Let's Design and Build a (mostly) Digital Theremin!

Ender 3 Work

Contemplating the mass printing of somewhat larger coil forms, I replaced the stock 0.4mm diameter nozzle on my Ender 3 with a 1.0mm nozzle to see how that might work.  The larger test coil forms I printed and wound earlier utilized a 4 x 0.4mm pass wall to give a total 1.6mm wall thickness, which didn't seem sufficiently rigid, and particularly so without an added inside rib stiffener or two.  Two passes of 1.0mm for a total of 2.0mm wall thickness should be stiffer and print much faster too.  Another possibility is 3 x 0.8mm passes, which would give an even beefier wall thickness of 2.4mm.  We'll have to see where this ends up.

One fear with the larger nozzle was that the extruder heating capacity wouldn't be able to keep up with the required PETG filament volume demand, but this is offset by the small-ish layer z-step size matching the wire diameter (plus a tiny bit of oversize) in order to guide and facilitate the winding process.  I haven't printed a coil form with the 1.0mm nozzle yet, but I have printed a test piece and that worked out well (see below).  

Another fear was that the resulting print would somehow be too cosmetically rough to use.  The zig-zag lines on the first layer fill are definitely more obvious, as are the tiny voids where the fill meets the perimeter walls.  I think this could probably be worked on to some degree in order to make it look better, but I'm not sure at this point if I'd use the 1.0mm nozzle for aesthetically fine finished work.  Sharp exterior corners are obviously right out when using a 1.0mm nozzle.

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Previously, after printing the larger test coil forms with the stock 0.4mm nozzle, I noticed that printed end mating parts to the coil forms only fit when rotated a certain way.  So the Z axis was out of square with the X and/or Y axis.  To check this I designed a test jig and printed it with the 1.0mm nozzle:

Above: Z-axis test jig being checked with a transparent plastic drafting square.

The test jig features a 50mm diameter / 1mm tall round base, 2mm wide / 8mm tall bracing ribs, and a 10mm square / 75mm tall hollow tower.  There are cutouts in the base in the +X and +Y directions that allow the drafting square to rest on the table.  Since the pattern here is obvious, the axes didn't need to be labeled in any way.

It turns out that both the X and Y axes were out of square with the Z axis.  To remedy this I watched a couple of YouTube videos which didn't actually help very much.  So I took matters (literally) into my own hands by moving the Ender from the top of my toolbox over to a very flat work table top in order to operate on it. The fact that it rocked when placed on the table was a strong indication that it needed adjustment.  It's super important to be working on a very flat surface when doing the assembly, something they maybe don't emphasize in the instruction manual.

Above: My Ender 3, front view, sitting on my toolbox.  The plastic bag (pulled back here) helps to keep the heat inside uniform, which keeps thin-walled parts from warping, and larger parts from warping off the print surface.

Above: Side view.

I started by removing the control panel and the very top rail (4x hex cap head screws @ red arrows).  I removed the X-axis gantry and rail by sliding it all the way to the top.  I also unplugged the Z-axis limit switch cable.  This allowed me to check the squareness of the vertical Z-axis rails against the horizontal base rails using my drafting square.

It turns out that the left vertical rail was square to the base rail, but the right was noticeably tilted to the left (i.e. in the -Y direction).  So I removed the right side assembly by removing the two screws at the green arrows.  I loosened the two screws on the bottom (blue arrow) and inserted a piece of 0.05mm shim stock (the advantages of having once worked in a machine shop) on the left side of the joint (sorta visible to the right of the left green arrow) then re-tightened the screws, which squared things up.  The Z-axis rails were perpendicular to the base rails and work table at this point, so I reattached the right side assembly to the unit.

To square up the Z and Y axes, I loosened the green arrow screws on both sides, leaving them a bit snug, and then used a small machinist's ruler to measure the distance from both ends of the Y-axis rail (poking out to the left of the printing table in the side view) to the table.  I tapped the rail a bit to make the distances equal, then tightened up the green arrow screws to lock things.

To square up the X and Z axes I loosened up the two screws (not shown) on both ends of the X-axis rail, with the screws somewhat snug.  I also loosened up the X-axis belt tensioner and let the belt go slack.  Then I reinstalled the X-axis rail and used a longer machinist's ruler to measure from both ends of it down to the tops of the foot rails, tapping on the right side of the X-axis rail to make the distances equal, then I removed the rail and tightened the screws at both ends.  I double checked the distances for equality, then re-tensioned the X-axis belt.  It would have been great if Creality had provided access holes in the Z-axis rails for this, having to removing the X-axis rail in order to adjust it is a pain.

Finally, I replaced the very top rail (red arrows), checking with a ruler to make sure the top and bottom distances between the two vertical rails were equal, and also reattached the Z-axis limit switch cable (important!).  Printing another test jig showed the Z-axis squareness to be right on the money.

I checked the squareness of the X and Y axes by simply pushing the printing table all the way in and holding the square edges of a small plastic drafting template against the Y-axis rail and the cross support rail underneath.  It wasn't off by enough to bother with, but adjustment can be done by simply loosening the two screws that hold them together, with no other disassembly required.

For those curious as to what other mods I've done to my Ender 3:
1. Replaced the controller board: https://www.amazon.com/dp/B07TMX9WFW
2. Extruder direct drive: https://www.thingiverse.com/thing:3589452
3. Magnetic textured printing surface (a gift from Roger Hess).

