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Tablets and smartphones are everywhere and prices are dropping fast.  A tablet provides a great user interface: it’s inexpensive, has a high resolution color touchscreen and it’s an ideal method to control an embedded system.

The most straightforward way to do this is with an embedded system exposing a web  interface over Wi-Fi. In this case, the tablet only needs a browser to connect. However, smaller embedded systems may not have this luxury. Here we will look at using an Android tablet to connect to a small embedded system using Bluetooth.

We won’t get into the details of the embedded system but for clarity’s sake, let’s say it’s a small Arduino measuring room temperature and connected to a Bluetooth transmitter. It sends a reading automatically once per second.

There is a lot of information online about using Bluetooth with the Android. The problem is that it is fragmented and few sites have all the information in one place. So I figured I’d compile a set of the major points you need to know to get the Bluetooth Serial Port Protocol (SPP) working on an Android app.

First, your app needs the BLUETOOTH and BLUETOOTH_ADMIN permissions. This goes in yourAndroidManifest.xml file.

<uses-permission android:name="android.permission.BLUETOOTH" />
<uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />

Now that we have given the app permission to access the Bluetooth API, let’s look at the classes that are relevant. We’ll need the BluetoothAdapter, BluetoothDevice and BluetoothSocket classes to connect to the external system.

First, we need to find and pair with the device. The following code snippet builds a collection of Bluetooth devices that were discovered by your Android device.

    BluetoothAdapter bta = BluetoothAdapter.getDefaultAdapter();
    bta.startDiscovery();
    Set deviceList = bta.getBondedDevices();

Now that we have a list of external devices, we can iterate over the collection and extract the name and address of each device like this.

    for (BluetoothDevice i : deviceList)
    {
        String name = i.getName();
        String address = i.getAddress();
    }

You can select which one you need from the list and connect to it. Selecting the device is something you can decide how to handle yourself. Connecting to the device is done using its Bluetooth address. Once you have a selected device, we connect to it by requesting the device through its address. The address is a unique 48-bit ID assigned to each Bluetooth device.
Once we have the device, then we’ll open a socket using the standard UUID for the SPP serial port protocol to indicate that we want to connect to the device as if it were a standard serial port.

    BluetoothAdapter bta = BluetoothAdapter.getDefaultAdapter();
    BluetoothDevice device = bta.getRemoteDevice(address);
    BluetoothSocket socket = null;
    if (device != null)
    {
        UUID serialID = UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");
        try
        {
            socket = device.createRfcommSocketToServiceRecord(serialID);
        }
        catch (java.io.IOException e)
        {
        }
    }

Next, we’ll handle sending and receiving data from the socket.

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Sometimes you want an operation to repeat periodically. Say you are building a parts washer that circulates cleaning fluid around the dirty parts. The cleaning cycle might run for an hour and in that time you want the circulation pump to run for 10 seconds, stop for 5 seconds for particles to settle, then run for 10 seconds and repeat for an hour.

We need a timer. The type of timer that does this is called a Cycle Timer because it repeats a specific timing cycle and it’s pretty easy to build a cycle timer with an Arduino and a little bit of software programming. We’ll need an Arduino (any kind, from any manufacturer will work), a power supply, the power driver circuit, and the “load” which in this case is our pump.

Let’s get started.

// Which pin to use to control the load const int OUTPUT_PIN = 1; 
// Total number of cycles 
const int NUMBER_OF_CYCLES = 10; 
// On time per cycle in milliseconds 
const int CYCLE_TIME_ON = 5000; 
// Off time per cycle in milliseconds 
const int CYCLE_TIME_OFF = 2000; 

void setup() 
{
 pinMode(OUTPUT_PIN, OUTPUT);
 digitalWrite(OUTPUT_PIN, LOW);
} 

// Run the timer 
void loop() 
{
 int cycles = NUMBER_OF_CYCLES;
 while(cycles-- > 0)
 {
    // Turned timed output on
   digitalWrite(OUTPUT_PIN, HIGH);
   delay(CYCLE_TIME_ON);
   // Turn timed output off
   digitalWrite(OUTPUT_PIN, LOW);
   delay(CYCLE_TIME_OFF);
 }
 // Hold forever
 while(1);
}

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A very common task is to measure temperature at various points. In fact, temperature is the most commonly measured and controlled variable in Process Control. The BeagleBone single-board computer is becoming more popular because of its low cost and sophisticated capabilities. It’s one of the easiest ways to serve up a web page that allows the operator to monitor data and control devices.

Temperature can be measured with a variety of sensors. Here we are concerned with the range of liquids from around 0C to 80C. A straightforward way to measure temperatures in this range is with a semiconductor analog-output sensor. They are easy to use, accurate, and low cost.

I happened to have a number of AD592 temperature to current sensors on hand. These have been traditionally used for process control because their current output makes them very noise resistant when long cable are used. We convert the current (1 microvolt/Kelvin) to a voltage by using a resistor. In this case, we use a 3.3kohm resistor to produce a voltage of around 1V at room temperature. The 3.3k resistor gives a resolution of .003V/Kelvin which with the 273.15K offset gives 0.003V/degree Celsius.

Using the sensor connected to 5V, AGND and the analog input on P9, pin 36, we use this code snippet to read the sensor:

// Read an AD592 temp sensor and return degF
function readTemp()
{
   var volts = 1.8 * trap.analogRead("P9_36");
   // Convert to Kelvin. AD592 outputs 1uV/K
   var k = volts / 3300 / 0.000001;
   // Convert to Fahrenheit
   var f= (k - 273.15) * 9 / 5 + 32;
   // Log the measured values
   console.log("V,k,F: " + volts,k,f);
   return f;
}

For improved accuracy we can measure the actual resistance of the 3.3k resistor and use that measured value in the program.

This concept is easily extended to remote reading. Since the BeagleBone is a powerful little Linux computer, it can be used to serve web pages. We have a simple node.js webserver and sensor data reader program available at this download link

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