Using the HC-05 Bluetooth Module

In a previous post I showed how to use the HC-06 Bluetooth module. In this post we will be using the HC-05. They are very similar, but the HC-05 has more features. The ability to be a master is the main feature. A master can create a connection with other Bluetooth devices. In this post we will only being using it as a slave.

The basic module looks like this.

It is typically purchased soldered to carrier PCB with some additional electronics. The HC-05 typically has a 6 pin header, rather than the 4 pin HC-06 header. The two extra pins are state and enable (en). State gives the state of the Bluetooth connection and enable can power down the module by turning off the power regulator. I will not be using the enable. I will use the state to allow programming of the Arduino via Bluetooth.


Here is a schematic of the carrier board. Not all carrier boards are the same, though.

The parts on the carrier PCB are pretty basic

  • 3.3V Low Dropout Regulator, which allows you to power it from 3.6V to 6V.
  • An LED to show the mode.
    • Fast Blink = Waiting for Bluetooth connection3.6
    • Slow Blink = In AT command mode
    • Double Blink = Connected via Bluetooth
  • A button to enter AT Command Mode
  • A diode, probably for reverse voltage protection.
  • Various Pull Up/Down resistors and bypass capacitors.

Configuring the HC-05

Like the HC-06, the HC-05 has a AT command mode, but the commands are a little different. The HC-05 is put in the AT command mode by holding in the switch while applying power. It will do a slow blink when in AT mode. AT Mode accepts commands at 38400 baud , N,8,1 using the Rx and Tx pins. You should level shift the Tx out of your Arduino to 3.3V using a resistor divider. Commands are sent with line feed and carriage return appended to the end. Most serial monitors can do this for you including the Arduino Serial Monitor.

Any command that sets a parameter can also get it.

  • Set commands are in this format “AT+CMD=PARAM” like … AT+NAME=FRED to set the name to FRED. Some commands have multiple parameters that are separated by commas.
  • Get commands are in this format AT+CMD?” like AT+PSWD? to get the password. Weirdly, they all seem to work except AT+NAME?.

Here are the commands you needs for slave mode. Remember, each is followed by a line feed and carriage return.

  • AT (This is just a way to test the connection. It will respond “OK”)
  • AT+VERSION? (This returns firmware version info)
  • AT+ROLE=x (for x use 0 =Slave role, 1 = Master role, 2 = Slave-Loop role default = 0)
  • AT+NAME=xxxxx (to change name to xxxxx default=HC-05″)
  • AT+PSWD=nnnn (to change password to 4 digit nnnn default = 1234″)
  • AT+UART=nnnn,s,p (nnnn=Baud, s=stop bits (0=1, 1=2), p=parity (0=None, 1=Odd, 2=Even) Example AT+UART=115200,0,0
  • AT+POLAR=a,b (a=PIO8 (LED), b=PIO9 for both 0=low turn on, 1 = high turn on. (see below for how we use this)
  • AT+ORGL (reset all parameters to defaults)
  • AT+RESET (restarts the HC-05. Will not be in AT mode afterward unless button held”)

Using an Arduino to program the HC-05

We need some hardware to talk to the HC-05. An Arduino will easily do that. Here is a diagram and sketch to do this using an Arduino UNO.

This is the hardware diagram. I show an UNO, but virtually any hardware (Nano, Mega, etc) will work. The HC-05 is a 3.3V device so we need to level shift the Arduino 5V Tx signal down to 3.3V.  The diagram uses a resistor divider to do this. The Arduino should have no trouble reading the 3.3V Tx signal from the HC-05, so we don’t need to shift that.

The State connection through the capacitor is optional. This will force a reboot of the Arduino when a Bluetooth connection is made. More on that later.

BTW: A lot of people don’t bother to level shift and it appears to work fine, at least in the short term 🙂

The Arduino Sketch

Here is the sketch I use. We will be setting up 2 serial links. One link will be from the PC to the Arduino to send the commands from the keyboard over USB.  We also need a serial connection from the Arduino the HC-05. We will use a software serial port for this and can use any remaining pins to do this. HC-05 uses 38400 baud for AT commands, regardless of the what you set it to for Bluetooth operation.  I used 115200 for the PC to Arduino connection. Set the Serial monitor like this.

You can then type AT commands in the Sereial Monitor.