The new controller board was necessitated due to the failure of the micro SD card slot - it made all the stepper motors dead silent!  The extruder direct drive is something you can easily print, and gets rid of the stupid Bowden tube (they're really awful).  Textured magnetic printing surfaces give you the best of all possible worlds: a beautiful face finish, excellent adherence during printing, and excellent release at the end.  

All of these things should have been stock IMO.

Rods vs. Tubes

Over on the FB group, Gladys Hulot (Hyrtis) asked the question as to whether there was any difference between solid and hollow antennas.  I've been telling folks seemingly forever that there is zero difference, so you might as well go with the lightest thing for portability reasons.  But I've never actually tested this extremely basic assertion of Theremin physics.

Scrounging in the garage, I found a section of copper plumbing pipe, OD=15.94mm, ID=14mm, Length=280mm.  For better electrical connection, I cleaned one end of the OD, as well as the inside with steel wool.  To simulate a solid rod I stuffed it full of aluminum foil.  I also found a bit longer section of plastic plumbing pipe with identical OD, and to this I applied a single layer of conductive aluminum tape 280mm long.  The tape increased the OD slightly to 16.15mm.  The tape didn't quite make it all the way around the diameter, there was a ~2.5mm gap, which shouldn't bother anything:

Above: Aluminum foil stuffing (top), copper pipe (middle), plastic pipe with aluminum tape (bottom). The plastic taps box is empty, and just there to keep the antennas from rolling around.

I grabbed a D-Lev kit control panel and volume coil / AFE box and plugged the USB power & communication dongle into my PC.  The rod / tube under test was placed on a plastic storage box, the volume box positioned on top of an empty plastic coil box.  The interconnect to the antenna was the kit standard banana plug with alligator clip:

Above: Test setup.  Two test subjects are shown here, but in actuality I only tested one thing at a time.

I queried the operating frequency using the librarian software, and then using my t-coil program I worked that backwards from that given the coil inductance of 2mH to get antenna capacitance:

ANTENNA             MHz        pF
=================   ========   =====
Al filled Cu pipe   1.065361   11.16
Hollow Cu pipe      1.066355   11.14
Plastic & Al pipe   1.063839   11.19

My suggestion always (if you aren't going with plates, which I believe are superior) is to go with the largest diameter you can stand (to maximize the surface area) and to keep the pre-bent lengths very roughly the same.  Aluminum tubing is probably best for portability.  I also always recommend the antennas be painted, or covered in heatshrink tubing, or otherwise somehow insulated in order to protect against ESD, as well as oscillator unlock if touched.

So, no meaningful difference whatsoever. - dewster

The result shows that the physics is correct. However, the value itself appears to be too high; this could apply to the coil capacity as well.

"However, the value itself appears to be too high; this could apply to the coil capacity as well."  - JPascal

I double checked the math and it looks OK to me.  The coil is 2mH, and 11pF (total including stray) seems in the ballpark for this antenna.

Which value appears too high to you?

"However, the value itself appears to be too high; this could apply to the coil capacity as well."  - JPascalI double checked the math and it looks OK to me.  The coil is 2mH, and 11pF (total including stray) seems in the ballpark for this antenna.Which value appears too high to you? - dewster

"I mean, the isolated rod antenna geometry described does not result in 11 pF against environment, so some of the capacitance in the resonant circuit must come from the coil or the wiring."  - JPascal

Ah, I see.  Well, the AFE presents approx. 1.5pF load for sensing, and the AFE PCB is positioned rather close to the 2mH coil.

I just tested a 2mH coil alone using my function generator and scope.  Free air resonance is around 2.3MHz, which means the coil has about 2.4pF of self C.  Connecting it to the AL stuffed CU pipe the resonance is about 1.23MHz, or 8.37pF total C.  So the pipe itself might be around 6pF, or roughly 1/2 of what was tallied up above.

Does that make more sense?  I've read that the inductance of the coil itself can change rather dramatically as resonance is approached, so all of these experiments are rather crude, first order affairs.

Does that make more sense? - dewster 
Yes, now everything fits good with a theoretical consideration. The formula for the capacitance of the insulated rod in the “universe” is:

With your geometrical parameters we would get 15.58 pF / 3.25 = 4.8 pF. 

Btw: Anodized aluminum is corrosion-resistant, scratch-resistant, and wear-resistant thanks to the hard, natural oxide layer created by the anodizing process. It is electrically insulating on the surface. And thus a good choice für theremin antennas. I use it for my theremins. In Germany it is available in hardware stores. One manufacturer is alfer aluminium GmbH. It has a matte sheen.

Hi JPascal,
How do you make the contact between the antenna and the AFE ?
I never tried aluminium for antennas.
I'am afraid of contact issues because of the insulating oxyde layer.

How do you make the contact between the antenna and the AFE ?- André

Good question. Regardless of the design, whether as a plug connection or a fixed screw connection, contact must always be established with the inner surface or via the cut surfaces with sufficient pressure. For example, with a threaded screw or via a spring tongue, as with an audio jack. Cinch or coaxial connectors with a ribbed metal outer surface are also suitable for certain inner diameters of the aluminum tubes. I tighten the pitch antenna with an internal threaded rod, and contact is made via washers.

Thanks JPascal. I agree with all of that but prefer to avoid aluminium.