Building a Mobile App to Control Arduino Board Via Bluetooth

In this instructable I'll show you how to build a custom mobile app to control your Arduino project via Bluetooth. You don't need mobile programming experience to start since a visual App Inventor IDE will be used.

This is an easy to follow step-by-step tutorial to engage students who are not very familiar with programming. It is ideal for Hour of Code or other short coding activities. I tried to keep the Arduino sketch and the mobile app as compact as possible, yet sufficient to demonstrate the concept. You could easily extend them to control motors, visualize sensors data on your smartphone etc. If you need an inspiration, scroll to the last step.

1. Arduino board. Most of them would work. I used Arduino UNO. Serial interface will be used.
2. Bluetooth module. HC-06 or HC-05.
3. Wires.
4. Android device with camera, Internet access and QR-code scanner.
5. A computer or laptop with Arduino IDE and web browser.

Upload the attached sketch to Arduino board and connect a Bluetooth module to your Arduino board. Note that TX is connected to RX and vise versa. The sketch should be uploaded BEFORE wiring since Arduino uses the same serial interface to communicate with HC-06 and for sketch uploading.

int val;
int LED = 13;//built-in LED
void setup()
{
 Serial.begin(9600);//Opening serial port
 pinMode(LED, OUTPUT);//Initializing LED pin
 digitalWrite(LED, HIGH);//Turning built-in LED on
}
void loop()
{
 if (Serial.available())//If connected
 {
   val = Serial.read();
   if (val == '1')// If "1" received, turning LED on
   {
     digitalWrite(LED, HIGH);
     Serial.println("LED: ON");//Sending the confirmation
   }
   else if ( val == '0')// If "0" received, turning LED on
   {
     digitalWrite(LED, LOW);
     Serial.println("LED: OFF");//Sending the confirmation
   }
   else
   {
     Serial.print("Unknown command");//Sending a message about unsupported commands
     Serial.println(val);
   }
 }
}

The protocol is very simple. There are two supported one-character commands, '0' and '1' to turn built-in LED off and an. Read the comments for further explanations. In the following steps we'll create a mobile app to send these commands.

Register on http://ai2.appinventor.mit.edu or sign in with your Google account and create new project.

Drag a VerticalArrangement onto app's main screen and make it fill all available space. It will align all the controls vertically.

Drad a ListPicker onto Layout. This widget will be used for Bluetooth device selection.

Drag a BluetoothClient onto your screen. This invisible object will be used to control Bluetooth communication.

Switch to Blocks mode and create code for available devices list generation.

Now you should be able to upload your app to Android device. The easiest way is to generate QR code and download apk file. First go to Anrdoid Bluetooth settings and allow connecting to HC-06. If it asks password, the default one is "1234". Now you could run your app and open devices list. Make sure your HC-06 module is available there.

Return to Designer mode and drag Button just below the ListPicker. Make it fill all the screen width and call it "Connect".

Drag a Label just below your "Connect" Button. The label will be used for connection status indicating. Make it red and give it a text value "Disconnected".

Switch to Blocks mode and add Button.Click event handler. It will try to establish connection with the device user selected in the ListPicker. It will modify the Label's text and color if the attempt was successful.

Now you should be able to download your app and make sure you are able to connect to HC-06 module.

Return to Designer mode and drag Switch between your Button and Label. We'll use it to control the Arduino LED.

Switch to Blocks mode and add Switch.Changed event handler. It will send '0' or '1' commands t the board. Note that we used ASCII codes of these characters.

Now you may download your final app and test it.

If you have extra time, you could extend this project. For example, this mobile app is based on my initial example. It reads data from DS18B20 temperature sensor and draws a plot on Canvas. The sketch has to be modified too.

#include <OneWire.h> //a library for 1-Wire interface
OneWire ds(2); // OneWire object to interact with DS18B20 sensor
void setup(){
  Serial.begin(9600);
}
 
void loop(){
  // reading temperature from DS18b20
  byte data[2]; // an aray for storing temperature value
  ds.reset(); // reset the sensor
  ds.write(0xCC); // command for searching DS18b20 by address
  ds.write(0x44); // command to make DS18b20 measure temerature and store it in its internal memory
  delay(1000); // giving DS18b20 some time to process our command
  ds.reset(); // getting ready for temperature reading
  ds.write(0xCC); 
  ds.write(0xBE); // command to make DS18b20 send us data from its internal memory registers
  // reading the response
  data[0] = ds.read(); // reading the minor byte
  data[1] = ds.read(); // and the major byte
  // merging them to one value
  // 0,0625 is sensor's resolution
  float temperature =  ((data[1] << 8) | data[0]) * 0.0625;
  // sending obtained value to the serial port
  Serial.println(temperature);   
}