Bluetooth-Controlled Car

Learning Goals 5 min

Cluster E ends with the project everyone wants to build: a phone-controlled robot car. You already have all the pieces — L298N + chassis (L03-10), HC-05 + level shifter (L03-23), command protocol (L03-25). Today you combine them, install a real joystick-style app on your phone, and drive a robot around your living room from a remote. By the end of this lesson you will:

1. Wire and configure a fully untethered 2WD robot: chassis + L298N + HC-05 + UNO + battery, no USB cable required.
2. Use a published Android "Arduino BT Car Controller"-style app — or Adafruit Bluefruit Connect on iOS / Android — to drive with a touch joystick / d-pad.
3. Add real-time speed throttling via a slider, plus an emergency stop that overrides everything, plus a heartbeat that times out the motors if the phone disconnects.

Warm-Up 10 min

The full BOM (Bill of Materials) for today:

• 2WD chassis (L03-10 build).
• 2 × DC gearmotors.
• L298N motor driver.
• HC-05 Bluetooth module (renamed and PIN-set from L03-24).
• 1 kΩ + 2 kΩ resistors for the HC-05 RX level shift.
• 4 × AA pack for motors.
• Arduino UNO + USB cable (only for upload / fallback power).
• Android phone with "Serial Bluetooth Terminal" OR "Arduino BT Car Controller" OR Adafruit Bluefruit Connect installed.

App choices

For HC-05: use Serial Bluetooth Terminal or an Arduino-Car app. For BLE peripherals, use Bluefruit Connect or nRF Connect. Today's build assumes HC-05.

New Concept · Connection-aware safety 20 min

The new ideas vs L03-25

1. Live joystick mode: continuous "F"/"B"/"L"/"R" characters arrive every ~50–100 ms while you're pressing. When you release, the app stops sending — but our existing sketch keeps the last command active forever. Solution: implement a per-command timeout. If no command for > 300 ms, stop the motors.
2. Connection drop = stop. If the phone goes out of range mid-drive, the chassis would coast forever and crash. Solution: the HC-05's STATE pin (or a heartbeat timeout) detects disconnect → stop.
3. Speed slider: a SPEED command updates the duty for subsequent F/B/L/R, just like L03-25. Live throttling.

The watchdog pattern

unsigned long lastCmdAt = 0;
const unsigned long CMD_TIMEOUT_MS = 300;

void loop() {
  // ... existing parse + dispatch ...

  // Auto-stop if no command arrived recently
  if (millis() - lastCmdAt > CMD_TIMEOUT_MS) {
    stopAll();
  }
}

void dispatch(...) {
  lastCmdAt = millis();   // reset watchdog on every received command
  // ... actual dispatch ...
}

So the car's default state, in the absence of input, is "stopped". Hold the d-pad arrow → app sends F at 10 Hz → watchdog stays reset → motor runs. Release → app stops sending → 300 ms later → motor stops. Natural "dead-man's switch" behaviour.

Heartbeat for connection-loss detection

Many phone-side car-controller apps DO send a heartbeat character (often H or K) so the robot knows the connection is alive. We treat any character as a heartbeat — receiving F or even an unknown char counts. The CMD_TIMEOUT_MS handles both "no input from user" and "phone disconnected" with one mechanism.

Architecture diagram

The complete sketch has three concurrent jobs in one loop():

1. Bluetooth parser: drain incoming bytes, dispatch.
2. Watchdog: stop motors if no command for 300 ms.
3. Heartbeat output: send "# alive" every 2 s so the phone screen confirms the link.

All three non-blocking, all sharing the same loop. The pattern we've been building toward since L02-35.

Worked Example · The full chassis sketch 30 min

Step 1 — assemble

The chassis from L03-10 with the HC-05 from L03-25, all wired as before. Battery pack at the back, Arduino in the middle, HC-05 forward (the antenna trace likes a clear line-of-sight).

Step 2 — the production sketch

// L03-28 · Bluetooth-controlled robot car (HC-05 + L298N + 2WD chassis)
// Single-char drive commands: F B L R S
// Keyword config: SPEED nnn (0..255), STOP (alias for S)
// Watchdog: if no command for CMD_TIMEOUT_MS, motors stop.
// Heartbeat: '# alive' every 2 s to the phone.
#include <SoftwareSerial.h>
#include "motor.h"

const int BT_RX = 2, BT_TX = 3;
SoftwareSerial bt(BT_RX, BT_TX);

const Motor motorL = {9, 8, 5};
const Motor motorR = {7, 6, 11};

int driveDuty = 200;
String line;

