HC-05 Bluetooth Classic

Learning Goals 5 min

Cluster D taught you protocols on a wire. Cluster E takes the wire away. The HC-05 is the classroom Bluetooth module — a small blue PCB that, once paired with your phone, behaves exactly like a UART link over the air. Your sketch sees characters via Serial; your phone sees characters in a terminal app. By the end of this lesson you will:

1. Wire an HC-05 to a UNO using SoftwareSerial (so the hardware UART stays free for upload) plus power and a 5 V → 3.3 V level shifter for the module's RX pin.
2. Pair your phone to the HC-05 (default PIN 1234) and use a Bluetooth-serial app to send characters that your sketch reads — same code patterns as L01-26.
3. Build a Bluetooth-controlled LED: 1 from the phone turns it on, 0 turns it off.

Warm-Up 10 min

Pull out the HC-05. Identify the pins (usually 6, sometimes 4):

Pre-class question

Why does the HC-05's TX → Arduino RX direction not need a level shifter, but the Arduino TX → HC-05 RX direction does?

New Concept · Bluetooth Classic as a wireless UART 25 min

What an HC-05 actually is

Internally: a Bluetooth Classic SoC running Bluetooth's Serial Port Profile (SPP). To your sketch it presents a UART; to your phone it presents a Bluetooth serial endpoint. The radio negotiates the pairing and the encryption; the data layer is plain bytes-in, bytes-out, both directions.

Latency: ~10 ms per byte once paired. Throughput: up to ~10 KB/s with good RSSI. Range: ~10 m indoors. Power: ~30 mA when paired, ~5 mA idle. Plenty for remote-control toys, dashboards, sensor uploads.

HC-05 vs HC-06 vs BLE — quick decoder

The HC-05 is the cheapest and easiest for Android-based projects. For iOS, jump to L03-26 (ArduinoBLE).

Pairing flow

1. Power up the HC-05 (5 V to V_CC, GND to GND). The on-board red LED blinks rapidly (~5 Hz) = "not paired, advertising".
2. On your Android phone: Settings → Bluetooth → Scan → see "HC-05" in the list → tap → enter PIN 1234 (default) or 0000 (some clones).
3. Pairing complete. The LED's blink slows to ~once every 2 seconds = "paired".
4. Open a Bluetooth-serial app (e.g. "Serial Bluetooth Terminal" for Android). Connect to HC-05. The LED stays solid = "connected".
5. Type characters in the app → they appear on the HC-05's TX pin → on the Arduino's SoftwareSerial RX → in mySerial.read().

Wiring with a resistor divider for RX protection

The 1 kΩ / 2 kΩ divider takes 5 V from the UNO down to 5 V × (2 / (1+2)) ≈ 3.3 V at the HC-05's RXD. Safe.

Worked Example · Bluetooth-controlled LED 25 min

Step 1 — wire it (with the resistor divider)

HC-05 V_CC/GND to UNO 5V/GND. HC-05 TXD → UNO D2 (direct). UNO D3 → 1 kΩ → HC-05 RXD → 2 kΩ → GND. LED + 220 Ω resistor between UNO D6 and GND.

Step 2 — pair your phone

Power the UNO. The HC-05's red LED blinks rapidly. On your phone: Settings → Bluetooth → pair with HC-05 (PIN 1234 or 0000). Open "Serial Bluetooth Terminal" (or equivalent). Tap menu → Devices → tap HC-05. The terminal should connect and the HC-05's LED should slow / go solid.

Step 3 — the sketch

// L03-23 · Bluetooth-controlled LED via HC-05 + SoftwareSerial
#include <SoftwareSerial.h>

const int BT_RX = 2;      // UNO RX <- HC-05 TXD
const int BT_TX = 3;      // UNO TX -> HC-05 RXD (through 1k/2k divider)
const int LED   = 6;

SoftwareSerial bt(BT_RX, BT_TX);   // RX, TX

void setup() {
  Serial.begin(9600);
  bt.begin(9600);               // HC-05 default baud
  pinMode(LED, OUTPUT);
  Serial.println("# Bluetooth LED ready. From phone: '1' = on, '0' = off");
}

void loop() {
  if (bt.available()) {
    char c = bt.read();
    switch (c) {
      case '1':
        digitalWrite(LED, HIGH);
        bt.println("LED ON");
        Serial.println("# LED ON");
        break;
      case '0':
        digitalWrite(LED, LOW);
        bt.println("LED OFF");
        Serial.println("# LED OFF");
        break;
      default:
        bt.print("Unknown: ");
        bt.println(c);
    }
  }
}

Two Serials are open simultaneously: Serial (hardware, USB → laptop monitor) and bt (software, D2/D3 → HC-05 → phone). Lessons L03-15 / L03-16 enabled this.

