HC-05 Bluetooth Module

Overview

HC-05 Bluetooth Serial Communication Module designed for wireless UART communication between microcontrollers, computers, smartphones, and embedded systems using Bluetooth Classic technology. A widely used Bluetooth SPP (Serial Port Protocol) module capable of operating in both Master and Slave modes.

The HC-05 is commonly used in Arduino, ESP32, STM32, robotics, IoT devices, wireless control systems, industrial automation, smart home projects, and mobile-controlled electronics. The module supports AT command configuration, configurable baud rates, and reliable wireless serial communication up to approximately 10 meters.

Specifications

Parameter	Value
Model	HC-05
Bluetooth Version	Bluetooth 2.0 + EDR
Frequency	2.4GHz ISM Band
Communication Protocol	SPP (Serial Port Protocol)
Operating Voltage	3.6V – 6V
Logic Level	3.3V TTL
Default Baud Rate	9600bps
AT Command Baud Rate	38400bps
Operating Current	~30mA
Communication Range	Up to 10m
Transmission Power	≤4dBm
Sensitivity	-80dBm
Interface	UART TTL
Operating Temperature	-20°C to +75°C
Antenna	Onboard PCB Antenna

Pin Configuration

Pin	Description
EN / KEY	AT Command Mode Enable
VCC	Power Supply
GND	Ground
TXD	UART Transmit
RXD	UART Receive
STATE	Connection Status Output

Features

• Bluetooth Serial Communication — wireless UART replacement
• Master and Slave mode support
• AT command configuration
• Easy UART interfacing
• Stable wireless communication
• Compatible with Arduino, STM32, ESP32
• Built-in PCB antenna
• Low power consumption
• Wide baud rate support
• Compact breakout board design
• Reliable short-range wireless control
• Ideal for robotics and IoT projects

HC-05 vs HC-06

Feature	HC-05	HC-06
Bluetooth Version	2.0 + EDR	2.0 + EDR
Communication	Full Duplex	Full Duplex
Mode Support	Master + Slave	Slave Only
AT Commands	Advanced	Basic
Configuration Flexibility	High	Medium
Pins	6 Pins	4 Pins
Ease of Use	Medium	Easy
Best For	Device-to-device communication	Smartphone control

Pin Wiring with Arduino UNO

HC-05	Arduino UNO
VCC	5V
GND	GND
TXD	RX (D0)
RXD	TX (D1) via voltage divider

Pin Wiring with STM32 Blue Pill

HC-05	STM32
VCC	5V
GND	GND
TXD	PA10 (RX)
RXD	PA9 (TX)

Pin Wiring with ESP32

HC-05	ESP32
VCC	5V
GND	GND
TXD	GPIO16 (RX2)
RXD	GPIO17 (TX2)

HC-05 RX pin uses 3.3V logic. Use a voltage divider when connecting 5V TX signals.

Common Applications

• Wireless robot control
• Smartphone-controlled devices
• Wireless sensor systems
• Bluetooth serial terminal
• Home automation
• Industrial monitoring
• Embedded wireless communication
• RC vehicle control
• IoT devices
• Wireless debugging

AT Command Examples

Command	Function
AT	Test communication
AT+NAME=AtomsBT	Change Bluetooth name
AT+PSWD=1234	Change password
AT+UART=9600,0,0	Set baud rate
AT+ROLE=1	Set Master mode
AT+ROLE=0	Set Slave mode

Basic Arduino Example

#include <SoftwareSerial.h>

SoftwareSerial BT(10, 11);

void setup() {
    Serial.begin(9600);
    BT.begin(9600);

    Serial.println("HC-05 Ready");
}

void loop() {

    if (BT.available()) {
        Serial.write(BT.read());
    }

    if (Serial.available()) {
        BT.write(Serial.read());
    }
}

Package Includes

• 1× HC-05 Bluetooth Module

Notes

• Default pairing password is usually 1234 or 0000
• Default device name is typically HC-05
• RX pin is 3.3V logic level
• Suitable for Android devices
• Most HC-05 modules do not support iOS SPP communication
• Use AT mode for advanced configuration
• Communication range depends on environment and interference

Applications in Embedded Systems

The HC-05 is widely used in:

• Arduino robotics
• STM32 wireless control
• ESP32 communication systems
• Smart home automation
• Mobile-controlled electronics
• Wireless serial monitoring
• Industrial embedded systems

The module remains one of the most popular low-cost Bluetooth Classic solutions for embedded and robotics projects due to its simplicity and flexibility.