Modern Day Relay-based Computers
A small group of DIY inventors have built relay computers and calculators. This section does not cover every known system. However, it highlights a few notable projects by dedicated experimenters.
In general, “relay computers” fall into three broad classes (used here to add clarity).
These are:
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Pure relay-only for all functions (~300 to ~2K relays). Some have capacitor charge memory and may use diodes for some logic gating.
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Mostly relays with some diodes for logic steering and semiconductor RAM/ROM memory. It takes 1,000 relays to store 1,000 bits so using semiconductor memory offers a huge saving in size, power and cost.
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Hybrid designs. A microcontroller (Arduino, Raspberry PI, ESP32, …) assists with user interface (ex: tablet with WiFi), storage, some program control with relays for all math. Enables lightweight, portable, low power, 32b floating point, with excellent user interface.
Some designs use only one relay type repeatedly (say, SPDT) while others use SPDT, DPDT and even more complex contact forms in the same machine.
The following summaries are based on my current understanding of these machines. If you see an error please contact me and I will update the description. All images are taken from the inventor's respective published sites.
The R30 Electromechanical computer (Class 3) is a DIY project is covered here.
The famous Z3 computer was reconstucted by experts and a coverage follows the DIY inventers.
Review of some Noteworthy DIY Relay Computers
1-- Jeroen Brinkman (Netherlands), MERCIA all Relay Computer, 2016. ~2,000 relays. 10 bits instruction width, 1K storage, integer math. No semiconductors except for diodes. One relay type (SPDT) is repeatedly used for all logic circuits.
Capacitors are used for 1/0 storage and refreshed if needed. This is a Class 1 design and an amazing effort to show all the key aspects of relay computing.
It is strictly a fixed-point/integer 10-bit machine. Any fractional or extended-precision arithmetic could be implemented in software/microcode.
2-- Simon Winder (UK), 8 bits, floating point, 480 relays. Computes square roots only. Note the value in the Nixie tubes below, the square root of 2. Simon started the project in 2009 and finished it in 2014. Digit entry uses a telephone dial. This is a Class 1 design. See a video here.
3—Dr. Harry Porter’s relay computer. 2015, 415 relays, Data Bus (8 bits), Address Bus (16-bits). Uses the Konrad Zuse Z3 relay adder circuit. Uses semiconductor SRAM for program storage to reduce relay use so falls into a Class 2 design. It supports 8-bit integer math.
4—DipDot, 2026 (YouTube). A DIY relay computer, evolving story. It appears that this is a Class 1 relay computer. Initially, the designer used semiconductor static RAM then later transitioned to a period-accurate 1940s-style Capacitor-Diode Memory. ~450 DPDT relays fully populated.
Very clean design with many custom PC boards. He uses an Arduino to assist in testing new designs quickly, but this is not needed for real-time math calculations.
Uses 8-bit integer math with a 16-bit address bus for storage access.
5-- Paul Law's architecture is heavily inspired by Dr. Harry Porter's Relay Computer. Law’s machine reportedly has about 400 relays. So, it supports 8-bit integer math.
Pragmatically, it uses semiconductor memory and transistor arrays to interface between SRAM and the relays. It uses a 5-relay per bit, full adder design. It was designed for portability and has the feeling of the MITS Altair 8800 computer (1975) user control panel. It is packaged beautifully and portable.
Because the hardware ALU can only operate on 8 bits at a time, calculations like division or long addition are chained together using multi-byte software algorithms (e.g., repeating 8-bit addition with carry).
In a Youtube video Paul shows an example of calculating PI to 20 decimal places. He needs an external controller (PC or other) that “assembles” a complex math routine into 8-bit operations that are loaded into the machine’s semiconductor ROM. So, without external assembly of microcode this is a Class 2 machine. For complex math operations this looks like a Class 3 machine.
Paul Law’s relay computer, front side
Z3 Reconstructions by Experts
There is a special class of "DIY" relay computer that is built by experts from universities and museums. Outstanding examples are the four versions of the Z3 relay computer originally by Konrad Zuse. For more info on the original Z3 (1941) see the article on this site - From Pascal to Zuse. Of the four, two are practical replicas and two are hybrid machine reconstructions. In 2027 each of these are in museums in Germany and Switzerland.
The first replica was built by Konrad Zuse in 1961 for the Deutsches Museum in Munich. This is a Class 1 all-relay machine with electromechanical stepping switches for memory. Because the original machine was destroyed by wartime bombing in 1943, Konrad Zuse recreated his masterpiece entirely from memory and surviving documents to act as an historically accurate, structurally identical clone. Pictures of this version are often considered the original since there are no images remaining of the 1941 version.
A second major reconstruction (Z3r) was engineered and built by Konrad Zuse’s eldest son, Professor Dr. Horst Zuse, between 2008 and 2010. Permanently housed at the Deutsches Technikmuseum in Berlin, this version deliberately diverges from the original Z3 blueprint for educational purposes. Horst added diagnostic LEDs to the relay blocks so live museum audiences could physically trace the movement of bits. He made other changes too for demo and simplification purposes.
It uses a Siemens Simatic PLC (S7) electronic controller to assist in stepping through program sequences. This is a Class 3 machine.
See a video of Horst explaining, in German and English, his version of the Z3.
Two other versions were build by Dr. Raúl Rojas and Dr. Christof Traber respectively. See below.
Snapshot of Horst Zuse in front of his version of the Z3, 2022 at the Museum
The four reconstructions of the Z3:
1. 1961 (Zuse KG / Konrad Zuse): Built by Zuse's own company to serve as technical evidence for patent claims and historical priority. It reproduced the logical operation using newer, smaller relays in a single memory cabinet configuration and is on permanent display at the Deutsches Museum in Munich. This uses the very clever 2 relay full adder circuit compared to the 5 relay 1-bit adder design for the original Z3.
2. 2001 (Dr. Raúl Rojas / Free University of Berlin): An educational, hybrid reconstruction using relays for the arithmetic and memory units combined with digital components and a PC for user I/O. It is housed at the Konrad Zuse Museum in Hünfeld, Germany.
3. 2010 (Dr. Horst Zuse's Z3r): Built at approximately full historical scale using around 2,500 modern industrial relays to mark the centenary of Konrad Zuse’s birth. It features LED indicators and manual single-bit stepping controls, and is located at the German Deutsches Technikmuseum in Berlin.
4. 2021–2024 (Dr. Christof Traber): A fully operational electromechanical reconstruction based on the 1941 patent application, using modern relays, stepping switches, and relay-chain clock generation. It is housed and regularly demonstrated at the Enter Technikwelt Solothurn in Derendingen, Switzerland.
The 2001 reconstruction of the Z3 by Raul Rojas and team. Modern relays were interconnected using circuit boards. The console and punched tape were simulated by means of a modern display. Photograph by R. Rojas.
