Bluetooth Robot Car v2

Learning Goals 5 min

L03-28 was "the chassis works". v2 is "the chassis works well" — PWM-throttled speed, gentle turns, obstacle avoidance, fancier phone control. You'll combine Cluster B (motors), Cluster E (Bluetooth) and Cluster H (classes) into one polished build. By the end of this lesson you will:

1. Refactor v1's flat sketch into a class-driven architecture: Motor class, Vehicle class, BluetoothLink class, plus a RangeFinder from L02-21.
2. Add PWM speed control as a continuous variable from the phone's slider, plus gentle turn behaviour (one wheel slower while moving forward).
3. Add an ultrasonic obstacle-stop: if anything is within 15 cm in front while driving forward, stop and report.

Warm-Up 10 min

The chassis from L03-28 plus an HC-SR04 ultrasonic sensor mounted on the front, pointing forward.

Mount the ultrasonic sensor at the front of the chassis, ~5–10 cm above the ground.

New Concept · Architecture in classes 20 min

Four classes

The .ino is now mostly setup() + loop() that talks to these four objects.

Command protocol (v2)

Same shape as L03-25; new keyword ARC for gentle turns.

Obstacle stop policy

Run RangeFinder.readCm() every 100 ms. If currently driving forward AND distance < 15 cm, force stop and emit !OBSTACLE message. Re-allow forward only when distance > 25 cm (hysteresis).

Worked Example · The v2 sketch 30 min

Step 1 — assume the four header files exist

You wrote Motor.h and Blinker.h in L03-41/42. RangeFinder.h and BluetoothLink.h are similar wrappers — write them or download from your personal library. Skeleton:

// RangeFinder.h
class RangeFinder {
public:
  RangeFinder(int trig, int echo) : trig_(trig), echo_(echo) {}
  void begin() { pinMode(trig_, OUTPUT); pinMode(echo_, INPUT); }
  float readCm() {
    digitalWrite(trig_, LOW);  delayMicroseconds(2);
    digitalWrite(trig_, HIGH); delayMicroseconds(10);
    digitalWrite(trig_, LOW);
    unsigned long w = pulseIn(echo_, HIGH, 25000);
    if (w == 0) return -1;
    return w / 58.0;
  }
private:
  int trig_, echo_;
};

Step 2 — the main sketch

// L03-43 · Bluetooth Robot Car v2
#include <SoftwareSerial.h>
#include "Motor.h"
#include "RangeFinder.h"

Motor motorL(4, 7, 5);
Motor motorR(8, 9, 6);
RangeFinder front(A1, A0);
SoftwareSerial bt(2, 3);

int driveDuty = 200;
unsigned long lastCmdAt = 0;
unsigned long lastRangeAt = 0;
float lastDistCm = -1;
bool obstaclePresent = false;
String line, lastDir = "S";

void driveForward()  { motorL.setSpeed( driveDuty); motorR.setSpeed( driveDuty); lastDir = "F"; }
void driveBackward() { motorL.setSpeed(-driveDuty); motorR.setSpeed(-driveDuty); lastDir = "B"; }
void spinLeft()      { motorL.setSpeed(-driveDuty); motorR.setSpeed( driveDuty); lastDir = "L"; }
void spinRight()     { motorL.setSpeed( driveDuty); motorR.setSpeed(-driveDuty); lastDir = "R"; }
void stopAll()       { motorL.setSpeed(0); motorR.setSpeed(0); lastDir = "S"; }
void arcLeft()       { motorL.setSpeed( driveDuty/2); motorR.setSpeed( driveDuty); lastDir = "AL"; }
void arcRight()      { motorL.setSpeed( driveDuty);   motorR.setSpeed( driveDuty/2); lastDir = "AR"; }

void applyDir(char d) {
  switch (d) {
    case 'F': driveForward();  break;
    case 'B': driveBackward(); break;
    case 'L': spinLeft();      break;
    case 'R': spinRight();     break;
    case 'S': default: stopAll();
  }
}

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); }
    return;
  }
  if (cmd == "ARC L") { arcLeft();  bt.println("OK ARC L"); return; }
  if (cmd == "ARC R") { arcRight(); bt.println("OK ARC R"); return; }
  if (cmd == "?")     {
    bt.print("dir="); bt.print(lastDir);
    bt.print(" speed="); bt.print(driveDuty);
    bt.print(" dist="); bt.println(lastDistCm);
    return;
  }
  applyDir(cmd[0]);
  bt.print("OK "); bt.println(lastDir);
}

