Introduction: Minivac 601 (Version 1.0) Motorized Rotary Switch

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!

This is the promised follow-up to my Minivac 601 Replica (Version 0.9) Instructable. This came together faster than anticipated and I'm pretty happy with the result. The Decimal Input-Output panel described here is a drop-in replacement for the manual version described in the Version 0.9 Instructable. As the title says it adds the motorized feature to the sixteen position rotary switch. As with the original Minivac 601 this is accomplished using a friction drive implementation. Here is a video of the Motorized Rotary Switch in action:

The new design begins with a more robust Rotary Switch using real bearings for a much smoother operation. One of the problems with the previous design was that the tolerances on the switch itself were pretty loose. As a result the motor required too much torque to turn the switch properly.

In addition with the new solenoid based design, the motor is only engaged when it is active. When not in use it is detached from the switch allowing for a great "feel" when operating the switch manually.


For this Instructable, in addition to the 3D printed parts you will need the following:

    • 1 4 mm Urethane Round Belting - about 300 mm required (Amazon)
    • 2 F684ZZ Double-Shielded Flanged Ball Bearings 4x9x4 mm (Amazon)
    • 1 12V Solenoid Uxcell a14032200ux0084 (Amazon)
    • 1 Yosoo Micro DC 12V Speed Reduction Motor (Amazon)
    • 2 M3 x 10 mm bolts with nuts
    • 8 M3 x 8 mm bolts with nuts
    • 2 M3 x 6 mm bolts
    • 4 M2 x 6 mm screws
    • 1 Small piece of rubber hose with ID of about 7 mm and OD 10 mm or so
    • 1 General purpose 12V DPDT signal relay - Digi-Key part number 399-11029-5-ND
    • 16 Reed Switches - Digi-Key part number 2010-1087-ND
    • 19 Disk Magnets - 6 mm (diameter) x 3 mm (height)
    • 1 85 mm length of 4 mm piano wire
    • 1 65 mm length of .8 mm piano wire

    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 and White for the panel, any color(s) can be used for the interior parts

    Notes: All parts were printed in PLA with no supports.The following parts are required for this Instructable:

    • 1 MV601 Decimal Input-Output Panel
    • 1 MV601 Friction Drive Motor Wheel
    • 1 MV601 Motor Mount
    • 1 RS Body
    • 1 RS Flywheel
    • 3 RS Gasket
    • 1 RS Knob
    • 1 RS Rotor
    • 1 RS Top

    Step 2: Prepare the Input-Output Panel

    Following the instructions from Step 3 of the Minivac 601 Replica (Version 0.9) Instructable, add the rivets and solder lugs to the Input-Output Panel.

    In addition from Step 6:

    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.

    Step 3: Prepare the Rotary Switch Flywheel, Knob, and Rotor

    Push six of the M3 nuts into the slots on the tops of the Flywheel and Rotor, and the bottom of the Knob. Screw the M3 x 8 mm bolts from the sides into these nuts till they just reach the shaft hole to act as set screws. At this point I used a 3D pen to fill in the slots with filament, holding the nuts in securely. You could accomplish the same thing with a bit of hot glue.

    Cut a length of the 4 mm Urethane Round Belting just a few mm shorter than the circumference of the flywheel. Using a candle, heat the ends of the cord until they just melt, then join the two ends together quickly. Hold the joined ends together for about 30-60 seconds while the plastic cools, trying to keep them as aligned as possible. There are many good YouTube videos available on how to do this. Stretch your new rubber "o-ring" over the flywheel into the groove around the edge as in the picture above.

    Insert two disk magnets into the bottom of the RS Rotor. It's very important that the polarity of the magnets that will be on the bottom of the RS Rotor disk is the opposite of the polarity of the magnets secured in the RS Body. In other words they should attract! Also insert a disk magnet into the hole on the side of the Rotor. Use a little glue if necessary to secure these magnets in place.

    Slide an 85 mm length of 4 mm piano wire into the shaft of the Rotor. Leave about 18 mm protruding from the bottom of the Rotor as pictured above. Tighten the set screws on the shaft. Do not over tighten.

    Step 4: Prepare the Rotary Switch Top

    From the inside of the Rotary Switch Top press one of the Flanged Ball Bearings into the center hole. If properly installed it should be flush on both the inside and outside of the RS Body.

    On the outside attach the 12V Solenoid to the Rotary Switch Top using two M3 x 6 mm bolts, the holes provided, and the threaded holes in the solenoid itself. See the picture above.

