Solenoid Based Shutoff Switch

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About: Finishing up a long career in software development. Looking for new challenges.

I created this simple device as part of my larger Minivac 601 Replica project. The Minivac 601 was an electro-mechanical computer from 1961 (see the Wikipedia entry for details). This Instructable is also related to my Mostly 3D Printed Rotary Switch.

The Minivac 601 has a sixteen position rotary switch that was used for manual input and as an output device. You can see what the panel containing the switch looks like in the photo above. To accomplish the output function, the rotary switch was motor driven (something that I am working on right now with my replica). The motor is RUN by applying power through the eyelets labeled 17 and 18 using patch wires to create the circuit. The polarity of the wires determined which direction the rotor would turn.

There was also the ability to STOP the motor by creating a circuit between eyelets 18 an 19. On the original Minivac this was done by "shorting out" the motor. The circuit diagram above shows how this was accomplished.

For my implementation I went in a different direction, which led to this Instructable. I had a 12 V solenoid on hand as well as a few extra magnetic reed switches and magnets from the Rotary Switch project. The idea is simple. When the solenoid is not powered, a small magnet is positioned near the reed switch, enabling any circuit connected through the reed switch. When power is applied, the solenoid shaft retracts moving the magnet away from the reed switch and disabling the circuit. For the Minivac the motor will be wired through the reed switch to implement the STOP function.

Now I know I could have accomplished this any number of ways (with a relay for instance), but this approach:

  • makes good use of parts I had on hand,
  • feels in keeping with the overall Minivac vibe,
  • and most importantly was fun to do.

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

Both the solenoid mounting bracket and the magnet holder are in the following STL file:

  • Solenoid Bracket

Step 2: Obtain the Other Hardware

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

  • 1 Reed Switch - Digi-Key part number 2010-1087-ND
  • 1 Disk Magnet - 6 mm (diameter) x 3 mm (height)
  • 1 12V Solenoid - Amazon.ca - Uxcell a14032200ux0084
  • 2 M3 x 8mm bolts

Step 3: Assemble the Cutoff Switch

Assembly is pretty straight forward:

  1. Insert the reed switch onto the end plate of the Solenoid Bracket. The pins for the switch should pass easily through the holes from the inside to the outside. Carefully bend the pins a bit to hold the reed switch in place.
  2. Solder a couple of wires to the reed switch pins on the outside of the bracket. I used some hot glue to secure them in place.
  3. Use the cylindrical holder to attach the magnet to the end of the solenoid shaft.
  4. Attach the solenoid to the bracket with the two M3 bolts.

Step 4: Testing

Connect a multi-meter to the leads of the reed switch. The multi-meter should indicate that the circuit is closed (normally closed). Apply power to the solenoid. You should see the solenoid shaft retract and the multi-meter should show an open circuit.

Step 5: Final Thoughts

You might be wondering why the side walls of the Solenoid Bracket are different widths. The simple answer is that I only had 8mm M3 bolts on hand so I made one wall thicker than it had to be to accommodate this :-) Got to love the power of 3D printing.

The shutoff switch is now ready to perform the STOP functionality for my Minivac 601 project. Like with the Rotary Switch, this actual implementation is probably not as important as the idea behind it. I think that strategically placed magnets and switches can be a powerful tool for your designs.

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