HC-05 4-Pin Bluetooth Module (Master/Slave) w/ Button

To switch modes, ensure the module is powered on but not paired. Press and hold the onboard button, then power cycle the module. The LED will blink at a slower rate (approx. 2 seconds on/off) indicating entry into AT command mode. Connect the module's TXD/RXD to a USB-UART adapter, open a serial terminal at 38400 baud (8N1), and send 'AT+ROLE=1' for Master or 'AT+ROLE=0' for Slave. Remember to save with 'AT+SAVE' and reset the module for changes to take effect.

Connect the HC-05's VCC pin to the Arduino's 5V pin (or 3.3V for 3.3V logic boards), and GND to GND. For serial communication, connect the HC-05's TXD pin to the Arduino's RX pin (Digital Pin 0) and the HC-05's RXD pin to the Arduino's TX pin (Digital Pin 1). It is recommended to use a voltage divider (e.g., two resistors) on the line from Arduino TX to HC-05 RXD if using a 5V Arduino, as the HC-05's logic input is 3.3V tolerant but not 5V tolerant. Always double-check connections before powering on.

Enter AT command mode by holding the button while powering on until the LED blinks slowly. Using a serial terminal at 38400 baud, send 'AT+UART=115200,0,0' to change the data baud rate to 115200 (format: baud,stop bits,parity). Change the device name with 'AT+NAME=MyDevice'. Modify the PIN with 'AT+PSWD=4321'. After each change, send 'AT+SAVE' to store settings in non-volatile memory, then power cycle the module. The new settings will be active after reboot.

The HC-05's Class 2 radio provides a typical open-space range of up to 10 meters. To improve range, ensure a clear line of sight between devices and minimize interference from other 2.4GHz sources like Wi-Fi routers or microwaves. You can solder a wire antenna (approx. 31mm for 2.4GHz) to the designated antenna pad on the PCB or connect an external 2.4GHz antenna via a compatible connector if your module variant has one. Using a higher-gain external antenna can significantly extend the operational distance.