HC-05 6-Pin Bluetooth Serial Module with Button

To enter AT command mode, ensure the module is powered on but not paired. Press and hold the onboard button, then apply power until the LED blinks slowly (approx. 2 seconds per blink). Connect the module's TXD and RXD pins to your microcontroller's UART crosswise (TXD to RX, RXD to TX). Open a serial terminal at 38400 baud (default AT mode baud rate) and send commands like 'AT' to test, 'AT+NAME?' to check the device name, or 'AT+PSWD?' to view the PIN. Remember to save changes with 'AT+RESET'.

The primary difference is the operational mode. The HC-05 supports both master and slave roles, allowing it to initiate connections to other devices or accept incoming pairings. The HC-06 is a slave-only device, meaning it can only wait to be connected by a master (like a smartphone or PC). Therefore, for creating a link between two microcontrollers, you need at least one HC-05 in master mode. The HC-05 also features a dedicated KEY pin for mode switching, while the HC-06 typically lacks this hardware control.

Connect VCC to Arduino's 5V pin (if using the module's 3.6-6V range) or 3.3V for lower power. Connect GND to GND. For data, link the HC-05's TXD pin to Arduino's RX (pin 0) and the HC-05's RXD pin to Arduino's TX (pin 1). The KEY pin can be left unconnected for normal operation or connected to a digital pin to control command mode via software. Ensure you use a serial terminal on your PC (via USB) to see data sent from the HC-05, as it will communicate over the same hardware UART as the USB connection.

First, verify the baud rate. While the default data baud rate is 9600, the AT command mode often uses 38400 baud. Ensure you are sending 'AT' followed by a carriage return. Second, confirm the mode switch procedure: power must be applied while the button is pressed. The LED should indicate a slow blink rate. Third, check your wiring; TX and RX must be crossed between your USB-to-serial adapter and the module. Finally, note that some cloned modules may have non-standard AT command sets or require a specific voltage (like 3.3V) on the KEY pin to activate command mode.