Computing stuff tied to the physical world

Connecting a Blink Plug to a web browser

In Hardware, Software on Dec 23, 2012 at 00:01

Here’s a fun experiment – using Node.js with SocketStream as web server to directly control the LEDs on a Blink Plug and read out the button states via a JeeNode USB:

JC's Grid, page 51

This is the web interface I hacked together:

Screen Shot 2012-12-21 at 23.34.20

The red background comes from pressing button #2, and LED 1 is currently on – so this is bi-directional & real-time communication. There’s no polling: signalling is instant in both directions, due to the magic of WebSockets (this page lists supported browsers).

I’m running blink_serial.ino on the JeeNode, which does nothing more than pass some short messages back and forth over the USB serial connection.

The rest is a matter of getting all the pieces in the right place in the SocketStream framework. There’s no AngularJS in here yet, so getting data in and out of the actual web page is a bit clumsy. The total code is under 100 lines of CoffeeScript – the entire application can be downloaded as ZIP archive.

Here’s the main client-side code from the client/code/app/ source file:

Screen Shot 2012-12-22 at 00.48.12

(some old stuff and weird coding in there… hey, it’s just an experiment, ok?)

The client side, i.e. the browser, can receive “blink:button” events via WebSockets (these are set up and fully managed by SocketStream, including reconnects), as well as the usual DOM events such as changing the state of a checkbox element on the page.

And this is the main server-side logic, contained in the server/rpc/ file:

Screen Shot 2012-12-22 at 00.54.07

The server uses the node-serialport module to gain access to serial ports on the server, where the JeeNode USB is plugged in. And it defines a “sendCommand” which can be called via RPC by each connected web browser.

Most of the work is really figuring out where things go and how to get at the different bits of data and code. It’s all in JavaScript CoffeeScript on both client and server, but you still need to know all the concepts to get to grips with it – there is no magic pill!

Tomorrow, I’ll describe how I created this app, and how to run it.

Update – The code is now on GitHub.

  1. Hi Jean-Claude, while I was trying something similar (on a JeeLink v2) I noticed, that the setup() function was being executed everytime, the serial port was opened from the linux-webserver side. While not tragic, is this the intended behaviour? From the Arduino documentation I assumed that the setup() routine is only being called once and thereafter only the loop is being executed… If I am not wrong, that means that all global variables are reset and one cannot retain values between subsequent openings of SerialPort.SerialPort. Thanks for your thoughts! Michael.

    • That’s normal behavior for all Arduino-like boards – opening the serial port generates a reset. This “side-effect” is what allows uploading new sketches.

      You need to keep the serial port open (and have a process listening to it). In normal use this is necessary anyway, to receive incoming notifications.

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