STM32 Blue Pill HC-05 Bluetooth on USART2: Wiring, Code and Simulator

To build this STM32 Blue Pill HC-05 Bluetooth on USART2 project you need an STM32 Blue Pill (STM32F103C8) and a Bluetooth HC-05. USART2 (PA2/PA3) talks to an HC-05 while Serial (USART1) reports what arrives: switch the PC13 LED from the phone terminal. Make the 4 connections in the wiring table, upload the Arduino C++ code, and pair from the Radio tab and send 1. Remember PC13 is active-low: LOW turns the LED on.

Open this circuit in the free STM32 Blue Pill (STM32F103C8) simulator

Parts list

Qty	Part	What it does
1	STM32 Blue Pill (STM32F103C8)	the microcontroller board
1	Bluetooth HC-05	Bluetooth serial module: VCC 5 V, GND, TXD → the board’s RX pin, RXD ← the board’s TX pin (through a 1k/2k divider on a 5 V board). Starts at 9600 baud; hold EN high at power-up for AT commands at 38400. Pair it from the Radio tab’s phone terminal, or with a master HC-05 in another circuit (open it in a second tab).

Wiring

#	From	To
1	STM32 Blue Pill (STM32F103C8) 5V	Bluetooth HC-05 VCC
2	STM32 Blue Pill (STM32F103C8) GND	Bluetooth HC-05 GND
3	STM32 Blue Pill (STM32F103C8) PA3	Bluetooth HC-05 TXD
4	STM32 Blue Pill (STM32F103C8) PA2	Bluetooth HC-05 RXD

Step by step

Open the simulator. Open the BitForBytes Embedded Lab in your browser and start from the STM32 Blue Pill (STM32F103C8). No install or account is needed.
Add the parts. Add a Bluetooth HC-05 from the parts palette.
Wire 1. Connect STM32 Blue Pill (STM32F103C8) 5V to Bluetooth HC-05 VCC.
Wire 2. Connect STM32 Blue Pill (STM32F103C8) GND to Bluetooth HC-05 GND.
Wire 3. Connect STM32 Blue Pill (STM32F103C8) PA3 to Bluetooth HC-05 TXD.
Wire 4. Connect STM32 Blue Pill (STM32F103C8) PA2 to Bluetooth HC-05 RXD.
Paste the Arduino C++ code. Paste the Arduino C++ code below into the code editor.
Run it. Press Run. Pair from the Radio tab and send 1. Remember PC13 is active-low: LOW turns the LED on.

Arduino C++ code

// Bluetooth on an STM32 Blue Pill: an HC-05 on USART2 (PA2 TX, PA3 RX).
// Pair from the Radio tab's phone terminal (PIN 1234), send 1 or 0 to
// switch the green LED; what the phone sends is also shown on Serial.
// Wiring: HC-05 VCC 5V, GND, TXD -> PA3, RXD <- PA2 (3.3 V: no divider).
Uart bt(PA3, PA2);   // RX, TX (STM32duino 3; older tutorials call it HardwareSerial)

void setup() {
  Serial.begin(9600);
  bt.begin(9600);
  pinMode(PC13, OUTPUT);
  digitalWrite(PC13, HIGH);    // the LED is wired to 3.3 V: HIGH is off
  Serial.println("Blue Pill waiting for a phone...");
}

void loop() {
  if (bt.available()) {
    char c = bt.read();
    Serial.print("Phone sent: ");
    Serial.println(c);
    if (c == '1') { digitalWrite(PC13, LOW); bt.println("LED is ON"); }
    else if (c == '0') { digitalWrite(PC13, HIGH); bt.println("LED is OFF"); }
  }
}

What to try

Pair from the Radio tab and send 1. Remember PC13 is active-low: LOW turns the LED on.

Frequently asked questions

How do I wire the HC-05 Bluetooth on USART2 project on an STM32 Blue Pill (STM32F103C8)?

Make 4 connections: STM32 Blue Pill (STM32F103C8) 5V to Bluetooth HC-05 VCC; STM32 Blue Pill (STM32F103C8) GND to Bluetooth HC-05 GND; STM32 Blue Pill (STM32F103C8) PA3 to Bluetooth HC-05 TXD; STM32 Blue Pill (STM32F103C8) PA2 to Bluetooth HC-05 RXD.

What parts do I need for this STM32 Blue Pill project?

An STM32 Blue Pill (STM32F103C8) and a Bluetooth HC-05.

Can I simulate this STM32 Blue Pill project online for free?

Yes. The BitForBytes Embedded Lab runs this exact circuit and Arduino C++ code in your browser for free, with no install and no hardware. The sketch is built with the STM32duino core and runs on an STM32F103 emulated instruction by instruction: its ARM Cortex-M3 core, NVIC and peripherals (GPIO, EXTI, USART, timers, ADC and I²C).

What does the code do?

USART2 (PA2/PA3) talks to an HC-05 while Serial (USART1) reports what arrives: switch the PC13 LED from the phone terminal.