HC-05 Bluetooth Module

HC-05 Bluetooth Module – Serial Wireless Communication for Arduino & ESP32

The HC-05 Bluetooth Module is a classic, easy-to-use serial Bluetooth 2.0 module that enables wireless UART communication between microcontrollers and Bluetooth-enabled devices such as smartphones, laptops, and other HC-05/HC-06 modules. Unlike the HC-06, the HC-05 supports both Master and Slave modes, making it far more versatile for device-to-device communication projects.

Key Specifications

• Bluetooth Version: 2.0 + EDR (Enhanced Data Rate)
• Operating Voltage: 3.6V – 6V DC (onboard 3.3V regulator)
• Logic Level: 3.3V TX/RX (5V tolerant on most breakout boards)
• Interface: UART Serial (TX, RX, VCC, GND, EN, STATE)
• Default Baud Rate: 9600 bps (configurable via AT commands)
• Mode: Master / Slave (configurable via AT commands)
• Range: Up to 10 metres (line of sight)
• Pairing PIN: 1234 (default)

Compatibility & Use Cases

The HC-05 communicates via standard UART serial and is directly compatible with Arduino UNO, Arduino Mega, ESP32, STM32, Raspberry Pi Pico, and any microcontroller with a hardware or software serial port. Common applications include:

• Wireless serial monitor and data logging from Arduino/ESP32
• Bluetooth-controlled robots, cars, and drones
• Smartphone-to-microcontroller communication via Android apps
• Device-to-device wireless data transfer (Master–Slave pairing)
• Home automation and IoT wireless control systems
• STEM and engineering project wireless communication demos

Use AT command mode (hold EN pin HIGH during power-on) to configure baud rate, device name, PIN, and master/slave role. Compatible with the Arduino SoftwareSerial library for easy integration.

Frequently Asked Questions

What is the difference between the HC-05 and HC-06?
The HC-05 can be configured as either a Master or a Slave using AT commands, so it can initiate connections to other Bluetooth devices. The HC-06 is Slave-only and simply waits for a phone or another module to connect to it.

Will the HC-05 connect to an iPhone?
No. The HC-05 uses classic Bluetooth 2.0 serial (SPP), which pairs with most Android phones, tablets, laptops and PCs. iPhones require a BLE (Bluetooth Low Energy) module instead.

How do I enter AT command mode?
Hold the onboard button (or pull the EN pin HIGH) while powering the module. The LED then blinks slowly, about once every 2 seconds, and the module accepts AT commands at 38400 baud to change its name, PIN and baud rate.

Can I connect it directly to a 5V Arduino?
Power the module through its VCC pin (3.6V–6V, regulator onboard). Its RX line is 3.3V logic, so add a simple resistor voltage divider between the Arduino TX pin and the module RX pin for reliable long-term use.

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