How to connect two Bluetooth serial modules

Here we are going to look, how we can connect two Bluetooth serial modules. 

Here I am taking the example of Bluetooth module HC 05 because it is a widely using Bluetooth serial module (Bluetooth SPP). Serial port Bluetooth module is 3Mbps Modulation with complete 2.4GHz radio transceiver and baseband. 

HC 05 Module and important pins

Some of other features of this module are

• ·         Up to +4dBm RF transmit power
• ·         Low Power 1.8V Operation ,1.8 to 3.6V I/O
• ·         Default Baud rate: 38400, Data bits:8, Stop bit:1, Parity:No parity, Data control: has. Supported baud rate: 9600,19200,38400,57600,115200,230400,460800.
• ·         Given a rising pulse in PIO0, device will be disconnected.
• ·         Status instruction port PIO1: low-disconnected, high-connected
• ·         PIO10 and PIO11 can be connected to red and blue led separately. When master and slave are paired, red and blue led blinks 1time/2s in interval, while disconnected only blue led blinks 2times/s.
  Auto-connect to the last device on power as default.
• ·         Permit pairing device to connect as default. Auto-pairing PINCODE:”0000” as default Auto-reconnect in 30 min when disconnected as a result of beyond the range of connection.
• ·         Bluetooth protocal: Bluetooth Specification v2.0+EDR
• ·         Frequency: 2.4GHz ISM band
• ·         Modulation: GFSK(Gaussian Frequency Shift Keying)
• ·         Emission power: ≤4dBm, Class 2
• ·         Sensitivity: ≤-84dBm at 0.1% BER
• ·         Speed: Asynchronous: 2.1Mbps(Max) / 160 kbps, Synchronous: 1Mbps/1Mbps
• ·         Security: Authentication and encryption
• ·         Profiles: Bluetooth serial port
• ·         Power supply: +3.3VDC 50mA
• ·         Working temperature: -20 ~ +75Centigrade
• ·         Dimension: 26.9mm x 13mm x 2.2 mm

You can easily connect your android phone/ PC to this HC 05 module. For that, just power ON your HC module, then switch on your PC/Phone Bluetooth. Now search from your PC or Android phone, it will show the device name. Select pair option and give pair password. The default password might be 1234 or 0000. If everything is OK, then your HC-05’s pair LED (if any) will glow.

But if you want to connect to two embedded system device using HC-05 we want to do following tricks.

 

How to connect two embedded system project using Bluetooth module

HC-05 comes from the factory as SLAVE mode; that means a master (Android/PC) can easily manage the connection between the HC slave connections. But if you are connecting two HC modules, for example, you want to connect two embedded system projects,  you should do some tricks!. Otherwise, it will not works.

The reason is that, Initially Both of the modules are in slave configuration and slave-slave (or master-master) communication is not possible. So we want to change the mode of one Bluetooth module for the communication.

For this you need following tools

1. 1.      TTL to RS232 converter

1. 2.      Terminal Software (you can download it from www.pravysoft.org website)

Normally HC-05 modules are TTL module means if we directly connected this module to PC serial port the RS232 signals from PC will damage our module. So you should use a TTL to RS232 converter. If you are familiar with MAX232 chip, you can make this using simple circuit (visit http://pravysoft.blogspot.org ) for making this module.

HC 05 With built in RS232 PORT

Now you need to connect the RS232 connector to your PC. Latest PC’s does not have serial port (DB9 pin), if so you need to use  RS232 to USB converter module, note that you should install proper drivers to work USB to serial converter. You can see the corresponding ports of USB to serial converter in DEVICE MANAGER as shown in the figure.

Serial to USB convertor (Black) and HC 05 module

Device Manager window to find serial port number

 If the port number is greater than COM7, change the port number to a lower one. Otherwise, the terminal software won't detects the PORT

Then take your HC05 module and feed a 3.3V voltage for its KEY pin (Please see the first figure).

Some HC05 RS232 modules are coming with KEY PINs, for that you just place a jumper there.

HC05 with Key Jumber is shorted

Open terminal software select correct serial port

Please note that you should select baud rate of  38400.

To test all connections are ok or not, you just type AT and press enter

You will get continuous OK string, don’t worry please power off your HC05 module and reopen terminal now.

Again connect the module using terminal software and sent AT+ROLE=1

Terminal software window

If everything is ok, then your module become MASTER now. To check it is master or not just power ON the device and search it using a phone.

Alternatively power on another HC module (slave config) and Power on configure HC module, if it is MASTER, it will automatically connect to the slave module (Check PAIR LED status)

TIP: Power on slave module before Master one for fast pairing