void loop() {
  if (!bluetooth.available()) return;
  char c = bluetooth.read();
  if (c == '\r' || c == '\n') return;

  if (c >= '0' && c <= '9') {
    speed = (c - '0') * 25;  // 0, 25, 50 ... 225
  } else if (c == 'q') {
    speed = 255;
  } else {
    move(c);
    Serial.print("drive ");
    Serial.print(moving);
    Serial.print(" at ");
    Serial.println(speed);
    return;
  }
  // A new speed applies to whatever the car is doing now.
  move(moving);
  Serial.print("speed ");
  Serial.println(speed);
  bluetooth.print("speed ");
  bluetooth.println(speed);
}