GK-HC-05 geeekus HC-05 Bluetooth Transmission Module for Arduino Master Slave

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The geeekus HC-05 bluetooth module can add two-way (full-duplex) wireless functionality to your bluetooth enabled devices, like a smartphone, or other microcontrollers using serial communication (USART).

You can use this module to communicate between two microcontrollers like Arduino or communicate with any device with Bluetooth functionality like a Phone or Laptop. There are many android applications that are already available which makes this process a lot easier. The module communicates with the help of USART at 9600 baud rate hence it is easy to interface with any microcontroller that supports USART. The default values of the module can be configured by using the command mode. So if you looking for a Wireless module that could transfer data from your computer or mobile phone to microcontroller or vice versa then this module might be the right choice for you. However do not expect this module to transfer multimedia like photos or songs.

HC-05 Bluetooth module

The HC-05 has two operating modes: one is the Data mode in which it can send and receive data from other Bluetooth devices and the other is the AT Command mode where the default device settings can be changed. The device can be operated in either of these two modes by using the key pin as explained in the pin description.

It is very easy to pair the HC-05 module with microcontrollers because it operates using the Serial Port Protocol (SPP). Simply power the module with +5V and connect the Rx pin of the module to the Tx of MCU and Tx pin of module to Rx of MCU.


1. core module uses HC-05 slave module, pins include VCC, GND, TXD, RXD, KEY, Bluetooth state pin (STATE), output low if not connected, high if connected
2. LED indicates the connection status of Bluetooth, quick flash means no connectivity, slow flash means entering AT mode, double flash means Bluetooth is connected and the port is open.
3. BOttom plate has anti-reverse diode,with 3.3V LDO. input volt is 3.6~6V; current about 30mA with no pairing, current about 10mA after pairing. Input volt should be within 7V!
4. Pin level is 3.3V. It can be directly connected with various MCUs (51, AVR, PIC, ARM, MSP43 etc.) , also 5V MCUs, no need for MAX232 and cannot be connected with MAX232!
5. In open area, effective range is 10m (power level is CLASS 2). It’s possible to be more than 10m. But the connecting quality cannot be guaranteed.
6. After paring , it can be used as full duplex serial port. No need for knowing any Bluetooth protocol. It supports 8-bit data bits, 1 stop bit; can set odd-even check communication format, which is the most common communication format; do not support other formats.
7. Can access AT command to set parameters, and query information by pulling high pin 34.
8. Small size (3.57cm*1.52cm), SMD production, ensure quality; with transparent heat shrinkable film for dustproof and anti-static function.
9. Use AT-command to switch from master or slave mode, and to connect to specific devices.
10. Support standard baud rate of 4800bps~1382400bps.

Connection Diagram

HC-05 Connection Diagram

Sample Code

int val; 
int ledpin=13; 
void setup() 
} void loop()
{ val=Serial.read(); 
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