void readBluetooth() {
  while (bt.available()) {
    char c = bt.read();
    lastCmdAt = millis();
    if (c == 'S' || c == 's') { stopAll(); line = ""; bt.println("OK STOP"); continue; }
    if (c == '\r') continue;
    if (c == '\n') { dispatch(line); line = ""; continue; }
    if (line.length() == 0 && (c == 'F' || c == 'B' || c == 'L' || c == 'R')) { applyDir(c); continue; }
    line += c;
    if (line.length() > 32) line = "";
  }
}

void readRange() {
  if (millis() - lastRangeAt < 100) return;
  lastRangeAt = millis();
  float d = front.readCm();
  lastDistCm = d;

  bool wasObstacle = obstaclePresent;
  if (d > 0 && d < 15)      obstaclePresent = true;
  else if (d > 25 || d < 0) obstaclePresent = false;

  if (obstaclePresent && !wasObstacle) {
    if (lastDir == "F" || lastDir == "AL" || lastDir == "AR") {
      stopAll();
      bt.println("!OBSTACLE - stop");
    }
  }
}

void setup() {
  Serial.begin(9600);
  bt.begin(9600);
  front.begin();
  stopAll();
  bt.println("# Robot car v2 armed. F/B/L/R/S, SPEED 0..255, ARC L/R, ?");
}

void loop() {
  readBluetooth();
  readRange();

  // Watchdog: stop if no command for 300 ms in joystick mode
  if (millis() - lastCmdAt > 300 && lastDir != "S") {
    stopAll();
    bt.println("# watchdog -> STOP");
  }

  // Refuse forward while obstacle is too close
  if (obstaclePresent && (lastDir == "F" || lastDir == "AL" || lastDir == "AR")) {
    stopAll();
  }
}

Step 3 — bench test

Wheels off the floor. Pair phone. Test every command. With the chassis held in the air, wave your hand < 15 cm in front of the sensor → the watchdog plus obstacle stop fires.

Step 4 — floor test, fast lap

Set on floor with a clear lane. Drive forward with F. Place an obstacle 30 cm in front. The chassis approaches, the obstacle-detection fires at 15 cm, the chassis stops. Phone shows !OBSTACLE - stop. Move the obstacle away → it stays stopped (still lastDir = S from the auto-stop). Press F again → chassis resumes.

Step 5 — speed test

Try SPEED 100 (slow), SPEED 200 (default), SPEED 255 (max). Notice the difference; recall the dead band from L03-09 means speeds below ~50 won't start the motors from stop.

Step 6 — arcing test

ARC L while moving forward → chassis curves left. ARC R → curves right. Combined with obstacle stop, you can drive a smooth curved path around obstacles.

Try It Yourself 15 min

Mini-Challenge · Demo + measure 10 min

1. Tape an obstacle course on the floor — start line, finish line, a couple of obstacles.
2. Time yourself driving from start to finish with the phone.
3. Record a 60-second video showing: forward → obstacle stop → reverse-arc-around → finish.
4. Note in your notebook: total course time, number of times obstacle stop fired, anything that surprised you about v2 vs v1.

If you finish early, swap chassis with a classmate and drive each other's. Comparing build quality and code feel is the fastest way to spot polishing opportunities.

Recap 5 min

v2 = v1 + class architecture + PWM + obstacle stop. The class refactor means the .ino stays small even as features grow. Obstacle stop is a tiny safety layer that transforms the toy from "guided missile" to "sensible robot". Cluster I's pattern: take an earlier project, scale it with Cluster H ideas, ship a finished v2. Tomorrow: the plant monitor.

v2 / iteration

The polish pass on an earlier build. New features, cleaner architecture, real-world testing.

Class architecture

Project structure where each major concept (Motor, Vehicle, Sensor) is its own class. The .ino just wires them together.

Joystick mode

Single-character commands at ~10 Hz. Watchdog stops motors if input stops. From L03-28.

Obstacle stop

Hard rule: if a sensor reads too close in the direction of travel, stop. With hysteresis so the chassis doesn't oscillate.

Hysteresis

Different thresholds for entering and leaving a state. < 15 cm = obstacle; > 25 cm = clear. The 10 cm gap prevents flickering near the threshold.

Arcing turn

One wheel slower than the other while moving forward. Produces a smooth curve rather than an in-place pivot.

Calibration

Measuring real-world response (e.g. spin-time per degree) and storing as a constant. The bridge between "the code thinks it's 90°" and "the chassis actually turned 90°".

Homework 5 min

1. Finish v2. Video. Bring to next class.
2. Save as bt-car-v2.ino + the supporting class headers.
3. Read ahead to ARD-L03-44 (WiFi Smart-Plant Monitor). Bring a soil moisture probe + an ESP + an LDR. We'll wire a plant data logger with email alerts.

Bring back next class:

• Working v2 car + video.
• ESP + soil moisture probe.