Super Simple Design Idea

I use shellac, which is alcohol based and thus dissolves the wax a bit.     I typically mask off the areas not covered with the windings as to keep things tidy. 

As for the heat treatment- at best, I dont think the wire is expanding that much.  maybe just a bit to keep the windings snug. 

"... and the first example they give.. ?  ... Flashing a single LED!"  - FredM

I know what you mean.  Every time I get a new demo board I'm shocked at how little creativity goes into the physical "elevator pitch".  Even if only given 4 LEDs one can do quite a lot, particularly if PWM is employed.  I used the bouncing ball thing first in a CPLD for a piece of telecom equipment during the development phase (to indicate the processor hadn't taken control of the indicators yet) and no one told me to get rid of it so it must have made it to production that way (if it indeed made it to production, very little of what we worked on actually did).  One coworker consulted with me to get PWM working on his brother's parade entry (an "ant car" with large LED backlit ant cutouts covering a car).

"ps - thats a neat little board.. are those 0.1" headers on the underside? can the board simply plug into PCB mounted recepticle (sockets) on a board below it?"

Yes, it is meant to plug into a larger motherboard of sorts, with an ecosystem of other stuff (but unfortunately no 16 bit D/A board):

"Expanding copper will increase the wire length, which will (?) increase the coils inductance but I  dont see how expansion will change the effective former diameter (but here is where I am most likely to be wrong I think) as the expansion will occur on all axis, so as the wire lengthens it will also be getting thicker.. If the wire expands to a greater degree than the former, it will (I imagine) pack more tightly against the former as it heats up (?).. In which case it would be better to cool the wire (and former) when winding it (?)"  - FredM

I believe heating the wire (and not the coil former) should make it grip the former better when the wire cools, provided the coil former has a lower thermal expansion coefficient.

Here is my reasoning:

Imagine a ring made of copper, with thickness T and diameter D.  Now imagine we remove a section of the ring that is T wide forming a slug.  If we heat this slug it will grow by delta T in all directions.  This will increase the outer diameter of the ring by delta T, and will reduce the inner diameter by delta T.  But the ring is formed by a number of such slugs, the quantity of which is given by S.  From the math above I believe the overall increase in diameter will swamp the inner diameter decrease, particularly when the wire diameter is small compared to the former diameter.

[EDIT] I know the opposite (inverse? negative? contrapositive?) of this works from my years working in a machine shop.  We would super cool copper bushings (rings) that were made slightly oversize and stick them into bores in aluminum blocks.  When the temperature equalized they were really stuck together.

"Then I also wonder about the effect of bonding the winding to the former - If tight winding is beneficial, then why wouldnt rigid bonding of the wire with a hard adhesive like cyanoacrylate or a hard epoxy be just as effective?"

I don't know.  But binding agents increase the relative permittivity, which decreases the self-resonant frequency.  Probably not a huge deal on a single layer inductor.  If the inductor is a transformer there will be some kind of insulation layer between the primary and secondary, which will also increase relative permittivity.

Thanks for that Dewster - I (sort of) see the reasoning with regard to a section of the ring (or nearly equivalent single turn).. But its this latter "nearly" which still bugs me a bit.. For a "closed" ring, I see it.. Expansion on all axis will cause compression..

But for a coil I still find the whole process difficult to visualise - ok - if we boil it down to actual numbers (I think copper has a coefficient of 16.6 ppm / K) the 'real' length / width increases are incredibly tiny.. but.. with a coil, I would expect the expansion along its length to be somewhat determined by the freedom of movement available to the wire (?) - Oh hell - I dont know!  ;-)

I almost wish I hadnt brought this subject up, LOL ;-) ! ... All I wanted to do was to rectify my rash and incorrect statement about the pointlessness of winding coils - as I had proved (at least to myself) that this effort could be justified for high-end theremins - that it is at least one way to reduce thermal drift..

Now im rummaging through my notes and trying to find what I recorded about my haphazard experiments - and even looking at my fridge and ATE and wondering if I can set this up under my bench somewhere so that I can run (and record) the tests again without disrupting my present diversion.. LOL ;-)..

But no - It will have to wait till I am in my new lab away from my present insane hostile environment..

In terms of my present direction, the thermal stability related to coils is only of minor importance - the 6300 series of inductors and 42IF106 are more than adequate, as I am generating a DC control voltage from the front-end, this voltage is then fed to the required linearizing analogue circuitry, and temperature compensation is applied to this from thermal sensors at the coils - or alternatively the whole front end is encapsulated in an "oven"... The "oven" is by far the most stable solution, but its a bit power hungry - the linear temperature / inductance relationship of air inductors does make analogue correction easier (if its needed) - but I am fairly sure that I will be staying with the 6300/IFT  ferrites as only minor thermal correction is needed with these and errors in the correction due to their non-linear temperature relationship seems to be extremely marginal.. and a single 20 turn preset is all I need to trim out the major thermal error component... A lot simpler than using compensating capacitors!

That said - I do still want to crack the coil issue.. I still hope to build a low cost standard "lev clone" theremin one day, and I think this will need air coils.. In terms of cost, even with air coils, I think the original Lev designs implemented using transistors (BJT and JFET) will give the most cost effective good quality basic theremin - oh, it wont do anything fancy - maximum 5 octaves, no register switching, and a few "tone" controls to adjust drive to the mixers "virtual tube" (made from opto-coupled FETS), and a coupling control and a control to feed the audio back as a FM modulation signal..

