The HC-05 is a highly popular, low-cost Bluetooth Classic (2.0) module designed for wireless serial communication in microcontroller projects. It operates as a Master or Slave device, making it ideal for connecting microcontrollers like Arduino to smartphones, laptops, or other Bluetooth modules.Core Technical Specifications

Pinout ConfigurationThe module is typically mounted on a breakout board with 6 pins:

• VCC: Power supply input (3.6 V – 6 V).
• GND: Ground reference connection.
• TXD: Serial transmit pin (3.3 V logic) connects to MCU RX.
• RXD: Serial receive pin (3.3 V logic) connects to MCU TX (requires voltage divider).
• STATE: Indicates Bluetooth connection status (Low = disconnected, High = connected).
• EN / KEY: Toggle pin to enter AT Command mode (High = AT mode, Low/Floating = Data mode).

Connecting HC-05 to ArduinoWhen wiring the HC-05 to a 5 V Arduino, you must use a voltage divider on the HC-05 RXD pin because its logic level is strictly 3.3 V.textArduino 5V ————————> VCC (HC-05)Arduino GND ————————> GND (HC-05)Arduino TX (Pin 11) —> [ 1k ] —> RXD (HC-05) | [ 2k ] | GNDArduino RX (Pin 10) <---------------- TXD (HC-05)