const unsigned long CMD_TIMEOUT_MS  = 300;
const unsigned long HEARTBEAT_MS    = 2000;
unsigned long lastCmdAt = 0;
unsigned long lastHeartbeat = 0;
String lastDir = "S";

void driveForward()  { motorSpeed(motorL,  driveDuty);
motorSpeed(motorR,  driveDuty);
}
void driveBackward() { motorSpeed(motorL, -driveDuty);
motorSpeed(motorR, -driveDuty);
}
void spinLeft()      { motorSpeed(motorL, -driveDuty);
motorSpeed(motorR,  driveDuty);
}
void spinRight()     { motorSpeed(motorL,  driveDuty);
motorSpeed(motorR, -driveDuty);
}
void stopAll()       { motorSpeed(motorL, 0);
motorSpeed(motorR, 0);
}

void applyDir(char d) {
  switch (d) {
    case 'F': case 'f': driveForward();
    lastDir = "F";
    break;
    case 'B': case 'b': driveBackward();
    lastDir = "B";
    break;
    case 'L': case 'l': spinLeft();
    lastDir = "L";
    break;
    case 'R': case 'r': spinRight();
    lastDir = "R";
    break;
    case 'S': case 's':
    default:            stopAll();
    lastDir = "S";
    break;
  }
}

void dispatch(const String& cmd) {
  if (cmd.length() == 0) return;
  lastCmdAt = millis();

  if (cmd.startsWith("SPEED ")) {
    int n = cmd.substring(6).toInt();
    if (n >= 0 && n <= 255) {
      driveDuty = n;
      bt.print("OK SPEED ");
      bt.println(n);
    } else bt.println("BAD SPEED");
    return;
  }

  if (cmd.equalsIgnoreCase("STOP")) { stopAll();
  lastDir = "S";
  bt.println("OK STOP");
  return;
  }

  applyDir(cmd[0]);
  bt.print("OK ");
  bt.println(lastDir);
}

void setup() {
  Serial.begin(9600);
  bt.begin(9600);
  motorInit(motorL);
  motorInit(motorR);
  stopAll();
  bt.println("# BT Car armed. F B L R S | SPEED 0..255 | STOP");
}

void loop() {
  unsigned long now = millis();

  // 1. Drain incoming bytes
  while (bt.available()) {
    char c = bt.read();
    lastCmdAt = now;             // any byte resets the watchdog

    // Emergency-stop
    if (c == 'S' || c == 's') {
      stopAll();
      lastDir = "S";
      line = "";
      bt.println("OK STOP (immediate)");
      continue;
    }

    if (c == '\r') continue;
    if (c == '\n') { dispatch(line);
    line = "";
    continue;
    }

    // Joystick mode — single-char direction with no terminator
    if (line.length() == 0 && (c == 'F' || c == 'B' || c == 'L' || c == 'R' ||
                               c == 'f' || c == 'b' || c == 'l' || c == 'r')) {
      applyDir(c);
      continue;
    }

    line += c;
    if (line.length() > 32) line = "";
  }

  // 2. Watchdog
  if (now - lastCmdAt > CMD_TIMEOUT_MS && lastDir != "S") {
    stopAll();
    lastDir = "S";
    bt.println("# watchdog -> STOP");
  }

  // 3. Heartbeat
  if (now - lastHeartbeat >= HEARTBEAT_MS) {
    lastHeartbeat = now;
    bt.print("# alive  speed=");
    bt.print(driveDuty);
    bt.print("  dir=");
    bt.println(lastDir);
  }
}

Step 3 — bench test (wheels off the floor)

1. Power up. Pair the phone. Open Serial Bluetooth Terminal at 9600 baud, line ending "None".
2. Press "F". Both wheels spin forward.
3. Wait 1 second without sending anything. Within 300 ms, the watchdog fires and wheels stop.
4. Repeat with "B", "L", "R" — all should briefly spin then auto-stop.
5. Type SPEED 100\n. Then press "F". Slower spin.

Step 4 — install a controller app (Android)

On the Play Store, search "Arduino Bluetooth Controller" or "RC Bluetooth". Most send single chars on button press and resend at ~10 Hz while held. Configure: F, B, L, R, S as buttons. Connect to your HC-05. Press and hold "forward" → the chassis drives; release → it stops (watchdog).

Step 5 — drive on the floor

Set the chassis on a clear floor. USB cable disconnected. Power: the L298N's 5V regulator (with the 5V-enable jumper installed) feeds the UNO. The HC-05 takes its 5V from the same rail.

