Bluetooth module is useful for wireless communication for short range of distances (up-to 10m). These modules can be used to control devices like speakers , motors , and for sending and receiving data wireless . This is one of the important and useful module to know about . 

How it works 

To understand well , lets assume the below condition . 

Suppose , Bluetooth terminals are connected to arduino.

Bluetooth is connected to mobile wireless.

Now , the bluetooth module need to receive and transmit signals through wireless communication with mobile . It also has to transmit and recieve signals with arduino through wired communication. 

•  The  communication between (arduino – BT module) is digital and type is UART(Universal Asynchronous Receiver/Transmitter). In UART communication, there is no shared clock between transmitter and receiver (here bt module and arduino) . For data transmission between two devices , we need clock which synchronizes rate of data transmission between transmitter , if there is no clock , we can’t get synchronization. So , in UART , the communication can only happen with predefined rate or baud rate ( bits per second). For this , the both transmitter(arduino) and receiver(bluetooth module) should operate at same and fixed baud rate. If there is difference in  baud rate , the data will crash and communication doesn’t happen.  The concept is like both devices should communicate at same rate , so if we provide a clock for both , they can see it and adjust themselves to relative rates . But we don’t have clock in UART communication. So they operate at fixed rate.

• (Bluetooth to mobile communication) : This communication is wireless. wireless transmission happens through electro-magnetic waves ( analog communication) . so we must convert the digital signals to analog signals using modulation techniques like FSK(frequency shift keying) and PSK(phase shift keying) . Which converts digital into analog signals of Radio frequency. And then we connect that output to antena to transfer signals wireless. 

• Bluetooth modules operate in the 2.4 GHz ISM (Industrial, Scientific, and Medical) band. This frequency band is globally available and unlicensed, allowing Bluetooth devices to be used worldwide without interference concerns.

pin out 

• STATE (Status Pin): This pin indicates the status of the module (connected or disconnected).
• VCC (Power): Connect this pin to the power supply (typically 3.3V or 5V).
• GND (Ground): Connect this pin to the ground of your circuit.
• TXD (Transmit Data): Connect this pin to the RX (receive) pin of the device you want to communicate with.
• RXD (Receive Data): Connect this pin to the TX (transmit) pin of the device you want to communicate with.
• EN (Enable/Key Pin): This pin can be used to enter AT mode or control the state of the module. When pulled HIGH, the module is in command mode; when pulled LOW, it enters data mode.