How Does the LED Sign Work?

The infrastructure to print pages from the SCE website works well to control other peripherals. Writing to the LED sign is slightly simpler than printing and only involves Core-v4, AWS SQS and a Quasar. Below is a high level diagram of the sign messaging system.

To demonstrate the functionality of the LED sign, let's start at Core-v4's webpage to update the sign.

Explanation of Each Component

Core-v4

A request is made through Core-v4's led sign page:

When the user fills out the required fields and hits submit, it will make a post request to the SCE backend in Core-v4/src/APIFunctions/LedSign.js:53. For more information about Core-v4's code structure, check the respective wiki page.

Core-v4's backend recieves the request in Core-v4/api/peripheral_api/routes/LedSign.js:22 which sends the update sign request to AWS SQS

Pushing Messages to SQS

After the user submits data to update the sign, Core-v4's backend pushes a message to a Queue in Amazon's Simple Queue Service (SQS). Below is an example message pushed to the queue:

{
    "scrollSpeed": 25,
    "backgroundColor": "#0000FF",
    "textColor": "#00FF00",
    "borderColor": "#FF0000",
    "text": "Welcome to SCE!",
}

• Note: the range of the scrollSpeed field is 0-50

Once Quasar recieves a message from the LED queue, the message's data is sent to the LED sign in the form of an HTTP request. The LED sign has a REST API Quasar uses to write messages to it. If the LED sign has an IP address of 10.8.0.1, Quasar would send a request to http://10.8.0.1/api/update-sign, with the message data encoded as JSON in the HTTP request body.

The Physical LED Sign

The LED sign is running a Python server using Flask and can be found at SCE-Development/rpi-led-controller. The code to recieve HTTP request from Quasar can be found on SCE-Development/rpi-led-controller/server.py:60. The handler makes use of the SignMessage class to write to the sign. It converts the data from the HTTP request to a shell command on SCE-Development/rpi-led-controller/sign_message.py:18.

On the sign, the received message translates to the command:

./sce_sign.exe \
 --set-speed <speed> px/vsync \
 --set-background-color <background color>
 --set-font-color <font color> \
 --set-border-color <border color> \
 --set-font-filename ./10x20.bdf \
 --set-brightness 66% \
 --set-text <text here>

the sign would show the message with the below colors:

Fun Facts

1. This sign was built as a group project lead by @kammce a long long time ago.
2. The actual text scrolling algorithm can be found in the aforementioned project's repo, kammce/sce-led-sign.
3. SCE initially controlled the LED sign remotely with gRPC, specifically SCE-Development/led_sign/led_sign_server.py:42