Drive around. Notice:

• Press-and-hold = drive; release = stop.
• If you walk out of range, the connection drops and the chassis stops within ~1 s (Bluetooth disconnect + watchdog).
• Type SPEED 255 for max speed bursts; SPEED 120 for fine maneuvering.

Step 6 — emergency stop test

Drive forward at full speed. Tap "S" in your app. Wheels stop instantly — the 'S' bypasses the line buffer and fires immediately. Don't skip this test; you need to trust it before driving in tight spaces.

Basic 5 min

Challenge 1 5 min

Challenge 2 5 min

Challenge 3 · Fix the watchdog 10 min

A classmate wrote a cut-down car sketch with a watchdog. It only knows F and S. Two things go wrong:

• Holding F on the app, the wheels only twitch.
• After fixing that, closing the app leaves the car driving forever.

#include <SoftwareSerial.h>
#include "motor.h"

SoftwareSerial bt(2, 3);

const Motor motorL = {9, 8, 5};
const Motor motorR = {7, 6, 11};
const int DRIVE_DUTY = 200;
const unsigned long CMD_TIMEOUT_MS = 300;

unsigned long lastCmdAt = 0;
bool moving = false;

void stopAll() {
  motorSpeed(motorL, 0);
  motorSpeed(motorR, 0);
  moving = false;
}

void setup() {
  bt.begin(9600);
  motorInit(motorL);
  motorInit(motorR);
  stopAll();
}

void loop() {
  unsigned long now = millis();
  lastCmdAt = now;

  while (bt.available()) {
    char c = bt.read();
    if (c == 'F') {
      motorSpeed(motorL, DRIVE_DUTY);
      motorSpeed(motorR, DRIVE_DUTY);
      moving = true;
    }
    if (c == 'S') {
      stopAll();
    }
  }

  if (now - lastCmdAt < CMD_TIMEOUT_MS && moving) {
    stopAll();
    bt.println("# watchdog -> STOP");
  }
}

Find both bugs. Explain which symptom each one causes. Fix them, upload, and test with the wheels off the floor.

It works if holding F drives smoothly, and the wheels stop within 300 ms of letting go or closing the app.

Recap 5 min

Cluster E ends with a real robot. The new ideas compared to L03-10's wired version were all about safety in the wireless world: a watchdog so "no command" means "stop", an emergency stop that bypasses buffering, a heartbeat for confirming the link, and connection-loss detection via the same watchdog. The bigger Cluster E lesson: Bluetooth Classic is great for Android phones streaming joystick commands; BLE is better for iOS, sensors, and battery-powered builds. We'll come back to this robot in L03-43 (Build) with PWM speed, BLE app, and obstacle avoidance. Next cluster: leave Bluetooth's 10 m range behind and step onto the internet with WiFi.

Watchdog timer

A piece of code that automatically stops or resets the system if it doesn't see an expected event within a deadline. For our car: if no Bluetooth byte for 300 ms, motors stop.

Dead-man's switch

Any control scheme where action requires continuous input. Press-and-hold drives the car; release stops it. Standard in tools, vehicles, industrial machines.

Emergency stop

A path that bypasses normal command processing and immediately halts actuators. Should work from any input state — half-typed buffer included.

Heartbeat

Periodic message from the device confirming "I'm alive and you're connected". Useful for the operator who can't see the device.

Joystick mode

Single-character commands arriving at high rate (~10/s) for as long as the user holds a button. Contrasts with line-buffered "type a verb + Enter" mode.

Connection-loss detection

The mechanism that notices the phone has disconnected. Cheapest path: same watchdog as the no-input one (no bytes for N ms = disconnect = stop).

Bench test vs floor test

Bench test = chassis on a desk, wheels free, you can grab it. Floor test = chassis on the floor, can crash. Always bench-test new code first.

Ship-ready

Another person can use the device without your instructions. Cluster E's capstone bar — same as Cluster A (turntable) and Cluster B (rover).

Extra Mission 5 min

Part 1 — Design a safe remote-controlled machine

Pick a machine that would be useful to drive from a phone: a toy crane, a camera dolly, a line-marking cart, a pet-treat launcher. On paper, design how it stays safe when something goes wrong.

Your design must include:

• The machine's name, its one job, and its command list.
• What it does when the user lets go of a button.
• What it does when the phone goes out of range, and how fast.
• An emergency stop, and the byte that triggers it.
• One extra safety sensor and the reading that makes it stop.

Part 2 — Make it

Add your design's safety rules to the car: the watchdog, the emergency stop, and your extra safety sensor (for example the HC-SR04 from Challenge 2). Test each one with the wheels off the floor first, then on a clear floor.

Bring back next class: your design, your uploaded sketch, and a short video showing each safety rule stopping the car.