MODI Building (5)

Next is the main display of the MODI building. I designed that part years ago and decided at that time to use a Raspberry Pi to drive the (relatively larger) 240x320 2.2” display. The advantage of using a Raspberry Pi is that they are much more powerful than microcontrollers like Arduinos. They can play mp4 / h.264 video at 30fps using very standard software. However, so far, I have avoided them in buildings because, as they are “full-fledged computers” running Linux, they need to be properly shut down before cutting the power off. This is annoying. On the layout, I have a main on-off switch and I want to be able to switch the power off at any time.

I have quickly tested an ESP32-based Arduino with that same display and I managed to make it work at 12fps (using the jpegdec library). The difference with 30fps is really noticeable. In addition, I’d have to add an external SD card module to store the video.

So I came back to the Raspberry Pi and found a solution to the shutdown issue: Make a Read-Only Raspberry Pi with the new Raspberry Pi Overlay File System. Read the details here: https://learn.adafruit.com/read-only-raspberry-pi/overview
Recent Raspberry Pi OS releases have an option to put the /boot partition (where the kernel image and other critical files reside) in a read-only mode. If software installed on this system only performs reading and playback operations, then we can just unplug the system when done without risk of corrupting the operating system’s files on the SD card. This is exactly what I need here.

This is a description of the setup I use in the MODI building

• Raspberry Pi Zero W (the newest Raspberry Pi Zero 2 W would of course be even better) in headless mode (no keyboard / no monitor), access using SSH over WiFi only
• 2.2” ILI9341-based TFT display, resolution 320x240
• Raspberry Pi OS 3 / Kernel 4.19.118 based on Debian 10 Buster
• mplayer software playing standard mp4 h.264 video files
• Note: That solution is based on the obsolete fbtft_device, which is not supported on newer kernels. I’ll have to find a new driver in the future in order to support the latest revisions of the Raspberry Pi OS
• “Vertical” video files are mostly found on TikTok, then cropped / resized using handbrake (https://handbrake.fr)

The Raspberry Pi (and the display) are powered by a local step-down DC-DC Buck Converter (https://www.aliexpress.com/item/1005007494413893.html), which provides a stable 5V power supply from the layout’s 12V supply.

Fitting the display to the frame of the building is a challenge. I have unglued the display from its control PCB (red in the photo below), but the flat cable connecting both is short and fragile. The display can only be placed a few millimetres (less than 10) from the PCB. I made rough 3D models of the display and its control PCB so that I could design a suitable slot for a display in the building’s frame. The control PCB is glued right behind the display, and the Raspberry Pi is attached to the other side of the building.