Introduction: Minivac 601 Replica (Version 0.9)

About: A retired software developer, living in Waterloo, Ontario, Canada, who appreciates having the time to make whatever the heck he damn well feels like!

Created by information theory pioneer Claude Shannon as an educational toy for teaching digital circuits, the Minivac 601 Digital Computer Kit was billed as an electromechanical digital computer system. Produced by Scientific Development Corporation in the early 60's it sold for $85.00 (about $720 today).

Minivac 601 used electromechanical relays as logic switches as well as for very basic storage. Simple DPDT switches and SPDT push buttons made up the binary inputs, with lights to represent the outputs. A large motorized dial allowed the user to enter decimal or hexadecimal numbers, and to output numbers, or to act as a clock signal generator. For more information about the Minivac 601 here are some additional references:

The Instructable presented here is for a full size replica of that Minivac 601 from 1961. I have tried to remain as true to the original as possible given the technologies and resources available to me. I don't have a "vintage" unit so this replica has been constructed based on photos and from the original manuals that were available online. I have included these manuals in PDF format as part of this project. I brought these files to a local copy center and had them printed as the spiral bound booklets you can see above. I'm really happy with the results.

So how close is this replica?

Since "frame" for the original Minivac 601 was made from wood I feel that I have made a pretty reasonable reproduction. I'm not sure what the top panels were constructed from but this replica's are 3D printed. The original had a built in transformer and used mains power. I chose to use a common 12V "wall wart" for safety reasons replacing the "fuse" on the Power Panel with a 2.1 mm power jack.

Sourcing the electronic parts was pretty straight forward, however finding parts that exactly matched the look of the original was harder. I lucked out on the slider switches finding some at my local surplus store that seem pretty much identical. The lights I found at the same surplus store did not have the right look but I was able to 3D print "caps" for them that were more in line with the photographs. I ended up sourcing relays that don't look too bad, and could be made to match even better by removing the clear dust covers (I chose not to). The push buttons are considerably larger than the original's, but they are readily available and robust "arcade" style switches so I went with them. I designed the rotary switch specifically for this project so it gets pretty high marks for an authentic look. See my Mostly 3D Printed Rotary Switch Instructable for details.

The biggest gap, and the reason I am calling this Version 0.9, is that the motorized rotary switch functionality has not been implemented yet. I had a version that sort of worked but wasn't really reliable enough. So I'm going back to the drawing board as they say to fix that. Meanwhile the rotary switch works great in manual mode. So in the interim I have implemented a signaling mechanism to indicate when the motor is "engaged" and in what direction the rotor should be turned. For this version I am asking the operator to be the motor. In this way all of the "experiments" listed in the three manuals can be performed. When completed the motorized version will be a "drop-in" replacement for just the Digital Input-Output panel.

Step 1: Print the Parts

Print Resolution: .2 mm

Infill: 20%

Perimeters: 5 (All of the holes in the top panels should be very "robust" to support the soldering of parts.)

Filament: AMZ3D PLA in Black, White, and Red

Notes: All parts were printed in PLA with no supports. To create a Minivac 601 you will need to print the following parts:

  • 1 Main Panel - Depending on the size of your print bed you can print the Main Panel as 1, 2 or 4 pieces. Most will probably be printing 4 pieces: Bottom Left and Right, and Top Left and Right. Print in black. I set a pause at the 2.20 mm mark to change the filament to white in order print the panel text.
  • 1 Binary Output Strip - Print as 1 or 2 pieces. Print in blue, pause at 1.20 mm and switch to white for text.
  • 1 Secondary Storage Strip - Print as 1 or 2 pieces. Print in blue, pause at 1.20 mm and switch to white for text.
  • 1 Binary Input Strip - Print as 1 or 2 pieces. Print in Blue.
  • 1 Decimal Input-Output Panel - Print in black, pause at 2.20 mm and switch to white for text.
  • 1 Power Panel - Print in black, pause at 2.20 mm and switch to white for text.
  • 1 Power Panel Strip - Print in blue, pause at 1.20 mm and switch to white for text.
  • 13 Light Cover (optional) - I printed these in red to cap the lights that I sourced to make them look more like the original.
  • 6 Relay Base (optional) - I'm assuming that the relays that others source will be quite varied in their mounting configurations, so I just put rectangular holes in the Main Panel for relays and printed these inserts for the relays that I used. They should just snap in to the Main Panel.
  • 1 Motor Direction Indicators - Print in red and pause at 0.60 mm and switch to black text.

