Sunday, July 27, 2014

Comparing the BeagleBone Black and the Spark Core: Remotely turning on an LED

After writing the Spark Core iOS library I started thinking more about being able to control the BeagleBone Black from my iOS devices.  The BeagleBone Black does not have built in Wi-Fi but I normally have my spare BeagleBone Black (non-robot one) connected to my Wi-Fi router with a network cable.  So I started asking myself; how could I control my spare BeagleBone Black from my iOS device with it connected directly to my router?  The answer came in the form of and bonescript.  

Lets look at how we can use with bonescript to control one of the user LEDs on the BeagleBone Black.  I used the Cloud9 IDE to write this application so the first thing that I needed to do was to install where Cloud9 can use it.  To install run the following commands (Note: I am using Agnstrom Linux, you may need to adjust the paths depending our you flavor of Linux):

cd /var/lib/cloud9
npm install

Once is installed, we can open the Cloud9 IDE in our favorite web browser by going to port 3000 of our BeagleBone.  You can do this by opening your favorite browser and entering the IP Address of your BeagleBone followed by colon and 3000 IE: 

Now that we have everything setup, lets write the application to control one of the user LEDs on the BeagleBone black.  In the Cloud9 IDE, create a folder called “RemoteLED”.  We will be creating two files within this folder called toggleled.js and index.html.  Lets look at the toggleled.js file first:

var app = require('http').createServer(handler);
var io = require('').listen(app);
var fs = require('fs');
var b = require('bonescript');

//Set to listen on port 2080

//my constants
var ledPin = "USR3";

//called to initilize the device
setup = function() {
    b.pinMode(ledPin, b.OUTPUT);
    b.digitalWrite(ledPin, STATEOFF);

//function called to setup our page
function handler (req, res) {
  //load our index.html page
  function (err, data) {
    if (err) {
      return res.end('Error loading page');


//on method listener
io.sockets.on('connection', function (socket) {
  socket.on('led', function (command) {
    if (command == 'on'){
        b.digitalWrite(ledPin, STATEON);
        socket.emit('ledstatus', 'LED is on');
        b.digitalWrite(ledPin, STATEOFF);
        socket.emit('ledstatus', 'LED is off');

We begin by adding the external modules needed using the require function that comes with node.js.  Next we set to listen for requests on port 3080 and define three constants.

The setup function is called when the application starts up.  It initializes the LED pin and also turns the LED off so the LED will begin in the OFF state.   The handler function is called when a HTTP request comes in and sends the index.html page, which we will look at shortly, to the client. 

Finally we add an “on” method listener that listens for incoming actions.  In this example, we listen for the ‘led’ action and when a ‘led’ action is received it runs the inner function.  This inner function will turn the LED on if it receives the “on” command otherwise it will turn the LED off.  It also uses the emit function to send a ‘ledstatus’ action back to the client with the current status of the LED.

Now lets look at the index.html file. 

        .button {height:100px;width:140px;font-size:34px;margin-left:30px;margin-top:50px;}
        .status {font-size:34px;margin-top:50px;margin-left:30px;}
    <script src = "/" > </script>
        var socket = io.connect();
            socket.on('ledstatus', function (data) {
                var statusDiv = document.getElementById("status");
                statusDiv.innerHTML = data;
        function ledOn(){
            socket.emit('led', 'on');  
        function ledOff(){
            socket.emit('led', 'off');

    <input type="button" name="on" id="onButton" class="button" value="ON" onClick="ledOn();">
    <input type="button" name="off" id="offButton" class="button" value="OFF" onClick="ledOff();">
    <div id="status" class="status">LED is off</div>

The interesting parts of this page are between the two “script” tags.  In the first “script” tag we set up a listener that listens for an ‘ledstatus’ action and when it is received it will call the inner function that sets the status label to the message that was received.  This action is sent from the toggleled.js script with the following command:  socket.emit('ledstatus', 'LED is on'); 

The second ”script” tag contains two functions.  These functions are called when the on and off buttons are pressed and uses the emit function to send a ‘led’ action back to the listener in the toggleled.js script that turns the LED on or off.

If you run this code within the Cloud9 IDE you should be able to control User LED from any computer or mobile device that can connect to the BeagleBone Black.  The web page should look like this: 

Now lets look at how we would control the User LED on the Spark Core. I will use Spark’s standard tools and then write an iOS app using my Spark Core – iOS Library that will control the User LED on the Spark Core.  If you do not know how to use Spark’s IDE or flash the Spark Core, please refer to Spark’s getting started page.

