HC-05 Bluetooth Shield based on Bluetooth SPP (Serial Port Protocol) module designed as Wireless Serial Communication allows use with any microcontroller. It supports the UART protocol for easy transfer and receives data wirelessly. This module works as both Master and Slave Mode and easy switchable between these two modes. By default, Slave mode is configured. And the configured Modes changed using AT Commands. While working in slave mode, HC-05 cannot initiate a connection to another Bluetooth device but can accept connections. Whereas in Master mode, it initiates a connection to other devices.
This serial port Bluetooth module features fully qualified Bluetooth V2.0+EDR (Enhanced Data Rate) 3Mbps Modulation with a complete 2.4GHz radio transceiver and baseband. The module communicates using the USART protocol at a baud rate of 9600. Hence it is easy to interface with any microcontroller that supports USART. The onboard LED present on the board indicates the connection status, whether the Bluetooth connection or not. This red LED blinks continuously in a periodic manner before connecting to the module. When connected to any other Bluetooth device, its blinking slows down to two seconds.
This Bluetooth protocol is an affordable communication method in PAN network, with a maximum data rate of 1Mb/S, working in a nominal range of 100 meters using 2.4 G frequency is a common way of wireless communicating. HC05 module consists of an internal 3.3v regulator and that is why you can connect it to 5v voltage. But it is highly recommended 3.3V voltage since the logic of HC05 serial communication pins is 3.3V. Supplying 5V to the module can cause damage to the module. In order to prevent the module from being damaged and make it work properly, you should use a resistance division circuit (5v to 3.3v ) between the Arduino TX pin and the module RX pin.