Post Printing: Install the blue strips into the appropriate slots on the various panels based on the pictures above. I used a small amount of Ultra Gel Super Glue to hold them in place. For the strips that have them, be sure that the rivet holes are lined up.

Step 2: Build the Box

I laser cut the console frame from a single 2 x 4 foot piece of 1/8 inch plywood. Attached you will find the cut file used. See the drawings above for the rough position of each piece within the frame. Reds are the tallest outside pieces, blues slightly shorter inside vertical supports, and yellows inside horizontal supports. Pieces were mostly glued in place with some 1/2 in square dowels and a few brad nails added for strength.

I used the printed panels from the previous step to determine the exact placement of the frame's support pieces. When done, the Main, Power, and Decimal Input-Output panels should fit into the frame and be well supported by the blue and yellow supports. You might have to sand the edges of some of the panels a bit to get them to fit, I did.

When finished I painted the console in a light blue color close to the original's.

Step 3: The Most Riveting Part of the Build

Sorry I couldn't resist! To build a Minivac 601 you need to add a lot of small rivets (388 by my count) to the front panels along with their corresponding solder lugs on the back. The rivets or eyelets provide a place to insert and connect the jumper wires used to create circuits, and the solder lugs allow you attach the rivets to the switches, lights, and such. Here are the parts that I used (with a few spares added to the count):

  • 400 Smooth Edge Lug Terminal Flat Connector - Digi-Key part number 36-4004-ND
  • 400 0.089" (2.26mm) Eyelets Brass, Tin Plated - Digi-Key part number 36-35-ND

They were not cheap, but I'm sure you can source them at a better price overseas if you are willing to wait on the shipping (I was too impatient). In addition you will need a tool to set the rivets. I purchased the following from Amazon (see the pictures above):

  • CRAFTMEmore Grommet Tool Eyelet Punch Setter Anvil and Hole Punch Cutter for Applying 0.08" (2 mm) & 0.12" (3 mm) Tiny Grommets

I made a jig to keep the panel I was working on level while I set the eyelet (see above). I have included the cut file for the jig which was made with 1/8 inch plywood and some scrap lumber for the risers. Cut two squares the same size, one with the hole included and one without, and glue them together. Add the risers to be level with the top of the anvil when it is seated in the hole.

To set a rivet push it through a hole from the front of the panel to the back. Place the small hole of a solder lug over the protruding rivet. Set the head of the rivet onto the center of the anvil (the panel back should be facing up). Insert the shaft of the rivet setter (called a hammer) into the rivet hole and when everything is lined up tap the rivet hammer sharply a few times with a small mallet. The solder lug should now be firmly attached to the panel and the rivet hole should be unobstructed. Repeat 387 more times. Trust me you will get very good at this!

Pictured above is the back of the Decimal Input-Output panel with all of it's rivets and lugs installed. See some of the following pictures in this Instructable to determine the optimal orientation of the solder lugs for the other panels.

Step 4: Procure the Parts

The main components used in this Minivac 601 build are as follows:

  • 13 12V Panel Mount Lights - I found these at a local surplus store for a couple of dollars each. They have a mounting diameter of about 15 mm.
  • 7 DPDT Panel Mount Slider Switches - Again found at the surplus store cheap. The mounting holes should be 28 mm apart center to center and they are mounted under the panels. Finding ones with red sliders to match the original was a bonus. I used M3 x 8 mm bolts with nuts to secure them in place.
  • 6 SPDT Panel Mount Push Button Switches - Digi-Key part number 1568-1476-ND (red).
  • 6 12V DPDT Relays - NTE Electronics R14-11D10-12 Series R14 General Purpose DC Relay from Amazon
  • 1 Panel Mount Power Jack - This should match the plug of whatever 12V 2A "wall wart" transformer you choose for this project.
  • 1 Rotary Switch - Use the Instructable Mostly 3D Printed Rotary Switch to create the rotary switch, but use the STL files provided here to do it. These files provide for a true 4 mm shaft (instead of 1/8 inch) and I have added set screws for both the Rotor and Knob parts.
  • 1 Shutoff Switch - Use the Instructable Solenoid Based Shutoff Switch to create the shutoff switch.

