Here’s a clear, practical guide to connect a USB–TTL converterwith a Bluetooth HC-05 module for configuration and testing. This is the most common setup used before connecting HC-05 to Arduino or ESP boards.
1. Know the HC-05 Pins (important)
Typical HC-05 breakout pins:
| HC-05 Pin | Function |
| --------- | -------------------------------- |
| EN / KEY | AT command mode enable |
| VCC | Power (3.6–6V on breakout board) |
| GND | Ground |
| TXD | Transmit data |
| RXD | Receive data |
| STATE | Connection status (optional) |
2. USB–TTL Converter Pins
Most USB-TTL converters (CP2102, CH340, FT232):
| USB-TTL Pin | Function |
| ----------- | ------------ |
| 5V or 3.3V | Power output |
| GND | Ground |
| TXD | Transmit |
| RXD | Receive |
3. Basic Wiring (Normal Mode / Communication Test)
🔌 Connection Table
| USB-TTL | HC-05 |
| ------- | ----- |
| TXD | RXD |
| RXD | TXD |
| GND | GND |
| 5V | VCC |
⚠️ Important notes
*TX ↔ RX must be crossed
* Most HC-05 boards accept **5V on VCC** (they have onboard regulators)
* HC-05 RXD is 3.3V logic, so use a voltage divider if your USB-TTL TX is 5V
Voltage Divider for RXD (recommended)
USB-TTL TX ---[1.8kΩ]---+--- HC-05 RX
|
[3.3kΩ]
|
GND
This drops 5V → ~3.3V safely.
4. AT Command Mode Connection (Configuration Mode)
To configure name, password, baud rate, etc., HC-05 must enter AT mode
Steps
1. Disconnect power
2. Press & hold the button on HC-05 (or connect KEY pin to 3.3V)
3. Apply power
4. LED will blink **slowly (once every ~2 sec)** → AT mode active
Wiring (same as normal mode)
| USB-TTL | HC-05 |
| ------- | ------------------------------ |
| TXD | RXD |
| RXD | TXD |
| GND | GND |
| 5V | VCC |
| KEY | 3.3V (or press onboard button) |
5. Serial Monitor Settings
Use:
* **Arduino Serial Monitor**
* **PuTTY**
* **Tera Term**
* **CoolTerm**
Baud Rates
| Mode | Baud Rate |
| ----------- | -------------- |
| AT Mode | 38400 |
| Normal Mode | 9600 (default) |
Serial Settings
Baud: 38400
Data bits: 8
Parity: None
Stop bits: 1
Line ending: Both NL & CR
6. Test AT Commands
Try these:
AT
Response:
OK
AT+NAME?
AT+ROLE?
AT+UART?
7. Common Problems & Fixes
| Problem | Cause | Fix |
| --------------- | -------------- | ------------------------ |
| No response | Wrong baud | Use 38400 |
| Garbage data | Baud mismatch | Change baud |
| HC-05 overheats | Wrong VCC | Use 5V only |
| AT not working | Not in AT mode | Hold button before power |
8. Quick Diagram (Text)
USB-TTL HC-05
TX ----------> RX
RX <---------- TX
GND ----------> GND
5V ----------> VCC