    Step 5: Prepare the Motor and Motor Mount

    Slide the Friction Drive Motor Wheel onto the shaft of the 12v Speed Reduction Motor. It should fit snugly. Once in place, cut a 9 mm length of suitably sized rubber hose and stretch it over the motor wheel just added. This should provide a lot of traction.

    Solder some wires onto the leads at the bottom of the motor. Be careful these are quite delicate.

    Using a couple of small zip ties, attach the motor to the Motor Mount in the slot provided as in the picture above.

    Step 6: Populate the Rotary Switch Body

    First push the other Flanged Ball Bearing into the center hole of the RS Body from the inside. If properly installed it should be flush on both the inside and outside of the RS Body. Mine fit snugly and didn't require and glue to stay in place.

    Insert the sixteen 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.

    Insert sixteen disk magnets into the RS Body. Be sure that the polarity of all sixteen 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 inside bottom of the RS Body when inserted.

    Step 7: Attach the RS Body to the Decimal Input-Output Panel

    Using four M3 x 8 mm bolts and nuts attach the RS Body to the back of the Input-Output panel as shown in the picture above.

    Step 8: Wire the Rotary Switch Onto the Decimal Input-Output Panel

    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 the picture 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.

    Step 9: Attach the Motor and Motor Mount

    Using the two M3 x 10 mm bolts and nuts attach the Motor and Motor Assembly to the back of the Digital Input-Output Panel. Use the picture above as a guide.

    Step 10: Install the Rotary Switch Rotor

    Add three Rotary Switch Gaskets onto the shaft on the bottom side of the Rotor. This will ensure the proper spacing between the magnets on the Body and the Rotor. Slide the Rotor and shaft into the bearing at the bottom of the Rotary Switch Body.

    Step 11: Attach the Rotary Switch Top

    Slide the Rotary Switch Top down the shaft and attach it to the Rotary Switch Body with four M2 Screws. Be sure that the Solenoid lines up with the Motor Mount.

    Step 12: Link the Solenoid and Motor Mount

    Use a piece of .8 mm piano wire to join the solenoid and the Motor Mount. As shown in the first picture above, the wire should be 35 mm on the long side and the shorter sides about 15 mm. Once installed bend the short ends of the wires to prevent them from slipping off the solenoid and Motor Mount. See the second picture.

    Step 13: Wire the Motor, Solenoid, and Relay

    Wire the motor, solenoid, and relay as in the picture above. The Solenoid leads (blue) are wired in parallel to the motor's (red and yellow). I just "dead bugged" the motor and solenoid through the normally closed switch of the relay (yellow, blue on one side white on the other to the RUN rivets (17)), then wired the relay coil to the STOP rivets (19) on the Decimal Input-Output Panel (orange). The motor, solenoid, and relay coil (red, blue, orange) all have a common lead wired to the shared rivets marked with the 18).

    Step 14: Testing

    Before you can attach the Rotary Switch Knob, you have to make sure that the knob's pointer will be aligned with the Rotary Switch Rotor magnet. I did this by connecting my multi-meter to the ARM and 0 points on the panel and turning the rotor until the circuit was closed. Slide the Rotary Switch Knob onto the 18 mm shaft till it's flush with the Decimal Input-Output's panel and tighten the set screws with the knob pointing at the 0.

    You should now be able to drop the new and improved motorized sixteen position rotary switch panel into the Minivac 601 frame. If you apply 12V power to the RUN terminals of the panel, the rotary switch should turn in one direction. Reverse the polarity of the power leads and the rotary switch should turn in the opposite direction.

    While powering the motor if you power the STOP leads, the motor should stop. See Experiments 12, 13, and 14 in the manual titled "Book 1 - Getting acquainted with the Minivac 601" for more details.

    NOTE: This method for stopping is slightly different than the "short circuit" method used in the original Minivac 601. The STOP here must be a proper powered circuit and not just a "wire" running from rivets 18 to 19.

    Step 15: Final Thoughts

    I went down a number of paths when trying to decide how best to "motorize" the Rotary Switch.

    My first (failed) design involved a DC motor connected to the Rotary Switch via pulleys. In fact the Flywheel from this Instructable was re purposed from that design.

    In one plan I considered using an Arduino and a motor controller with either a DC or stepper motor. The irony of using a microprocessor many orders of magnitude more powerful than the device I was trying to replicate was not lost on me.

    Another method involved a high torque slow speed DC motor with a gear and a solenoid based clutch mechanism.

    Ultimately I'm really glad that I was able to find a solution that was not only simpler than all of the above, but also more in line with the design of the original Minivac 601.