One-Bit Single-Board Computer Made with Just 24 Vacuum Tubes
UE-0.1 is a unique SBC that fits in the palm of your hand(s).
Building computers with minimal functional blocks can create interesting instructional devices. While some engineers might make those blocks with discrete transistors, Dave Lovett of the Usagi Electric YouTube channel goes even more fundamental. UE-0.1 is a one-bit single-board computer built with 24 vacuum tubes!
The UE-0.1 is a scaled-down version of the UE-1. The full-sized UE-1 fits on a wall and contains 192 vacuum tubes that implement a fully functional arithmetic logic unit (ALU.) Lovett based its design on a 1-bit Motorola MC14500 industrial processor. While it is highly functional, it is difficult for admirers unfamiliar with tubes or basic processor design to grasp what is happening within the array of boards.
Lovett wanted to make a much smaller unit that would be more accessible. The smaller UE-0.1 uses only 24 tubes to create a "computer" that is simple enough to understand, powerful enough to do something interesting, and small enough to fit in a person's hands.
UE-0.1's PCB measures 267 by 221 millimeters and is a modern design. Lovett gave the traces a visually striking style by making them 1 millimeter wide, separated by 2.54 millimeters, and curved. By having the boards fabricated without solder mask (or silkscreen), the combined effort results in a gorgeous-looking design.
Unlike its larger cousin, the 24-tube design does not implement a complete ALU. Instead, it only has a logic unit (LU) built from NOR gates. Most of UE-0.1's blocks contain only NOR gates because Lovett says these are very easy to implement with vacuum tubes.
Given the PCB's size, it literally fits in the palms of your hands, and it is safe to hold! Unlike many vacuum tube projects, UE-0.1 runs at relatively low voltages. There are only two supplies: positive 24 volts and negative 12 volts. When not holding the single-board vacuum tube computer, the switches allow you to enter data into the LU, and a vacuum fluorescent display (VFD) shows the status of the instruction bus, data bus, and registers.
In the video, Lovett demonstrates soldering the electronic components, making a single fix, and then ends with a demonstration of how to make UE-0.1 calculate 1 + 1! While even Lovett admits this is not a full-fledged computer, it is undoubtedly an excellent trainer tool to understand how a 1-bit computer works.
You can download the Gerbers and PCB design files in the UE-0.1 GitHub repository. Remember to see the single bodge (or correction) needed to make the board functional. If you do not want to build the UE-0.1 but still want to experience a 1-bit processor, the repo also contains a Logisim simulation project.