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

D-Lev PWBs Are A Go

The PWBs arrived on Friday, 3 days early - JLCPCB makes an excellent product quite quickly and relatively painlessly, and DHL really shags ass with international shipping (though one pays semi-dearly for it!):

Boards (clockwise from lower left): main, large tuner, tiny tuner, 4x encoder, tuner driver, and (center) AFE.

I spent Friday evening examining the physical stuff like overall dimensions, hole locations, and parts footprints, and did some electrical buzzing out comparisons with Roger's boards.  Saturday I built up a main board bit by careful bit, checking functions as I went, until it was complete and fully functional (unsoldering is difficult, and you don't want to waste parts on a dud).  Then I built up both large and tiny tuners, a couple of tuner driver boards, and a two AFE boards, also checking things carefully in steps. 

Mounted them in printed white PETG panels:

The controls are hooked up to a tiny tuner and I haven't seen anything show-stopping so far; I hooked one AFE to a random coil and rod and it's behaving like it should.  TOSLINK out works, MIDI out works, the footswitch inputs work.  The only thing I haven't tested yet are the SPDIF outputs, but they're pretty trivial.

I've been holding off on the coil winding because it felt premature.  Indeed, yesterday I looked into 1-1/2" polypropylene sink drain pipe instead of 1" schedule 40 PVC (slightly larger OD) and it looks like it may be a better fit.  The relative permittivity of 1.5 is about as low as it gets (minimum self capacitance) and the weight is less as it's a thin wall pipe (any significant mass experiencing sudden deceleration can cause trouble, so the less the better) though the coefficient of thermal expansion is likely a bit higher (which is probably one of the most important factors for long-term stability / drift).  I wound a pitch side 1mH coil this morning and am currently testing it, so far so good:

So things are definitely moving in a directionally correct direction!

Wow! They’re beautiful! The tiny tuner also looks great. It appears as if you’ve spent the entire weekend working on this. It’s really amazing what you’ve accomplished. The blue LEDs also look good. I especially like that one of them has been placed in the center of the tuner (as it really serves a merely a reference). You’re really an artist who works with electronics as your medium! 

"The tiny tuner also looks great."  - ContraDude

I think I prefer it to the large tuner, but haven't had a chance to actually play it yet.  The white PETG gives a nice glow around the LEDs, and looks spiffy with the blue LEDs.  The 8mm LEDs in the tiny tuner are noticeably brighter than the 10mm LEDs in the larger tuner, and the 1/2" 7-segment display has better contrast than the 1" version, another reason to pick the tiny tuner.  I've got enough parts for both, so I guess it will be an option for the kit, at least the first ones.

"It appears as if you’ve spent the entire weekend working on this. It’s really amazing what you’ve accomplished."

Yah, I was rather tense about how the PWBs turned out, so just kept going on Saturday until they were all built and functionally tested (~1AM).  I haven't played a lick for a couple of weeks, getting rusty.  Most of my days lately are test printing and ordering stuff on Amazon, there are a million little trivial things to get in the supply chain pipe (heatshrink and lockwashers arrived today).

"The blue LEDs also look good. I especially like that one of them has been placed in the center of the tuner (as it really serves a merely a reference)."

That's Roger's idea, and it probably helps when playing in the total darkness as it gives orientation info.  It would be nice to have some optional minimum brightness to the the tuner ring for super dark situations.

A Tale Of Two Drain Pipes

Went to Home Despot yesterday to pick up more PP (polypropylene) 1-1/2" drain pipe for the coils.  They had tons of 12" extensions and I had an armful when I noticed PVC versions right next to them:

https://www.homedepot.com/p/Everbilt-1-1-2-in-x-12-in-White-Plastic-Solvent-Weld-Sink-Drain-Tailpiece-Extension-Tube-C9787A/205154042#overlay

OD and ID looked identical, but the PVC is much more rigid than the PP.  The PVC OD is quite shiny, relatively flawless, and has no markings to remove, and the wall thickness is much more consistent (and quite a bit thinner) than schedule 40 PVC.  The mass is a tad higher than PP.  Since (IIRC) self-capacitance is a function of the wall thickness, it probably doesn't matter all that much if we pick PP or PVC, and it will be getting a layer of heatshrink PVC on top anyway.  Structural and thermal integrity are likely more important for minimal drift.

So I grabbed the 3 that they had left in stock and will give them a wind-up.  If they work out this will be my go-to coil form.

I'm curious about the air coils and forgot the mH and Q. Despite they look delicious and remind about Lev's theremin, what is the purpose to use not smaller cross wound ones? 

"I'm curious about the air coils and forgot the mH and Q. Despite they look delicious and remind about Lev's theremin, what is the purpose to use not smaller cross wound ones?" - JPascal

I'm using 1mH on the pitch side and 2mH on the volume side.  Q when loaded with a rod or plate is around 100, limited more I think by the capacitance than the inductance.  Availability isn't an issue as you can wind a good coil on just about any plastic pipe, and the winder itself is pretty simple to make.

Smaller cross-wound ferrite chokes would probably give lower Q, and would have more temperature drift - and who do you buy them from?  About the only good thing about them is they are physically small, and I suppose they contain the magnetic field better.  But their SRF is usually too close to the operating point, so you usually have to use several in series if you don't wind your own.  And the winder is much more complex.

I just wound these on 1-1/2" thin-wall drain PVC:

Top one is 240 turns of 30AWG (1mH).  Bottom is ~360 turns of 32AWG (1.87mH, a bit shy of the target).  Took me about an hour total to do it all, including heat-shrink on the outside.  It's not exactly fun work, but it isn't terrible either.  For reference they're obviously 1-1/2" in diameter (38.1mm OD) and 100mm long.

To accommodate the slightly larger diameter I modified the coil & AFE box:

White one on the left is the original, army green in center and light gray on right are new.  All are 60mm wide by 100mm tall, the length has gone from 80mm to 90mm, note the extra corrugation in that direction in the new design.  I think I'm going to go with light gray for the kits.

Army green and light grey ones look like medium voltage transformers.
(ok, MV Transformer are much bigger but they also contain coils after all)

I like this modern approach... The technical heart of the machine.

"I like this modern approach... The technical heart of the machine."  - Mr_Dham

It's working out pretty well!  Today I mounted both coils and AFEs in their boxes and hooked it all up for the first time:

If you squint you can see that the volume box on the left is 10mm longer than the pitch box on the right.  I could sort of play it even with no antennas or ground.  Tomorrow I'll stick it all in a cardboard box.

Also worked on a (stinkin') badge type emblem for the outside:

OpenSCAD pulls in fonts really well and it wasn't too difficult adding the other stuff.  Top 3 are army green PETG, bottom two are white PLA.  I think PLA is the way to go here as it's more rigid and sands easier.  The green looks better than the photo, the white looks worse.  The gold and black applied is just a Sharpie, obviously real paint would look better and be more permanent.  I need to try white lettering on black, but the gold on green isn't too ugly.  I tried something called "ironing" in the Cura slicer (2nd one down) but it just blobbed everything up.

D-Lev Kit (After Dark)

(The red glow is from the FPGA board SMD LEDs, and that's a bag of black 3D printed knobs to the left.)

The 4th badge appears like blue on white rather than black on white. Is it just my screen ?

Other question: what are the dimensions of front panel and tuner ?