HC-05 Bluetooth Module is a low-cost, easy-to-operate & small-sized module used for wireless communication in the Bluetooth spectrum.
It supports Serial Port Protocol (SPP), which helps in sending/receiving data to/from a microcontroller (i.e. Arduino UNO).
Its default baud rate is 9600 for data communication and 38400 for command mode communication.
Its operating frequency is 2.4 GHz ISM Band.
HC05 uses CSR Bluecore 04-External single-chip Bluetooth system with CMOS technology.
This module follows the IEEE (Institute of Electrical and Electronics Engineers) 802.15.1 standard protocol.
Dimensions of HC-05 are 12.7mmx27mm.
Its operating voltage is 5V. But data Tx/Rx levels at 3.3v. Use level shifter at Rx
It sends and receives data by UART, which is also used for setting the baud rate.
HC05 can operate in master/slave mode and thus multiple slave nodes can be controlled using a single master node (called mesh networking).
HC-05 supports "AT commands", controlled by TX (transmission) and RX (receiver) pinouts.
 
LED on module tells about different states of Bluetooth module:
If it's blinking once in two seconds, it has received a command.
If it's blinking continuously, means the module is waiting for input data.
If it's blinking twice in one second, means it's connected properly.