Paper Pi Is a Raspberry Pi-Powered Handheld with an E Ink Display

What Is E Ink?

Display matrix technologies come in all kinds of varieties. These include LCD, LED, and OLED. However, they tend to consume a lot of power and can lead to eye strain for many people. E Ink, on the other hand, has no backlight, and since the screen only draws meaningful amounts of power when its content is changed, devices using it can stay on for longer stretches of time before needing to be recharged.

It works by using a lattice of microcapsules that are suspended in a liquid. When a negative charge is applied, the white particles become visible at the surface. Conversely, a positive charge will cause black particles to rise. This means sharp, high-contrast text can be shown without drawing too much current, which is perfect for a Linux terminal like the Paper Pi. The project's creator, Daniel Norris, went with a Waveshare 4.2" E Ink screen for his design.

Pi to Screen Communication

With the display figured out, it was time to get the screen talking to the Raspberry Pi 4. Ordinarily, people simply plug in monitors via HDMI to get video output, but that's impractical for two reasons with this project. First, the connection would be far too bulky inside of a small enclosure, and second, the E Ink module only supports SPI. To get around this, Norris used the PaperTTY module, which enables a service on the Pi that essentially pipes the output from the terminal over the SPI bus to the screen. He also chose Ubuntu instead of Raspberry Pi OS because it supports a greater variety of terminal sizing within PaperTTY.

Keyboard Wiring

Placing a full, 87-key USB keyboard was out of the question, so how does one maintain all of the functionality while still saving space? The answer to this challenge was building a fully-custom keyboard that is split in half. The switches themselves are membrane surface mount buttons, and each one is soldered to a piece of perf board. They are arranged in the standard 5x3_3 layout, with a total of 10 columns and 4 rows.

After soldering on the wires, the matrices were connected to the Pi's GPIO header. Values are read via the QMK firmware. This lets each button act as a key like in any other keyboard.

Designing the Enclosure

Keeping the device's footprint small and handheld was the top priority, which means every feature of the enclosure was designed around this. It's primarily split into three different sections, with the ones on the left and right acting as both grips and battery holders, and the one in the middle as the bay for the Raspberry Pi 4. The cover simply has individual cutouts for the keyboard's buttons and the screen.

After Norris was finished designing the enclosure, he 3D printed the pieces with PLA filament and gave it some slight finishing. He then placed each component into its respective area and attached the whole thing together with machine screws.

Trying It Out

With the batteries charged, it was time to boot the Paper Pi. Starting up, the Pi runs a series of scripts that configure and initialize both the keyboard and screen, along with the battery management board. Then while using the handheld, the battery's voltage is measured and recorded to a file. If it goes below 3.2V, a warning is displayed in the terminal.

This gadget is a novel way to get acquainted with the GNU/Linux terminal, and when combined with networking, it can perform some very powerful tasks.