Here is the sketch…

#include <SoftwareSerial.h>

#define SOFT_RX 11
#define SOFT_TX 12

SoftwareSerial hcSerial(SOFT_RX, SOFT_TX); // RX, TX

String fromPC = "";

void setup() { 
  Serial.begin(115200); // hardware serial for the USB-PC
  hcSerial.begin(38400);  // software serial Arduino to HC-06 (38400 is default)

  // print instructions
  Serial.println("HC-05 AT Command Programmer V1.2");

  Serial.print("For Arduino Rx use pin ");
  Serial.print("For Arduino Tx use pin ");
  Serial.println(" -- Command Reference ---");
  Serial.println("To Read use '?', Like AT+PSWD?");
  Serial.println("AT (simply checks connection)");
  Serial.println("AT+VERSION (requests the firmware verison)");
  Serial.println("AT+ROLE=x (0 =Slave role, 1 =  Master role, 2 = Slave-Loop role  default = 0)");
  Serial.println("AT+NAME=xxxxx (to change name to xxxxx default=HC-05");
  Serial.println("AT+PSWD=nnnn (to change password to 4 digit nnnn default = 1234");
  Serial.println("AT+UART=nnnn,s,p (nnnn=Baud, s=stop bits (0=1, 1=2), p=parity (0=None, 1=Odd, 2=Even)");
  Serial.println("AT+POLAR=a,b (a=PIO8 (LED), b=PIO9 for both 0=low turn on, 1 = high turn on.");  
  Serial.println("AT+ORGL (reset all parameters to defaults)");
  Serial.println("AT+RESET (restarts the HC-05. Will not be in AT mode afterward unless button held");

void loop() {
  // Read from HC-05
  if (hcSerial.available()) {
    while(hcSerial.available()) { // While there is more to be read, keep reading.
      Serial.print((char); // send it to the PC
  // Read from PC
  if (Serial.available()){
    delay(10); //     
    fromPC = (char);    
      hcSerial.print(fromPC); // show the HC-05 responce
      Serial.print(fromPC); // echo it back to the PC

Arduino Programming over Bluetooth.

Arduinos are programmed over serial via a bootloader. A bootloader is program that runs for a few seconds whenever the Arduino is started. It looks for someone trying to program it. It runs in one part of the Arduino’s memory. If it does not detect an attempt to program the Arduino it switches to the part of memory where the last program (sketch) resides. If it does detect an attempt to program the Arduino, it reads the incoming program instructions over the serial port and writes them to that other part of memory where normal programs (sketches) reside. Once the upload is complete it switches to that program and runs it.

Therefore, in order to program the Arduino over a serial connection, you need to trigger a reboot. The Arduino USB creates a full RS232 connection. In addition to Rx and Tx is has other control lines like DTR (Data Terminal Ready). The Arduino uses the DTR signal to force a reset. To reset an Arduino you pull the reset line to ground. The DTR signal out of the USB chip is high when there is no connection and low (ground) when there is a connection.

If we directly connect DTR to the reset pin, the Arduino will be stuck in permanent reset mode whenever a serial connection is open. To correct that, a capacitor is inserted in the circuit. Capacitors block a continuous signal, but pass a quick transition. Therefore the the change from high to low will look like a quick pulse to ground at the reset pin. That pulse is exactly what is needed to reboot run the bootloader.

Here is what that circuit looks likes on an Arduino Nano schematic. The length of the pulse depend on the value of the capacitor and characteristics of the high to low transition.

The HC-05 state pin will work for this. In its normal mode it is high during a connection. We need that to be low (ground). Fortunately the HC-05 has the Polar command. That allows you to flip that logic. AT+POLAR=1,0 will do the trick. The first parameter is for the LED. We leave that at 1. The second parameter is the state and we switch that from the default of 1 to 0.

I found that the typical 0.1uF capacitor would not generate an adequate pulse to ground, so I bumped it up to 1.0uF. It occasionally does not work when uploading. I think a little less capacitance might be better. The Arduino uses the hardware serial connections for programming, so you use those pins. When programming the Arduino use the virtual serial port you got when pairing the Bluetooth. Do not use Bluetooth and the USB serial port at the same time. Both would be connected to the hardware Rx and Tx and conflict with each other and possibly cause damage.


Other Reset Features

You may not care about uploading code over Bluetooth, but some of your applications may expect that reboot on connect behavior. I have found this with some GCode senders. They open the serial port and expect to see the text Grbl spits out at startup. Without seeing this text, the program wrongly assumes there is a problem and closes the connection.


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