Mostly 3D Printed Rotary Switch

I am currently working on a replica of the Minivac 601, an electro-mechanical computer from 1961 (see the Wikipedia entry for details). For this project I require a rotary switch with the following characteristics:

• 1P16T
• Continuous Rotation
• Non-Shorting
• Panel Mount
• Motorized

This last requirement is the kicker. Without having to drive the switch via a motor, a fair number of parts meet the remaining criteria. However I was unable to see how any of them could be easily motorized. Some are physically too small, and most require a fair amount of torque to switch between positions.

So I came up with the following 3D printable design that uses reed switches and magnets.

Step 1: Print the Parts

I printed the parts with the following settings:

Print Resolution: .2 mm

Infill: 20%

Filament: AMZ3D PLA

Notes: No supports

To make the basic rotary switch you need the following parts:

• RS Body
• RS Knob (optional)
• RS Magnet Mount
• RS Rotor Gasket (optional)
• RS Rotor

Step 2: Obtain the Other Hardware

In addition to the printed parts you will need the following:

• 16 Reed Switches - Digi-Key part number 2010-1087-ND
• 18 Disk Magnets - 6 mm (diameter) x 3 mm (height)

Step 3: Add the Reed Switches to the Body

Start by inserting the reed switches into the slots around the RS Body. The pins for the switches should pass easily through the holes from the inside to the outside of the body, and can be carefully bent from the outside to keep the switch in place. I tried cheaper glass encased reed switches for my first prototype and found them to be inconsistent when operating the switch

Step 4: Populate the Magnet Mount

Insert 16 disk magnets into the RS Magnet Mount. Be sure that the polarity of all 16 magnets is the same. You can use a bit of glue to hold them in if they don't grab sufficiently on their own. They should be flush with the top of the RS Magnet Mount when inserted.

Step 5: Add the Rotor Magnets

Insert the 2 remaining disk magnets into the RS Rotor. It's very important that the polarity of the magnet that will be on the bottom of the RS Rotor disk is the opposite of the polarity of the magnets secured in the RS Magnet Mount. In other words they should attract! Note that in the picture above the RS Rotor is upside down to show both the reed actuator magnet on the side of the disk and the one that will "lock" the rotor into one of sixteen positions.

Step 6: Install the Magnet Mount in the Body

Carefully slide the RS Magnet Mount to the bottom of the RS Body making sure that the small tab on the edge of the RS Magnet Mount is inserted into a grove on the inside bottom of the RS Body (below a reed switch) first and that the magnets are facing up. This will ensure the the magnets are lined up properly with the switches.

Step 7: Add the Rotor Gasket

Add the RS Rotor Gasket into the RS Body now. I think that it helps the switch rotate more easily.

Step 8: Add the Knob

For testing purposes you can temporarily attach the RS Knob centered onto the RS Rotor with two sided tape. Note that the point on the knob should align with the magnet on the side of the RS Rotor.

Step 9: Insert the Rotor Into the Body

Carefully slide the RS Rotor into the RS Body. The magnet on the RS Rotor disk should be facing down. As you slowly turn the knob you should be able to feel the switch snap to the 16 magnetic "detents". There is even a satisfying click as the magnets come together.

Step 10: Testing

Connect a multimeter to the leads of each reed switch. When the arrow is pointing at the connected reed switch the multimeter should indicate that the circuit is closed. Conversely if not pointed at the connected reed switch it should show an open circuit.

Step 11: Final Thoughts

So the switch is now ready for the next part of my Minivac 601 project. I'll be running a 4 mm shaft through the RS Rotor. One end of the shaft will come up from the bottom of the Decimal Input-Output panel seen above and be attached to the RS Knob for manual input. The other end will be attached to a low speed high torque motor via a couple of gears for automatic manipulation.

I'd be very surprised if anyone actually needed a rotary switch exactly like the one I have described here (unless of course they end up building a Minivac 601). I do hope however that the ideas outlined here will be generally useful for anyone with a similar need.

Step 12: Updates

May 18, 2019

As can be seen in the pictures above, the rotary switch has been added to the Decimal Input-Output panel for manual mode only. The motor to automatically drive the rotor is yet to be installed which is why the shaft is sticking up (down?) so far. I have posted new RS Rotor and RS Knob STL files that have a set screw incorporated (I used 3mm x 8mm screws) to attach them to the shaft (which is just a 1/8 inch welding rod):

• RS Knob With Set Screw.stl
• RS Rotor With Set Screw.stl

It works great! I'm very happy with the result. More to come.

Comments

[AndrewW370]

Hey, I’ve been following your projects. I’m a big fan of these retro computers. I have a suggestion about motorizing the switch. I think it will be next to impossible to synchronize a high torque motor with the switch assembly and it will lose track of each position over time. I’m also concerned that a geared down high torque motor will make it impossible to manually actuate the switch.

I think a solution would be a solenoid that pushes an arm that actuated the switch one step at a time. Like a very primitive mechanical stepper motor. Each pulse actuated the solenoid, which in turn steps the motor one position. Clock movements can provide inspiration for the mechanism and it should be possible to slide the mechanism from one axis to another to drive it forwards or backwards if necessary.

Good luck on this. It’s a great project.

cheers,
Andrew

[AndrewW370]

I just read the latest version of the switch and see you came to the same conclusion. The video demo of your version 1.0 Minivac looks awesome. Well done.

[megardi]

Sorry missed this post before I replied to the previous.

[megardi]

Thanks for taking the time to post Andrew. I did get the switch motorized and the solution did involve a solenoid just a little bit different implementation. Check out https://www.instructables.com/id/Minivac-601-Version-10-Motorized-Rotary-Switch/

Mike

[rebeltaz]

This is a really cool idea. When you finish the minivac, will you be sharing your model/project so that us mere mortals can recreate one as well?

[megardi]

As promised. Should have posted this sooner:

https://www.instructables.com/id/Minivac-601-Version-10-Motorized-Rotary-Switch/

[megardi]

Thanks! As with my other projects for sure I’ll be sharing. I don’t want these great machines to be forgotten.

[rnakamoto13]

This looks great!

Question: it looks like you managed to get some text on top of what looks like a 3D printed surface. How did you go about doing that?

[megardi]

The panel you are referring to was modeled in Fusion 360. The text tool was used to add the labels to the top of the panel, then they were extruded up .4 mm or so from the panel itself. Most single extruder 3D printers today (like the Prusa MK3) that I use allow you to add a pause in the print so that you can change filaments (M600 in gcode). I set a pause for the first layer with the text and switched from black to white filament. Works quite well. They are very durable. Hope this helps.

[Sajal chuttani]

Please can you tell me in detail what this minivac 601 does and what are you going to achieve with it
Please can you also tell me about your idea and inspiration of making this because i am fascinated by your idea
Thanks in advance

[megardi]

The Minivac 601 was a device for teaching computer concepts sold back in the early 60’s. You “program” it by connecting the various parts of the machine like lights, switches, and relays with wires. There were extensive instruction manuals teaching you about computer circuits and logic.
See the Wikipedia article for more details.

I’m am creating a replica of the Minivac 601 because they are becoming very hard to find and very expensive when you do (about $600 US on eBay). I want one and I think others might too. My other Instructables were done for the same reason.

In short I am trying to help preserve these wonderful devices.

[Sajal chuttani]

Best of luck for your awesome project
Dont forget to put it on instructables

[megardi]

Thank you. Keep an eye open for the Instructable in a couple of months.