The HC-05 Bluetooth Module is a popular and low-cost device used for wireless communication between microcontrollers (like Arduino, ESP32, or Raspberry Pi) and other Bluetooth-enabled devices such as smartphones, laptops, or PCs. It is based on the Bluetooth SPP (Serial Port Protocol), meaning it can replace serial communication wires with a wireless connection. 🔹 Features of HC-05 Communication Protocol: Bluetooth 2.0 + EDR (Enhanced Data Rate). Operating Voltage: 3.6V – 6V (recommended 5V with onboard regulator). Current Consumption: ~30 mA (during data transmission). Interface: UART (TX, RX, GND, VCC). Default Baud Rate: 9600 bps (can be configured). Modes: Master → initiates connection with other Bluetooth devices. Slave → waits for connection requests (default mode). 🔹 Pin Configuration VCC → Power supply (3.6–6V). GND → Ground. TXD → Transmit serial data. RXD → Receive serial data (⚠️ Works on 3.3V logic, use a voltage divider if connecting to 5V Arduino). EN/KEY → Used to enter AT Command Mode for configuration. STATE → Indicates connection status (HIGH when connected). 🔹 Modes of Operation Data Mode (Default) Used for communication with another Bluetooth device. Simply sends and receives data over UART. AT Command Mode Used to configure the module (change name, password, baud rate, etc.). Entered by holding KEY pin HIGH while powering the module. 🔹 Applications Wireless data transfer between Arduino and phone (e.g., controlling robots with a mobile app). Home automation (switching appliances ON/OFF via Bluetooth). Wireless programming of microcontrollers. Sensor data logging to smartphones/laptops. Bluetooth-controlled cars, drones, or IoT devices. 🔹 Advantages Low cost and easy to use. Supports both master and slave modes. Can replace UART serial communication wires. Works with Arduino IDE’s Serial Monitor or Android apps like Bluetooth Terminal.