Over the last three weeks I’ve been playing with various breakout boards, microcontrollers and embedded linux. I’ve been trying to take time to learn how the technology works and how best to combine them to teach and produce something useful, different and entertaining. So this project initially started out when I was playing with the RTLSDR dongle, I thought it might be cool to display the messages as they got demodulated on a separate device that was dedicated to grabbing POCSAG messages, I slapped it all together, then when my son was over the other weekend we decided to modify the code to grab the top 25 articles from a given subreddit.

Eventually when I get time ill put together a video tutorial on setting this all up but in the meantime here are the notes …



You will need:

1 x Raspberry Pi model B.

1 x Wifi dongle or ethernet cable.

1 x Arduino deumilinove.

1 x USB lead (for serial communication with the arduino)

1 x USB lead (to power the RPi)

1 x USB Charger

1 x LED Matrix 8×8 x4 (I used the horizontal style layout)

So how does it work?

Its pretty simple really, the raspberry pi runs a small python script that grabs the top 25 articles from a predefined subreddit using the api (most of reddits api can be accessed just by post fixing .json to a lot of their URLS the data structures are also well documented and available on github.) The script strips out any weird characters as the JSON is returned in utf-8 format. A string is built containing the articles rank, total amount of upvotes and the article title. If the string is over 300 characters it gets trimmed and “…” appended to the end. The script creates a serial connection to the arduino (the device is usually addressable using /dev/ttyUSB0) The string is then sent to the arduino over serial. When the arduino receives a new string it displays it to the LED matrix after having scrolled any current messages.



What’s the python code?

Install and configure raspbian in the normal way. Before running the script make sure you have internet access otherwise the python script will fail when it trys to communicate with reddit. (todo: add better error handling.)

Python code – http://pastebin.com/shXyry5C

Copy the code in to a file called reddit.py to your home directory.

Edit the file so it points to the correct serial device.

nano reddit.py

If you’re unsure about the arduino serial port run

sudo ls /dev

Look for something like …

ttyUSB0

Edit the line in reddit.py where the serial communication is initialised ensure it matches the port for the arduino.

ser = serial.Serial('/dev/ttyUSB0','57600')

Save the file using …

ctrl+x

Make sure the file has execute permissions by issuing the following command …

chmod +x reddit.py

You can test the script by running it from the command line using the following command (*this needs to be run with superuser privs to access the serial port) …

sudo python reddit.py

Once you’re happy with the way the script is running you can make it autoboot with the pi by using the following command …

sudo crontab -e

If you are prompted which editor to use and are unsure use nano (option 2)

At the bottom of the file add the following line …

@reboot /local/bin/python /home/user/reddit.py

Save the file by pressing ctrl+x

Reboot and test your script is autoloading.

To display the messages on the matrix you’ll need to flash your arduino with a little code …

Wiring the arduino …

To connect the arduino to the matrix you will need to ensure you have the correct SPI pins as they differ between arduino boards the ones on the deumilinove are pins A4 and A5, A4 SDA and A5 SCL, you will also need to attach the CS (chip select) pin to pin number 13 on the arduino and also connect VCC to 5v and GND to GND on the arduino.

A4 to SDA

A5 to SCL

13 to CS

5v to VCC

GND to GND

(todo : will post a pic when i get time)

What’s the arduino code?

Arduino code – http://pastebin.com/pvkM2DUK

Copy and paste the arduino code in to your arduino IDE, you’ll also need to grab the md_max29xx library from git hub. (Download the zip file and unzip the files to your arduino IDE library folder usually /Documents/arduino-x.x.xx/libraries) This library is used to drive the LED matrix. Once you’ve installed the library you’ll have to specify how many matrices you are using in the arduino code. (How many LED matrix panels you will be displaying to my board had 4 8×8 LED panels.) Here’s the important bit though depending on what/where/who you bought your led matrix board from you might find you’ll have to edit way the arduino draws to the matrix otherwise you could be in for some very interesting results. This can be done in the md_max29xx lib folder. There are a few variables that define the matrix rotation and draw direction IE. right to left, left to right, top to bottom etc. You will need to play with these variables until you get the right combination for your matrix.

NB. When initially getting this running use the example grapicstest to make sure the leds are drawing as expected then flash the arduino reddit serial code once you have the library configured correctly.

Now all the hard work’s done

Once you’ve installed the library, edited the arduino code, updated the variables for the draw direction and successfully flashed the code, plug the arduino in to the raspberry pi and boot. There should be enough of a delay in the script to ensure the RPI has connected to your network before it makes its initial request.

If you have the arduino setup correctly you should see scrolling text on the matrix. “Waiting for serial data!” after a short while the rpi should connect and start displaying articles from reddit.

Concluding

Still yet to build an enclosure or decided how / where im going to use this yet but i’ve had fun making it. Initially getting the libraries setup and running correctly was by far the most frustrating part of this project writing / editing the code was pretty straight forward. There’s plenty of scope for improvement / modification too, if I get time I was thinking I’d get it to cycle multiple subreddits, but for the time being I’ve spent enough hours on this project.

NB. The original code running on the arduino was limited to 25 characters I’ve modified this to allow for messages up to 300 characters in length (the size of an article title from reddit) Since doing so some of the serial messages appear to glitch intermittently. I’ve been checking the code for errors but haven’t managed to pinpoint the problem yet. As far as I can tell the python code runs fine but there might be a problem when the arduino is receiving the serial messages. Possible solutions maybe change the baud rate and send the data slower or add a checksum. (*As suggested by advancednewbie) Example code an library for implementing a checksum. https://github.com/AdvancedNewbie/serialChecksum



IF YOU HAVE ANY QUESTIONS REGARDING THIS PROJECT PLEASE PUT THEM IN THE COMMENTS ON THE YOUTUBE VBLOG EPISODE AT THE TOP OF THIS POST.