Lets begin by looking at the code that we will flash to the Spark Core:

int userLed = D7;
int ledStatus = 0;

void setup()
   Spark.function("led", ledController);
   pinMode(userLed, OUTPUT);
   Spark.variable("ledStatus", &ledStatus, INT);

void loop()
   //doing nothing

int ledController(String param)
    if (param == "on")
        digitalWrite(userLed, HIGH);
        ledStatus = 1;
        return 1;

    if (param == "off")
        digitalWrite(userLed, LOW);
        ledStatus = 0;
        return 1;
    return -1;

In the setup function we begin by making the ledController function available through the Spark cloud by using the Spark.function.  We can send information to the ledController function using a POST request from our client. We also define a variable that can be read using the Spark.variable function using a GET request.  We will turn the LED On or Off with the ledController function and read the LED status from the ledStatus variable.

The ledController function looks at the parameter that was sent in the POST request.  If the parameter is “on”, the function turns the LED on and sets the ledStatus variable to 1.  If the parameter is “off”, the function turns the LED off and sets the ledStatus variable to 0.  If something besides “on” or “off” was sent, the function returns -1.  Once you have the code in Spark’s IDE, you should flash it to your Spark Core.

Now lets look at the iOS application to see how it works.  If you are not familiar with my iOS library for the Spark Core, you can read my blog post about it.  You can download the code from the GitHub repository.  The full source for the iOS application is in the Spark Core GitHub repository under the “SparkComparisonApp” directory.

I do not want to go over the full ViewController class here but I do want to highlight two methods.  

-(void)getStatus {
    SparkTransactionGet *getTransaction = [[SparkTransactionGet alloc] initWithAccessToken:ACCESS_TOKEN deviceId:DEVICE_ID andProperty:STATUS_VAR];
    [SparkCoreConnector connectToSparkAPIWithTransaction:getTransaction andHandler:^(NSURLResponse *response, NSDictionary *responseDictionary, NSError *error){
        if(error == nil) {
            NSLog(@"Response: %@",responseDictionary);
            int status = [[responseDictionary objectForKey:@"result"] intValue];
            NSString *statusStr = @"off";
            if (status == 1)
                statusStr = @"on";
            _statusLabel.text = [NSString stringWithFormat:@"LED is  %@", statusStr];
        } else {
            NSLog(@"Error: %@",error);
            _statusLabel.text = @"Error Getting Status";

The getStatus method makes a GET request to Sparks Cloud API and retrieves the current status of the LED.  When the status is returned it sets the statusLabel UILabel with the status of the LED.  If an error is returned it sets the statusLabel UILabel to an error message.

-(void)sendRequestWithParameter:(NSString *)parameter {
    NSLog(@"Params: %@",parameter);
    SparkTransactionPost *postTransaction = [[SparkTransactionPost alloc] initWithAccessToken:ACCESS_TOKEN deviceId:DEVICE_ID functionName:FUNCTION andParameters:parameter];
    [SparkCoreConnector connectToSparkAPIWithTransaction:postTransaction andHandler:^(NSURLResponse *response, NSDictionary *responseDictionary, NSError *error){
        if(error == nil) {
            NSLog(@"Response: %@",responseDictionary);
        } else {
            NSLog(@"Error: %@",error);
         [self getStatus];
    NSLog(@"Continueing with app");


The sendRequestWithParameter: method sends a POST request to Sparks Cloud API telling it to turn the LED either on or off depending on what the parameter is.  The parameter should be either “on” or “off” to match the commands on the Spark.  When the response comes back from Spark’s API, acknowledging that the on/off command was received, we then call the getStatus function to retrieve the current state of the LED.

One thing I did notice with the Spark Core is there is a noticeable delay from the time I pressed the on or off button to when the status label was updated.  The delay was less then a second but it was pretty obvious.  I would imagine this delay would not be a problem for most applications but you would just need to account for it.  The response from the BeagleBone Black was all but instantaneous which is what I expected from 

Spark’s IDE is all right.  I would definitely rate the Cloud9 IDE above Spark’s mainly because the error messages from Spark’s IDE are really unhelpful.  I wasted a lot of time trying to track down a simple typo.

The architecture of the BeagleBone Black’s application is a lot nicer because the client and server components are contained in one application making it easier to update.  However if you plan on making a commercial product, being able to upload the client application to Apple’s app store would be really nice.

The big advantage that the Spark Core has over the BeagleBone Black is the built in Wi-Fi and it was designed/built to communicate with remote devices.  Spark’s Cloud API is also very easy to use with the REST based API.  Both the BeagleBone Black and the Spark Core have advantages and disadvantages; you will need to see which one fits best with your project.  I will say that I wish the BeagleBone Black had built in Wi-Fi but it does not.

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