HC05 Bluetooth Module is used very widely as it full of features and is available at very cheap price. HC05 Module is a serial Bluetooth module with six pins, which are State, RXD, TXD, GND, VCC and En. State pin is connected to the onboard LED which indicates that Bluetooth is working or not. RXD and TXD receive and transmits serial data respectively. GND is connected to the ground of the circuit whereas VCC is connected to the 5V supply voltage. En is connected to the button present on the module which is used to switch between AT command mode and data mode. It is set low for data mode by default.

There is a LED on the Bluetooth module to indicate the status of the module. If the LED blinks once in every 2 seconds then the module has entered AT command mode. If the LED starts blinking repeatedly then it means the module is waiting for a connection in data mode. And if LED blinks twice in 1 second then the module is in data mode and connection is successful. HC05 uses the UART protocol to make it easy to send and receive data wirelessly. It uses the frequency band of 2.4GHz ISM and input supply voltage is 3.6V to 6V. Range of operation for the module is around 10m.

HC05 Bluetooth module can operate in Master, Slave or Master/Slave mode. By default, Slave mode is configured. In slave mode, it can accept but connection requests but cannot initiate connection request to any other Bluetooth device whereas in the master mode it can initiate connection request to other Bluetooth devices in the range. 

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