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

Hi Rich,

It's all quite malleable at this point as the first prototypes will be hand wired, but of course one needs an initial goal for the build. LEDs rotating clockwise as the hand moves closer will be my first shot, if that doesn't seem sufficiently useful I'll try other approaches. I do appreciate your ideas along those lines.

My hope is that anyone using my Theremin for the first time will immediately understand how the tuner functions, and find it increasingly useful and so come to rely on it more and more as they play.  That's the goal, and if it ends up looking like a turd with a light inside I'll be OK with it as long as it works well ;-).  My initial Xilinx board used the short string of tiny LEDs on the protoboard as a tuner and I found even that fairly intuitive, so better arrangements should be able to enhance that.

With the tuner deviating somewhat from keyboard layout instruments (but not necessarily from non-keyboard layout instruments like strings) I find myself edging into the space alternative keyboard people operate in.  On the one hand one wants to make playing easier, but that is often traded-off by abandoning hard-won skills people have acquired on traditional instruments.  The industry takes no risks it isn't forced to so we end up with the same old tired input devices year in / year out, ironically right when we have the technology and the means to easily try brand new approaches to musical controllers.

Right now there's a bit of wiring to do, and a lot of software to write.  I've got a new PLL construct that I'm quite anxious to try out in hardware, particularly with the reciprocal tank - if it works well I'll likely use it for all the sensors.  But the software is ditch digging so I keep putting it off.  And the LCD interface is so ugly even a mother couldn't love it.

Dewster,

I can't wait to see and more importantly hear your creation!  Just remember the most important thing by far is that it sounds good and the response is fluid. Frankly all this other stuff - just nits that can be overcome with practice.

Rich 

"I can't wait to see and more importantly hear your creation!"

Your proximity to the casa del dewster laboratory of evil & laundry room may make you an early guinea pig!  ;-) 

Bring it on! I work for kibble.

The LED assortment and prototyping boards arrived from China today.  The assortment is 20 each of red, yellow, blue, white, and green, with 100 470 Ohm resistors in a separate pack that I wasn't expecting.  The eBay ad claims these are 5000 mcd but I seriously doubt that as the red ones aren't as bright as some red 1000 mcd LEDs I bought from Adafruit.  But the price was right, and shipping was pretty fast considering.

The yellow is kind of amber, and the forward voltage is almost the same as for the red (both about 1.98V) so I could probably use either for the octave digit.  The green and white are the brighest, so I'm thinking white for the center and key of C, with green for the sharps/flats.  This forms a "W" of white - coincidentally my last name starts with W, so god told me to do this!  :-)

Explorations In Max Q Space

Been thinking a lot more about LC stimulus / sense.  It seems most likely that the best way to do both is in as "unloaded" a fashion as possible (least power passively dissipated) and with as few extraneous resonances as possible (due to the square wave stimulus). 

So along these lines I've been looking more at Q, which is the resonance frequency divided by the width of the -3dB points.  I'm rather enamored with the Colpitts approach as the inductor is pretty much a ground at DC and thereabouts, shorting out 60Hz hum, and parasitic self-capacitance (Ctank) of the coil isn't a big issue.  Directly stimulating and sensing via a tiny capacitance gives low damping, and the capacitive voltage divider on the sense side is similarly easy on damping.

R2 / C4 give the opportunity to roll off unwanted HF content a bit without phase shifting too much (though I'd prefer more roll off).

Q of the coil and sim is profoundly influenced by the inductor parasitic DC resistance Rtank, so I wound a few coils today to investigate this in the flesh.

The coils, from top to bottom:

1. Single winding ~250uH of 34AWG on 0.5" Schedule 40 CPVC, 3.0Vp-p.

2. Single winding ~250uH of 30AWG on 0.5" Schedule 40 CPVC, 3.12Vp-p.

3. Counter-wound ~500uH of 26AWG on 1.5" Schedule 40 PVC, 2.92Vp-p.

I fully expected coil #3, utilizing one of the single coils, to have the highest resonance voltage, but indeed #2 is the best on the bench, though not by much.  From this it seems DCR isn't a huge deal with this circuit, so one can use as fine a wire as one desires with relative impunity.  30AWG is the easiest to work with, 26 is kind of heavy, 34 is kind of thin.  I'm thinking 32AWG might be optimal, but don't have any on hand.  0.75" or 1" PVC would probably also be more ideal (but I don't seem to have any left in the garage, need to make an order and hit the hardware store).