Step 5: Populate the Power Panel

Additional parts required:

  • 1 12v Voltage Regulator - Use a T7812 part rated at 2A or better.
  • 1 .33 uF Capacitor
  • 1 .1 uF Capacitor (optional)

Start by mounting the light, slider switch, and power jack onto the panel as in the first picture above. To power the Minivac 601 I used a 12V voltage regulator that I just "dead bugged" to the underside of the panel components. See the schematic and pictures above for details. When finished wiring the power supply you should be able to plug in the transformer and the panel light should come on. With a multi-meter test that you get a 12V reading from the + and - points on the blue power strip.

Notes on the Power Panel:

  • I built the power panel before I started using solder lugs, so the construction was a little different. In this case I "wrapped" the rivets with bare 22 AWG copper wire then set them in place with the anvil and hammer. For the Matrix this works pretty well but the wrapping process was pretty tedious. If I had it to do again I would use the solder lugs.
  • The power supply is connected to the + and - power strip points on the panel, In addition there is a small molex connector to attach the power strip points from the Main Panel. This allows all the panels to be easily removed for maintenance.
  • There is an extra pair of power wires taped off for now that I may use for the Rotary Switch motor for Version 1.0.

Step 6: Populate the Main Panel

Prepare the solder lugs by rotating each pair of lugs on the Main Panel towards each other (using the rivet as a pivot) until the large holes align with each other. Carefully bend the ends of the aligned lugs up a few degrees (small needle nose pliers work well for this). Use the photos above to determine the optimal orientation for each lug.

Mount the lights, relays, slider and push button switches onto the Main Panel as in the first picture above. The slider switches are attached with M3 x 8mm bolts and nuts. Using the above photos as a guide, carefully solder the mounted parts to the lugs using short lengths of 22 AWG hookup wire (I used solid core). The rivets for each part are pretty well labeled as to what they are supposed to do in case you are having difficult figuring out the hookups from the pictures.

Connect all the + power strip lugs together and all the - power strip lugs together leaving enough extra wire to attach them to the Power Panel. I used small molex connectors to make disassembly for maintenance easier if necessary.

WARNING: While soldering the lugs, plastic around the corresponding rivet will get quite soft. Try not to put any pressure on the lug in any direction while in the act of soldering a wire to the lug and for 10 seconds or so after. Try to minimize the time that heat is applied when soldering the wires to the lugs. Make sure that the solder joins the wire and both solder lugs.

Step 7: Populate the Decimal Input-Output Panel

NOTE: A new motorized version of the Rotary Switch has been published as an Instructable. Please use:

Minivac 601 (Version 1.0) Motorized Rotary Switch

instead of this step (unless you are happy with the simpler manual version presented here).

Additional parts required:

  • 2 5 mm LEDs - These were out of my junk box so I'm not sure what the voltage rating was.
  • 1 Resistor (optional) - I used a 3.1K Ohm resistor and it seems to work fine with the 12V supply.
  • 1 4 mm Shaft - I used piano wire. It needs to be at at least 45 mm long.

Make the Mostly 3D Printer Rotary Switch and the Solenoid Based Shutoff Switch Instructables before you start this panel. Also prepare the solder lugs as in the previous step.

Glue the Rotary Switch Body to the back of the Decimal Input-Output Panel being careful to line up the hole in the bottom of the Body with the center hole of the Panel. Also be sure that the reed switches line up precisely between the 16 pairs of solder lugs around the circumference of the switch.

Glue the Shutoff Switch to the back as well as in the second picture above.

Drill two additional 5 mm holes in the Decimal Input-Output Panel just below the ARM text. They should be 14 mm apart center to center and line up with the circular arrows on the Motor Direction Indicators plate. "Dead bug" wire the LEDs together based on the circuit above making sure that the centers of the LEDs are 14 mm apart and glue them to the back of the Panel in the holes that you just drilled. They should only be set part way into the holes. Glue the Motor Direction Indicators plate to front of the Panel as in the picture above covering up the holes.

Wire in the Rotary Switch. First strip enough insulation from a 22 AWG solid core wire so that the bare copper wraps completely around the Rotary Switch Body and there is at least 3 inches of insulated wire left attached. Carefully solder the bare wire to the bottom leads of all 16 reed switches joining them together. You should start and end in the position shown by the yellow wire in picture 3 above so that the wire can be attached to the ARM solder lug of the panel. With short lengths of 22 AWG wire connect the top lead from each reed switch to it's corresponding solder lug (green wires above). These connections require a bit of a delicate touch so as not to melt the plastic.

Wire the Cutoff Switch and the LEDs for the Motion Direction Indicators based on the photos.

Push M3 nuts into the slots in bottom of the Rotary Switch Knob and top of the Rotary Switch Rotor. Screw four M3 x 8 mm bolts from the sides into these nuts till they just reach the shaft hole to act as set screws. Take the 4 mm shaft and attach the Rotary Switch Knob to one end using the set screws. Slide the shaft with Knob attached from the top of the Decimal Input-Output Panel through the center hole till the Knob is about 2 mm above the panel. From the back side of the panel, slide the Rotor down the shaft into the Rotary Switch Body as far as it will go but not too firmly. Line up the Knob so that it points in the same direction as the bottom magnet of the Rotor then tighten the Rotor set screws, The Rotary Switch should turn freely with "stops" at each of the 16 numbers.

Step 8: Final Assembly

Attach the populated panels to the console frame. I used Velcro strips to hold them in place so that I could pop them out to do maintenance if required. Run wires from the Power Panel to the other locations on the blue power strip. I drilled some holes through the vertical supports to run the wires. Pictured above is the cable that I used to do this using Molex connectors. Power up the Minivac 601 and use a multi-meter to test that all of the + and - points on the blue power strip register as 12V.

Step 9: Make the Patch Cables

You "program" the Minivac 601 by plugging wires into the rivet points for the various components thus creating circuits. To create these wire connectors you will need the following parts:

  • 100 22-16 Gauge Butt End Connectors - Hilitchi 100pcs 22-16 Gauge Butt Insulated Splice Terminals Electrical Wire Crimp Connectors from Amazon
  • 100 Taper Pins - Spaenaur part number 239-497
  • 75 feet (or so) of 20 AWG Hookup Wire

Also required is a crimping tool for the connectors. I purchased the following:

  • Titan 11477 Ratcheting Wire Terminal Crimper from Amazon

I created patch cables in 3 lengths, 8, 16, and 24 inches. I used 3 different colors yellow, red, and blue to make identification easier and because the original did so as well. I made 20 each of the 8 (yellow) and 16 (red) inch lengths, and 10 of the 24 (blue) inch wires. You may need more for some of the more advanced experiments.

Use the butt end connectors and attach a taper pin to the ends of each wire using the crimping tool. The larger diameter end of the taper pin gets inserted into the connector. See the picture above. I found that I got a more solid connection with the rivets if I only inserted the taper pin about 1/2 way into the butt connector before crimping (exposing a slightly larger diameter part of the taper). This didn't seem to affect the quality of the crimp.

Step 10: Testing

You should now have everything that you need to test drive the Minivac 601. I would suggest that you start by powering up your unit and working through the "experiments" spread throughout the 3 excellent manuals. Despite the large number of connections that make up a Minivac 601 it is still a pretty simple device. Aside from the occasional bad solder joint there is not a lot that can go wrong.

In the picture above, a variant of Experiment 9, the Minivac's relay 2 is "programmed" to act as a latch. Tapping the 2nd push button will enable the relay and turn on light 2. The light will stay on even after the button has been released. Pressing the 3rd button will release the relay and the light will turn off.

Step 11: Final Thoughts

The projects that I have been working on lately all follow the same pattern:

  • They are replicas of cool computer toys and devices from the 60's.
  • Have tremendous educational value.
  • Feature unique and noteworthy designs.
  • Due to their age they have become rare and thus expensive and hard to get.
  • And perhaps most importantly they and their designers are worthy to be remembered and honored.

The Minivac 601 checks all of these boxes and then some. Between the Minivac 601 and a subsequent product the Minivac 6010 which was sold into corporations, scores of people learned about digital circuits and computer concepts.

Truly a very cool device worth remembering.