Project to design a model Enigma machine



The purpose of this paper is to explore the design of a model reproduction of the Enigma machine.  We assume a basic understanding of the working of the Enigma machine.  I am not qualified to complete the design and i would like this to be a cooperative effort.  I invite all who might be interested to join and contribute their ideas and expertise.  The project especially requires some mechanical design so those who can design and/or build mechanical parts are very specially encouraged to contribute. 

The design should meet the following requirements:

  • Actually work so that it can be used in practice
  • Serve to illustrate and understand how the Enigma machine works
  • We are not concerned with reproducing the external aspect of the
    original Enigma but rather its workings and functions.
  • Simplest design possible so that it can be manufactured easily and inexpensively
Once designed it could be built and sold commercially if a party interested in producing and selling it could be found.  I believe there would be a consumer interest in such a device.



The following figure shows the simplified electrical diagram of an Enigma machine

Enigma simplified electrical diagram
From this diagram we see that it lends itself to be built using Printed Circuit Board (PCB). Let us start from the left.
  • The battery can be of any commercial type.
  • For the lightboard commercial lightbulbs or LEDs can be used. Probably LED's would be a better choice as they are smaller and more reliable.
  • The keyboard can be built using commercially available keyboard switches.
  • The plugboard and plug-cables can be built using commercially available plugs and jacks.
  • The rotors are the heart of the machine and the only parts in the electrical circuit which would have to be specially designed and manufactured. We will try to make the simplest mechanical design which fulfills the same function. Initially we will consider building them with double sided PCB.
The rotors and their mechanical associated parts are, undoubtedly, the most complex parts to design and build. Today this fuction could be designed and implemented much cheaper and better with an electronic circuit but I consider the mechanical rotors an essential feature of the machine, one that needs to be preserved if the model is to fulfill the function of illustrating how the Enigma worked.

The main problem or consideration is how to mechanically advance the rotors.  A mechanical link between all the keys and the rotor assembly, such as the original Enigma had, would be complex and expensive and would probably require specially designed switches.  Unless a really simple solution can be found I would consider this not strictly necessary.  I can think of two alternatives:

  • Using an electromagnet to advance the rotor. This also seems a bit complex and not the simplest alternative.
  • Using a separate push button to mechanically advance the rotor. When encoding/decoding each character the user would push the appropriate key on the keyboard plus, simultaneously, pressing the rotor advance mechanism.  The machine could be designed so that the character key could be actuated with the index finger while the rotor advance key could be similar to the space bar on a computer keyboard and be pushed simultaneously using the thumb.  This might be the simplest alternative.
At this point we can see the design and mechanical working of the rotors are the crucial part which will make the project viable or impossible. So let us talk in detail about


The Rotors

to be continued. . .