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

3D Printing Coil Forms

I didn't have plumbing handy for sarob's coils, and while it's not a big deal to run down to the Home Despot and grab some, I thought this might be a good opportunity to instead do a bit of experimentation.  So I whipped up a simple 38mm diameter cylinder in OpenSCAD, sliced it with 4 outer walls, no end walls, and no fill in Prusa slicer, and printed one 99mm tall and another 109mm tall.  

Winding the 99mm with 30AWG was a little tricky, I had to keep pushing the windings together to make them tight.  I realized what was fighting me here was the printing grooves associated with the printing layer height of 0.25mm vs the 30AWG single coat diameter of 0.28mm.  Anyway, that coil measured a tad over the target 1mH, which was fine. 

Winding the 109mm with 32AWG went considerably smoother, but at the end it measured 1.62mH instead of the expected 2mH - what's going on?  The OD of 32AWG single coat is 0.22mm, but that measured fine.  Taking a jeweler's loupe to the coil revealed tiny spaces between the turns - the wire was riding the printed layer grooves like a phonograph needle!

So I printed two more forms, the 99mm with 0.28mm layers, the 109mm with 0.22mm layers, and this time both coils wound pretty easy and measured correctly.  I spray satin varnished them, and when that was dry I applied the heat shrink tubing.  I was a little worried that the PETG, being a thermoplastic and all, might deform during the heating of the heatshrink, but it went fine.  I think the nail polish and varnish doping stick better to PETG than to ABS / PVC.

Above, left to right: (good) 1mH w/ 0.28mm layers, 2mH w/0.22mm layers; (ok) 1mH w/ 0.25mm layers; (bad) 1.62mH w/ 0.25mm layers (which I didn't bother to heatshrink).

The forms take on the order of 2 hours to print.  If I had a faster printer I'd probably go for a double wall with cubic fill, just to make things more rigid.  PLA would be more rigid than PETG and can be printed faster, so maybe I'll give that a go next time.  Printing is pretty inexpensive, each form is like 75 cents of plastic; plumbing costs somewhat more though it's not prohibitive, and it certainly is convenient to buy something more or less ready-made.  The printed layer grooves are a nice guide to get things going with the wind, but they fight you somewhat during the wind, so it's a bit of a mixed bag.  I've found that printing just about everything for the kit has been the best way to go: highly custom parts with no reliance on anything except the raw material.

Wrap It Up

High Q coils apparently require a certain distance between adjacent windings.  Scramble / basket winding can do that because it spaces out the windings, makes them somewhat non-parallel, and introduces gaps.  Wire insulation / varnish / coil dope has a higher dielectric constant than air / vacuum, but from our simple testing it maybe doesn't seem to be a huge issue?  So instead of a complex & difficult winding process, could we instead use wire with a thicker insulation?  If so, where might we buy super heavy build copper wire?

Wire wrap wire is one possibility:

Above: On the left is a spool I bought off Amazon recently, the copper wire inside measures 0.22mm in diameter (~AWG 31), the insulation 0.68mm in diameter, which I believe is PVC because it feels kinda soft, and I wouldn't recommend this wire.  On the right are some old spools I bought long ago at Radio Shack (RIP) with 0.26mm (AWG 30) wire / 0.45mm insulation, which I believe is Kynar, a trade name and a specific formulation of PVDF [LINK].

Kynar, being a harder insulation, would probably make for a more physically stable wind, and the somewhat thicker CU would give lower DCR and hopefully higher Q.  I see spools of 1000 feet (300m) of what they claim to be Kynar 30AWG selling for $40USD on eBay.  A 20mH coil would consume most of it, making for a rather expensive wind!