You know you can make logic gates out of relays, right? How about a whole computer! The Single Board Relay Computer (or Relay Trainer) is a computer whose CPU is made out of electromechanical relays (like Konrad Zuse's Z3 from 1941), but with memory and front panel interface built from modern semiconductors and integrated circuits (like the 1970s and 80s single board "trainer" computers, such as the KIM-1).
83 relays are enough for a simple but surprisingly powerful CPU. The semiconductor memory allows it to be small and practical, replacing the equivalent of 14,000 relays.
There is much more information at the project website.
Over the years others have built a number of awesome hand-wired relay-based computers. Why? Many reasons! To learn computer architecture, to design and build a fun and challenging project, to create a nice teaching and demonstration tool, and all in homage to Konrad Zuse's Z3, which was the first working programmable computer.
The Single Board Relay Computer is my take on this idea. I was originally going to make a relay version of one of the smallest computers I knew, the classic PDP-8. But after running through the design I had a number of realizations: at 227 relays it would have been fairly large and expensive, yet too slow to run existing programs in any reasonable amount of time- meaning that although it was a nice goal, cloning an existing ISA (instruction set architecture) is not all that useful. This led me to clarify my goals: what I really wanted was a small computer that would be fairly inexpensive, still easy to program (not merely Turing complete), and not too slow (so parallel, not bit serial and it had to be some kind of single cycle RISC machine). To be really practical it should use printed circuit board technology and fit on a single board.
The Single Board Relay Computer is the design I eventually came up with. It took three PCB revisions to a get a working manufacturable computer, but now it is ready.
Here are some of the basic characteristics of the Single Board Relay Compter:
Other basic features:
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I am a computer engineer with many years of experience in FPGA, software and hardware design. I'm also a contributor to a number of open source projects- my most well known is Joe's Own Editor. I'm trying out Tindie as a way to sell some of my side projects.