Tuesday, April 4, 2017

The MeinEnigma Enigma Machine Kit - Assembly



In this post I'll continue the discussion of the MeinEnigma kit, this time describing my experience in building the kit.

It should be noted that this was for a beta version of the kit. Some changes to the construction method, assembly procedure, and physical parts are planned to be made before the kit goes into general release.

I was the first customer for a beta release of the kit; my unit was serial number 16. As I have a lot of experience building kits and live in the same city (Ottawa, Canada) as the developer, I was a good candidate to try out an early kit and give feedback on it.

This was the hybrid version of the kit, with all through-hole components with the exception of three SMT ICs that need to be soldered.


The kit came in a box with a nice printed label. All parts were included with the exception of batteries.

The PCBs are double-sided with plated-through holes, soldermask, and silkscreen on both sides. The design is somewhat unique in that there are pads for both SMT and through-hole parts.


All the parts come organized in small bags with identifiers on them that match the instructions in the assembly manual. This goes a long way in minimizing any errors in components or needing to identify them.

I made an inventory against the provided parts list and confirmed that all parts were there. The parts are grouped into five sub-assemblies that are built separately (corresponding in most cases to different circuit boards). I separated the parts for each assembly.


One of the reasons this was still considered a beta kit was that the assembly manual was not yet complete. It was in pretty good shape though, with detailed instructions and pictures for all the steps of the construction.

Lamp and Key Board


Construction started with the lamp and key board, the largest PCB but one without too many parts - mostly the 26 keyboard keys and 26 LEDs.

The first step was to install the HT16K33 IC, a 28-pin SMT device. I was a little anxious about this as I have only limited experience with soldering SMT devices by hand. Before installing it I refreshed my memory of SMT soldering techniques by watching some YouTube videos and doing some practice with some SMT parts on a piece of bare copper PCB.

As well as using a temperature controlled soldering iron and small diameter solder, I used some flux paste, applying it to the pads on the board. The initial alignment of the IC is critical. You solder one corner pin, get the chip correctly aligned, then solder the opposite corner. It is important not to proceed with soldering the remaining pins until it is correctly aligned. Soldering the remaining pins should then be quite easy. In the past I have done SMT soldering without paste; I believe the solder paste really helped the solder flow and avoid solder bridges between pins.


I used a digital multimeter in continuity test mode to verify that all the IC pins were making contact with the board and that there were no shorts between pins.

The assembly continued with two diodes and 13 resistors. These were through-hole parts. The SMT version should also be straightforward to solder. Often the biggest challenge with SMT is being able to identify the parts and avoid dropping and losing them.


Next there are 26 keyboard switches and 26 LEDs to install, and then some connectors. The last step is to install the four standoff feet and then assembly of this board is complete.

Encoder Boards


The four wheels, known as rotors, use rotary encoders. Each encoder is installed on a small custom PCB which is later connected to the main board using a four pin connector and a plastic bracket. Each board needs to have the encoder and connector soldered in. They are then set aside until final assembly. This step went smoothly.

Main Board


The main board has the most components. It starts with some through-hole components including fuses, a diode, and resistors. Two DIP ICs are installed using sockets. There are a number of connectors to install, and then an Arduino Nano (or equivalent clone) which is mounted on headers so it can be removed if needed. A sound board is installed, then the DC-DC boost convertor which is on a small PCB.


Other parts installed are a large filter capacitor, four keyswitches, a small speaker and piezo buzzer, the "big red switch", rotary function switch, and the four alphanumeric LED displays.

Construction wraps up with the battery holder and real-time clock module.


You can now complete the mechanical assembly of the main board by installing the standoff feet and assemble the rotors by inserting the wooden disks on the encoders, and then installing each of the four rotors to the main board using a plastic bracket.


Incidently, the four rotors are made from wood and laser cut. This is one of the features that makes the kit a more realistic replica of the original Enigma than some designs that just use buttons.

At this point the two PCBS can be connected using a ribbon cable and the system can be tested. Before doing so, I carefully inspected everything for opens, shorts, and incorrect components or orientation.

Plugboard and Cables


There is still one PCB to be assembled, the plugboard.

It has two more SMT ICs to solder. I wasn't entirely pleased with the job I did on the first IC on the lamp/key board, but I must have improved with practice because these ICs were better aligned. I did have to reflow the solder on some of the pins on the SMT ICs to ensure all were making good connections.

This board also has 26 suppressor diodes, a connector, and 26 banana jacks to install (the latter do not need to be soldered).


The plugboard also includes 10 patch cables. These are assembled by cutting the wire into 30 cm lengths and installing a banana jack on each end. The cloth-covered wire that is supplied is a nice touch and looks similar to the wiring typically used in a real 1940s era Enigma machine.

Final Assembly and Testing


The plugboard physically connects to the lamp/key board using some brackets and electrically connects with a small ribbon cable. This completes the assembly. You are now ready to install batteries and test the unit. The Arduino comes preprogrammed with the software, as well as the microSD card containing the sound files for audio output.

In general, I found that assembly process was straightforward despite the assembly manual not yet being complete.


There was one issue with the LEDs. It seems in the last batch of parts that were received, the manufacturer changed the orientation of the LEDs so that they don't follow the standard convention as indicated by the longer and shorter leads and flat side on the component.

Fortunately it didn't take long to figure this out, and it was relatively easy to unsolder and reinstall the LEDs with the correct orientation. I also had one bad LED which did not work at all.

It seems that there could be a reasonably high failure rate with these LEDs, so Peter Sjoberg will be individually testing all the LEDs shipped in the kits to avoid any problems (as well as correcting the assembly procedure).

Summary


Building this kit was a lot of fun. I took my time, and built it in three sessions of a few hours each, over a weekend.

The kit should be buildable by anyone with a moderate experience in soldering. If you aren't experienced with SMT soldering you could opt for the through-hole version of kit where the SMT parts are already installed.

In a future blog post I'll go over the operation of the MeinEnigma, its features, controls, and examples of how to encrypt and decrypt messages.

References

  1. MeinEnigma website: http://meinenigma.com
  2. MeinEnigma YouTube channel: https://www.youtube.com/channel/UC3s44doR_q1fKQ4ZMUeAlHQ

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