Arduino Tutorial: Blink an LED using Bluetooth

Bluetooth is a wireless technology standard for exchanging data over short distances (using short-wavelength UHF radio waves in the ISM band from 2.4 to 2.485 GHz) from fixed and mobile devices and building personal area networks (PANs). Invented by telecom vendor Ericsson in 1994, it was originally conceived as a wireless alternative to RS-232 data cables. It can connect several devices, overcoming problems of synchronization.

Prototype of Android controlled Arduino LED Controller

Fig. 1: Prototype of Android controlled Arduino LED Controller

This project aims to control an LED via Android Phone using Bluetooth Communication through commonly used module HC-05. For this, we use an Arduino board with an Atmega8 sitting on it.

HC-05 module is an easy-to-use Bluetooth SPP (Serial Port Protocol) module, designed for transparent wireless serial connection setup. It contains 6 pins which are labelled on the back, but most modules only have 4 of those populated with pogo pins. KEY & STATE are not required, as KEY is used for flashing the device and STATE simply indicates whether the device is awake or not. So that leaves only GND, VCC, TXD, RXD.

Typical Image of HC-05 Bluetooth Module

Fig. 2: Typical Image of HC-05 Bluetooth Module

For connecting the module with Arduino, we need to use the Serial (Tx and Rx) pins provided on the board.ard.

Making Connections with HC-05:

Some modules have VCC labelled for working voltages up to ~6 volts. These modules DO NOT like anything except 3.3 volts on the VCC line. We should use a level converter to 3.3V on the RXD line. Use two resistors, as a simple voltage divider to make the TTL level conversion. Wire one 2.2k ohm resistor to ground and another 1k ohm resistor to the TXD line on the MCU. Connect the RXD pin in between the two resistors for an output of approx 3.4 volts.

Connect RXD pin of module to TXD of Arduino (Digital Pin 1) through the voltage divider configuration shown below:

Image showing circuit connections of Arduino Uno and HC-05 Bluetooth Module

Fig. 3: Image showing circuit connections of Arduino Uno and HC-05 Bluetooth Module

Now connect TXD of module to RXD of Arduino (Digital Pin 0).

After making the above connections, you’ll see blinking led on the module. This just ensures your device is powered properly. After that pick your Android phone, and get the app- “Arduino Bluetooth Terminal” from the Google Play Store. This app is available for free, and is easy to use.

Screenshot of Android App used to control LED

Fig. 4: Screenshot of Android App used to control LED

After installation, turn ON Bluetooth of your phone. Now search for devices until you find HC-05 and then start pairing with it. Enter Password 1234, when asked.

If properly paired, the LED blinking on Bluetooth module will Stop. If not, check the connections again. Get back to the App you just downloaded. It’ll automatically find the device, match the key it displays, and start conversation. Make sure that the phone’s Bluetooth remains ON.

Now let’s transfer the code to Arduino, wherein, we try to blink an LED using the Android App.

How Code Works:

When we send ‘1’ to the Arduino, the LED connected at Pin-7 turns ON and when ‘2’ is sent the LED turns OFF. The serial data is read by Arduino, through Serial.read() function and is stored in the integer type variable state declared in the program. After this in the loop(), we just compare state value with 1 or 2, and perform respective operation.

Project Source Code

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int led = 7;                //led connected at digital Pin 7
int state;
int flag=0;       
void setup()
{
    pinMode(led, OUTPUT);                      // LED declared as O/P
    Serial.begin(9600);                        // Baud rate set to 9600bps
}
void loop() {
    if(Serial.available() > 0)
    {    
      state = Serial.read();  
      flag=0;
   }  
    if (state == '1')
    {
       digitalWrite(led1, HIGH);
       if(flag == 0){
       Serial.println("LED is ON");
       flag=1;
    }
    }
    else if (state == '2')
    {
       digitalWrite(led2, HIGH);
       if(flag == 0)
{
       Serial.println("LED is OFF");
       flag=1;
}
}
}           //loop() ends here

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