I said: "ps - thats a neat little board.. are those 0.1" headers on the underside? can the board simply plug into PCB mounted recepticle (sockets) on a board below it?"

Dewster replied: "Yes, it is meant to plug into a larger motherboard of sorts, with an ecosystem of other stuff (but unfortunately no 16 bit D/A board):"

I do like it! One of the big problems IMO with most DKs, is that once you have the DK doing what you want (often using only a tiny fraction of the on-board stuff that comes with it) you must design a board incoroprating the essential bits you need (CPU/CPLD/FPGA, regulators, capacitors, crystal etc) which is inevitably SMD, then add all the off-board stuff from your prototype.. This little board you are using neatly overcomes that 'problem' - you can just develop your external hardware and put this onto a board the "DK heart" plugs into.. No major up-front investement or SMD hassle.. and when your volume increases and you can justify making a reasonably large batch of boards, you can then just copy the DK board..

I was a little unfair in my comments about stupid DK demos in my last post... Cypress did one extremely good demo programs for their extremely low cost PSoC 3 "first touch" board.. But the "first touch" boards are, in my expierience, a big exception to the rule ...

Fred, that is indeed a very good demo of the PSoC 3 board!

"But for a coil I still find the whole process difficult to visualise - ok - if we boil it down to actual numbers (I think copper has a coefficient of 16.6 ppm / K) the 'real' length / width increases are incredibly tiny.. but.. with a coil, I would expect the expansion along its length to be somewhat determined by the freedom of movement available to the wire (?) - Oh hell - I dont know!  ;-)"  - FredM

It does kind of come down to freedom of movement. [EDIT] I guess what I mean is that you are certainly correct in thinking there are two competing effects at work, but the one that shrinks the coil diameter with increasing temperature isn't the dominant one when it comes to the average inductor.

Instead of a ring, it's probably easier to think of heating a long thin wire that is S in diameter and 1000 S in length.  The wire diameter will only expand delta S whereas the wire length will expand 1000 delta S.  Connect the ends together to form a ring, and the (+) ring inner diameter change due to the wire length change swamps any (-) ring inner diameter change due to the wire thickness change.

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I'm updating my Theremin design spreadsheet a bit, mainly cosmetic and text edits (though linearity is calling again). 

Playing around with the air core single layer inductor calculator worksheet, maximal inductance for a given wire length (giving the lowest DC resistance, highest Q, and lowest cost in terms of wire and possibly labor) favors squat coils, with a broad peak where the coil height = ~0.4 times the coil diameter.  Minimum self capacitance favors tallish coils with a similarly broad peak where the coil diameter = ~0.6 the coil height.  Given this, I believe Theremin was endeavoring to minimize self capacitance in the pitch side EQ coil by making it tall.

Kind of weird how all this stuff just falls out when examining the basic physics.  Theremin really knew what he was doing.

"Instead of a ring, it's probably easier to think of heating a long thin wire that is S in diameter and 1000 S in length.  The wire diameter will only expand delta S whereas the wire length will expand 1000 delta S.  Connect the ends together to form a ring, and the (+) ring inner diameter change due to the wire length change swamps any (-) ring inner diameter change due to the wire thickness change." - Dewster

Agreed.

What follows might be utter baloney - I am just floating my incomprehension or doubts.. So not only is everythin that follows "IMO" - its "This is probably just nonsense!" (TIPJN)  ;-()

My "comprehension" has no difficulty when thinking about a length which has no constraints on its movement / expansion.

however, if one has a straight length of 1000*S, and rigidly fixes this at intervals of say 100*S so that the movement is constrained at these points, one will (?) get a degree of 'buckeling' or distortion in each constrained section? - if one takes a 3d perspective on this, and simultaniously constrains movement on any axis, the distortion will be forced into any free space available..

My imagination / visualization of a coil rigidly bonded to a former leads me to "see" the "packing" of the windings becoming tighter as a temperature increases.. Its almost, I think (?) an inverse of the way you have presented for a straight unconstrained length.. The resistance to change in length will (?) be 1000*S if one compares this against bonding resistance to expansion of S.. the cumulative bonding strength (and therefore resistance to length-wise change) will I think cause the actual change in inductance to be much lower than one would compute if one took the thermal expansion of a straight unconstrained length and converted this to increase in turns.

One cannot stop expansion - the thermal effects must present in some mechanical manner.. But my mind cannot construct a simple picture of what goes on with a coil... And it certainly cannot see the expansion resulting in a tidy proportional linear increase in coil length /number of turns/inductance.. I can see this as more likely if one lubricated the winding and the expansion was free to 'distribute' without distortion over the entire length (although, even here - what happens to the expanded wire at the ends, and whether this increases the effective turns, is I think going to be determined by the construction.. If the pick-up points for the starting / ending winding are placed so that increase in length does not translate into increased turns, for example)

There are other possible thermal factors though.. Fortunately these are, I think, a lot less complex - One of these factors is the change in conductivity due to change in temperature - this impacts Q, and any variation in Q is highly significant if one is using a fixed or sequenced frequency capacitive sensing scheme as I am doing. Fortunately, the change in resistance is entirely predictable and repeatable, so simple monitoring of temperature allows easy correction of these errors..

But I do wonder whether change in coil resistance could significantly affect the Q or operating frequency of conventional theremin front-ends.

Not that I really think any of the above is likely to be important if well designed and constructed air coils are used.. AFAICS even a worst-case air coil will beat every available ferrite cored coil when it comes to thermal stability.

Fred