We will link the LEDs of every pixel using relays, then link those relays to a mainframe computer.
Regarding inputs/outputs, the peripherals (eg. a joystick) would be linked to the mainframe, which would in turn order the micro-controlers, themselves powering the LEDs.
1. Use case diagram
We have three users groups ; we are interested by the interactions they can have with the Wall.
- Internet users
→ Publish new animations
→ Simulate (before publishing)
- SNCF tech support
2. Main topics
2.1. LED & Alimentation
Multiple types of LED exists, 3V, 12V or 220V. Because a tension of more than 50V is dangerous for humans, we will not use more than 12V.
LEDs are sold either in RUBANS of 30 LED/m, 50 LED/m or 60 LED/m.
Regarding the alimentation, we need to convert 230V~ in 12V-. We also need to be sure that the intensity of the circuit will never be more than 2A (fire risk).
The ERP (Établissement Recevant Public) defines five levels of security regarding arsons, from M0 (unflammable) to M4 (easily flammable).
Firstly, we chose to build the mainframe in fireproof wood (M1) ; but the delegate of the GMP department (Génie Mécanique Productique), which will build it, has recommended an alloy of zinc and steel, or stainless steel instead of wood, because this is what they are specialized into. Also, stainless steel doesn’t need to be painted, but is more expensive.
He also recommended the use of aluminium for the grid pattern between the pixels, and we are still unsure of the position of the main computer.
Comment l’utilisateur va-t-il intéragir avec Arbalet ?
How will the user interact with Arbalet ?
We thought of joystick or a console gamepad. These are cheap and easy to handle both for us and the user, but they do not add to the design of Arbalet.
A touchscreen tablet could also be used to choose the animation, etc. The thouchscreen tech is fragile though, which can become a problem in a train station.
Leap Motion might be interesting ; their design and operating make it a really futuristic object which would fit very well the wall. However, the prerequisite are important ; Windows 7+ or Mac only, 2GB of RAM, and an i3, i5 or i7 core.
A Wi-Fi Acces Point with a simplified internet interface could be good too. Easy to create, it permits to anyone with a smartphone to connect to the Wall, without needing to download anything, which is always a good thing for the user. It could, for instance, let the user choose the animation they want to see.
The wall will be controlled by a Raspberry Pi (computer equiped to interact with micro-controlers). Raspberries run under Raspbian (Linux), and have a GPIO port (General Purpose In Out) to communicate using the daisy chain architecture.
Every micro-controler would be equiped of 4 LEDs and would have a unique ID to able the Raspberry to communicate with only one of them at a time.
The website could allow the users to modify their own programs or to publish them on Arbalet’s database to make them usable in the station. It will have an area to edit the code, a difficulty setting (to use a version more or less simplified and high-level of the API), and a simulator to try your code at home.
Published programs will be sent to the moderators, which will then test them, and either accept or refuse them. From this point onward, it will be possible to play them directly on the Arbalet Wall in the station.
Next time ...
We are going to write the bill of specifications to GMP and meet them to get their opinion on the project.