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.

Basic 5 min

Challenge 1 5 min

Challenge 2 5 min

Challenge 3 · Fix the silent Bluetooth LED 10 min

A classmate rebuilt the Bluetooth LED. Nothing works. Here is their wiring and their sketch.

• HC-05 V_CC → 5V, HC-05 GND → GND.
• HC-05 TXD → D3.
• D2 → 2 kΩ → HC-05 RXD; HC-05 RXD → 1 kΩ → GND.
• D6 → 220 Ω → LED → GND.

#include <SoftwareSerial.h>

const int BT_RX = 2;
const int BT_TX = 3;
const int LED_PIN = 6;

SoftwareSerial bt(BT_RX, BT_TX);

void setup() {
  Serial.begin(9600);
  bt.begin(38400);
  pinMode(LED_PIN, OUTPUT);
}

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

There are four faults: two in the wiring and two in the code. For each one, say what it causes and give the fix. For the divider, work out the voltage the HC-05's RXD actually gets.

It works if the phone app (line ending set to none) can send 1 and 0. The LED must follow, and the phone must show LED ON or LED OFF each time.

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.

Extra Mission 5 min

Part 1 — Design a phone-controlled gadget and its command list

Your sketch is now a tiny remote-controlled device. Design one you would really use: a night light, a "do not disturb" sign, or a desk fan.

Your design must include:

• The gadget's name and its one job.
• Its command list as a table: each command, what the gadget does, and the exact reply.
• What it does on power-up, and its reply to an unknown command.
• The pairing details: the baud rate and the PIN.
• A wiring drawing, including the 1 kΩ / 2 kΩ divider.

Part 2 — Make it

Build your gadget with the HC-05 and outputs you already know: LEDs, an RGB LED or a servo. Make the sketch follow your command list exactly. Every command must send its reply.

It works if every command in your table gives the action and the exact reply you wrote down.

Bring back next class: your command list, your sketch and a phone screenshot showing commands and replies. Keep the HC-05 wired for L03-24.