Bluetooth-controlled car

Learn

What you’ll build

Your robot car, driven from a phone. The phone sends letters (Forward, Back, Left, Right, Stop) over Bluetooth; the Arduino reads them and drives the motors.

Parts

Your 2WD car from the previous projects, plus:

Part	Notes
HC-05 or HC-06 Bluetooth serial module	Classic Bluetooth: works with Android “Bluetooth serial / RC car” apps
1 kΩ and 2 kΩ resistors	Voltage divider for the module’s RX pin
A phone with a Bluetooth controller app	Many free “Arduino Bluetooth RC car” apps send single letters

iPhone users

iPhones don’t support classic Bluetooth serial modules like the HC-05. Use a BLE module (such as HM-10) with a BLE app, or an ESP32, which can host a web page you drive from any phone’s browser (see the ESP32 lesson).

Wiring

HC-05 pin	Connect to
VCC	5V
GND	GND
TXD	Arduino pin 10 (SoftwareSerial RX)
RXD	Arduino pin 11 through a divider: pin 11 → 1 kΩ → RXD, and RXD → 2 kΩ → GND (its RX is 3.3 V)

Pair the phone with the module (often PIN 1234 or 0000), then connect from the app.

Safety: a failsafe

If the connection drops, the car must stop. Stop the motors if no command arrives for 500 ms.

Examples

The program

bluetooth-car.ino

#include <SoftwareSerial.h>
const int ENA = 5, IN1 = 7, IN2 = 8, ENB = 6, IN3 = 9, IN4 = 10;

void setMotor(int en, int a, int b, int speed) {   // speed -255..255
  digitalWrite(a, speed > 0);
  digitalWrite(b, speed < 0);
  analogWrite(en, min(abs(speed), 255));
}

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

void setupMotors() {
  int pins[] = {ENA, IN1, IN2, ENB, IN3, IN4};
  for (int p : pins) pinMode(p, OUTPUT);
}

SoftwareSerial bt(10, 11);   // RX, TX
unsigned long lastCmd = 0;

void setup() {
  setupMotors();
  bt.begin(9600);            // HC-05 default
}

void loop() {
  if (bt.available()) {
    char c = bt.read();
    lastCmd = millis();
    if (c == 'F') drive(200, 200);
    else if (c == 'B') drive(-200, -200);
    else if (c == 'L') drive(-150, 150);
    else if (c == 'R') drive(150, -150);
    else if (c == 'S') drive(0, 0);
  }
  if (millis() - lastCmd > 500) drive(0, 0);   // failsafe: no signal, stop
}

Many apps send the command repeatedly while a button is held, which keeps the failsafe happy. If yours only sends once, make the timeout longer.

Practice

Try it in the virtual Arduino

Press Start simulator, paste the sketch into sketch.ino (no extra wiring needed) and press ▶. Type a letter into the Serial Monitor box under the board and press Enter.

diagram.json

{
  "version": 1,
  "author": "RoboLearn",
  "editor": "wokwi",
  "parts": [
    {
      "type": "wokwi-arduino-uno",
      "id": "uno",
      "top": 0,
      "left": 0,
      "attrs": {}
    }
  ],
  "connections": [],
  "dependencies": {}
}

sketch.ino

// In the simulator the Serial Monitor stands in for Bluetooth:
// type F, B, L, R or S and press Enter.
void setup() {
  Serial.begin(9600);
  Serial.println("Ready: send F B L R S");
}

void loop() {
  if (Serial.available()) {
    char c = Serial.read();
    if (c == 'F') Serial.println("forward  -> drive(200, 200)");
    else if (c == 'B') Serial.println("backward -> drive(-200, -200)");
    else if (c == 'L') Serial.println("left     -> drive(-150, 150)");
    else if (c == 'R') Serial.println("right    -> drive(150, -150)");
    else if (c == 'S') Serial.println("stop     -> drive(0, 0)");
  }
}

Simulated remote control over SerialOpen full screen ↗

Start simulator

Runs on Wokwi, free, no account needed.

Add a hornEasy

Add an H command that prints “beep” (on the real car: a buzzer tone()). else if (c == 'H') Serial.println("beep");

Speed gearsMedium

Add commands 1, 2 and 3 that set a speed variable to 120, 180 and 255, used by the drive commands. Add int spd = 180; then else if (c == '1') spd = 120; etc., and use drive(spd, spd) for F.

Test the failsafeHard

In the simulator version, print “STOP (failsafe)” when no command has arrived for 2 seconds after the last one. Track lastCmd and a stopped flag so it prints only once.

Quick check

1. Why put a voltage divider on the HC-05’s RXD pin?

Its RX pin expects 3.3 VTo make it fasterTo pair itIt isn’t needed

2. What should the car do if the connection drops?

Keep the last commandStopSpeed upReverse

3. Can an iPhone talk to an HC-05?

Yes, alwaysNo; use BLE or an ESP32 web page insteadOnly with a cableOnly in airplane mode

Check my answers