[EDIT] This exercise also taught me the value of "melt it off with solder" varnish insulation on magnet wire.  The insulation on the 26AWG and 30AWG wire had to be scraped off prior to soldering, which was a pain, and nicks can seriously weaken the wire.  155C wire seems to be meltable with solder and 200C not, so I'd recommend the 155C stuff for those who are planning to purchase.  I get mine from Tech Fixx at eBay.

The 32AWG wire arrived several days ago.  Wound a couple of coils with it:

The one on the left is the 34AWG shown in the previous post, the ones in the center and on the right are 32AWG.  3/4" PVC wall thickness (coil on right) is quite a bit thicker than 1/2" CPVC (the other two coils) and probably overkill for supporting fine wire like this.  Not a real wire or space savings either, so I'll probably stick to 1/2" CPVC.  32AWG is definitely easier to work with than 34AWG.

Great, dewster!

Do you have a logged data (dimensions, SRF etc.) for all your coils? How well they are consistent with the David Knight's formulas ?

Good question ILYA.  I record the dimensions & specs in my notebook, and measure the inductance and DCR after I wind them.  My function generator doesn't go high enough for me to measure the SRF of the smaller coils, something I should probably remedy somehow, but in a Colpitts grounded coil tank, SRF isn't very important.  The targets for the above coils were 250, 200, and 200 uH, so you can see they are somewhat off of the mark in terms of measurement.  I'm pretty careful to count the turns while winding - counting the linear length of wire used would be preferable but I don't know how one might do that accurately.  Not sure how accurate my LC meter is either with these small inductances.

Though of course I would prefer the coils to be spot on, it fortunately doesn't matter very much.  The operating point is non-critical, and the antenna static capacitance isn't an exact thing either.

================

Those looking for suitcase type enclosures to build Theremins and other electronic projects in might try the Seahorse brand.  I stumbled across this Pelican knock-off a couple of days ago, the SE models 120, 300, 520, 540, 710, 720, 920, and 1220 have panel mount flange molded in, screw holes and all, and you can purchase pre-cut waterproof aluminum panels.  There is a red model SE120 (internal dimensions 7.57" x 5.00" x 3.23") on Amazon for $11.76 with free shipping that I'll be ordering soon I just ordered to check them out:

The larger models seem just the ticket to build a suitcase PC in.

[EDIT] Synthrotek uses the above case (in yellow) for a small portable synth rack, which sports a detachable lid via removable hinge pins.

Sorry about the similar ~double post.  My first post went to moderation, and I got impatient so I tried a second time which was the charm.  I wonder what trips the auto moderation script?

The Seahorse case above arrived last week.  It's really nice, and indeed has mounting flanges and pre-drilled holes in both the case bottom and top.  If you open it fully and set it on its top it looks like it has the makings of a portable tiny PC (with a keyboard mounted in the top and the guts & screen in the bottom).

=========

Also am sampling CCTV camera mounts for the antenna holders.  Bought a couple off of eBay and they seem like they might work:

Would mount them horizontally left and right off of the central control box and affix the antennas on the ends.  The antennas won't be like traditional Theremin antennas, the pitch side will most likely be a rectangular plate positioned diagonally inside square plastic tubing, the volume side in who knows what (some kind of plastic container).  I'd mount the antennas in a way that would allow for them to be aligned in parallel with the collapsed shafts for portability.  The only thing I don't like about these is the camera mount ball end being made out of aluminum, the threads might break off kind of easy if the thing hits the floor.

I hope I end up using them - with all the prototype stuff I've bought and ended up not using I could have bought a new scope.  (Those looking for a scope should seriously consider the $399 Rigol DS1054Z.  There are hacks to unlock everything in there including doubling the bandwidth.  4 channels, 100MHz, wow!)

As for the central control box, it needs to have the screen and control area at 45 degrees or thereabouts, I'm pretty much giving up on finding anything pre-made that might work, though 6x9 wedge type speaker boxes are very close: