Description
HC‐05 module is an easy chip to use Bluetooth SPP (Serial Port Protocol) module,designed for transparent wireless serial connection setup.The HC-05 Bluetooth Module can be used in a Master or Slave configuration, making it a great solution for wireless communication.This serial port bluetooth module is fully qualified Bluetooth V2.0+EDR (Enhanced Data Rate) 3Mbps Modulation with complete 2.4GHz radio transceiver and baseband. It uses CSR Bluecore 04‐External single chip Bluetooth system with CMOS technology and with AFH (Adaptive Frequency Hopping Feature).
 
Hardware Features
Typical ‐80dBm sensitivity.
Up to +4dBm RF transmit power.
3.3 to 5 V I/O.
PIO(Programmable Input/Output) control.
UART interface with programmable baud rate.
With integrated antenna.
With edge connector.
Software Features
Slave default Baud rate: 9600, Data bits:8, Stop bit:1,Parity:No parity.
Auto‐connect to the last device on power as default.
Permit pairing device to connect as default.
Auto‐pairing PINCODE:”1234” as default.
 
Resources:
1. Arduino and HC-05 Bluetooth Module Complete Tutorial
2. HC-05 - Bluetooth Module
3. How to Configure HC-05 Bluetooth Module As Master and Slave Via AT Command
 
NB: 
The HC05 Enable pin (pin 1, just before Vcc) must be held high to process AT commands. In normal use (transferring data) it needs to be low.
If it’s floating or low, AT commands won’t work.