Step 4 — upload and test

Upload the sketch. The terminal app on your phone should be connected to the HC-05. Type 1 → the LED on D6 lights up; the phone shows "LED ON". Type 0 → LED off. Type anything else → "Unknown: x" on the phone.

Step 5 — try the laptop Serial Monitor too

With the sketch running and the phone connected, open the laptop's Serial Monitor at 9600 baud. Press 1 on the phone → you see "# LED ON" on the laptop monitor AND "LED ON" on the phone. Two windows into the same sketch.

Step 6 — extend with a real action

Replace the LED with a servo from L03-01. F = forward 90°, B = backward 90°, S = stop. Now you have a Bluetooth-controlled servo — the foundation for the BLE robot car of L03-28.

#include <Servo.h>
Servo s;

void setup() {
  bt.begin(9600);
  s.attach(9);
  s.write(90);
}

void loop() {
  if (bt.available()) {
    char c = bt.read();
    if      (c == 'F') s.write(180);
    else if (c == 'B') s.write(0);
    else if (c == 'S') s.write(90);
  }
}

Walk away from your laptop with the phone in hand. The Arduino runs on USB power (or 9 V battery); you control it from up to ~10 m away. First wireless project of the course.

Try It Yourself 15 min

unsigned long lastTick = 0;
int tickCount = 0;

void loop() {
  if (millis() - lastTick >= 2000) {
    lastTick = millis();
    tickCount++;
    bt.print("tick ");
    bt.println(tickCount);
  }
  // ... existing read code ...
}

Mini-Challenge · Document your Bluetooth API 10 min

Your sketch is now a tiny remote-controlled device. Document its "API" — the commands it accepts — in plain language.

1. Open a fresh page in your engineering notebook.
2. Title it "[Device Name] · Bluetooth API · v1".
3. For each command, list: character/string, what it does, what the device responds with.
4. Note: default PIN, baud rate, expected behaviour on power-up.

Bluetooth LED · API v1
======================
Pair: PIN 1234, baud 9600.
Connect: any Bluetooth serial terminal at 9600 baud.

Commands (case-sensitive):
  '1'  Turn LED on.       Responds: "LED ON"
  '0'  Turn LED off.      Responds: "LED OFF"
  any  Unknown command.   Responds: "Unknown: X"

Heartbeat: none in v1.

The Bluetooth car (L03-28) will have a richer API — direction, speed, sensors. Documenting at this stage makes it easy to extend later. Same idea as writing a README for your code.

Recap 5 min

The HC-05 is "UART, but wireless". Wire 4 pins (V_CC, GND, TX, RX — with a level divider on RX), pair the phone, open a terminal app, and your sketch's SoftwareSerial reads characters that came from your phone. Default PIN 1234 (sometimes 0000); default baud 9600. iOS users need BLE instead — covered in L03-26. The full pattern shape is what you already know — read a character, switch on it, do something — just over the air instead of a USB cable. Tomorrow we learn to talk to the HC-05 itself (not just through it) using AT commands — rename it, change its PIN, change its baud.

HC-05

Cheap Bluetooth Classic module with SPP firmware. The classroom Bluetooth standard for Android-based projects.

SPP (Serial Port Profile)

A Bluetooth Classic profile that emulates a serial port over the radio link. Your sketch reads/writes bytes; the radio handles delivery.

Pairing

The one-time security handshake between phone and module. Establishes a shared key (using the PIN), saved for future quick connections.

PIN

The 4-digit number you enter during pairing. Default for HC-05: 1234. Change it for any real project; cover in L03-24.

SoftwareSerial

The library that gives you a second UART on any pin pair. Used to talk to the HC-05 without taking pins 0/1 (which would block USB upload).

Level shifter / resistor divider

A small circuit that drops a 5 V signal to 3.3 V to protect a 3.3 V chip. Resistor divider (1k + 2k) is the cheapest; dedicated breakouts (TXS0102) are cleaner.

RSSI (Received Signal Strength Indicator)

A measure of how strong the Bluetooth radio link is. Larger values (closer to 0) = better signal. Lower values (negative, far from 0) = weak signal.

Bluetooth Classic vs BLE

Classic: higher throughput, SPP, NO iOS support. BLE: lower throughput, GATT services / characteristics, full iOS support. Different radios, different APIs.

Homework 5 min

1. Save the Bluetooth LED sketch as bt-led.ino.
2. Document its API in your notebook (Mini-Challenge above).
3. Tomorrow we'll enter AT mode to rename the HC-05 and change its baud / PIN. Bring: the HC-05 still wired up, and have your phone's Bluetooth settings handy so you can re-pair under the new name.
4. Read ahead to ARD-L03-24 (AT Commands).

Bring back next class:

• Working bt-led.ino sketch and wiring.
• Bluetooth API page in your notebook.