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

Posted: 3/25/2024 12:12:30 PM

From: Northern NJ, USA

Joined: 2/17/2012


Here's the tcoil help screen:

Usage: TCOIL_EXE [ command ] [ -flag [option] -flag [option] ... ]


  [command] -h                                              Help with individual command flags
  help  [-v]                                                Command line help with optional examples

  wire [-awg gauge] [-wcd wire ID mm] [-b build]            Wire specs
  lcf [-l mH] [-c pF] [-f kHz] [-r Ohms]                    Given 2 of LCF, solve for the 3rd
  pvc [-i]                                                  PVC tubing table

  • ...                                            Solve brooks coil   solenoid
  • ...                                          Solve single layer solenoid   donut
  • ...                                             Solve multi layer donut   choke
  • ...                                             Solve multi donut choke   xfmr
  • ...                                              Solve transformer (given N, N2 turns only)     
  • [-file file] [-wd wire OD mm] [-wcd CU dia mm]           commands: choke, donut, solenoid, brooks, xfmr    [-f khz] [-c pF]                                         commands: choke, donut, solenoid, brooks    [-fd form dia mm]                                        commands: choke, donut, solenoid, xfmr    [-ow donut width turns]                                  commands: choke, donut, xfmr    [-oo donuts offset mm]                                   commands: choke, xfmr    [-o donuts]                                              commands: choke    [-wd2 mm] [-wcd2 mm] [-fd2 mm] [-ow2 turns] [-n2 turns]  commands: xfmr    [-l mH]                                                  Solve given L    [-n turns]                                               Solve given N turns    [-wl m]                                                  Solve given wire length

    And of course it got all munged up because the posting tool hates the greater and less than symbols, so I replaced them here with [].  The posting tool is giving me an error when I post, but the post itself happens OK.

    Posted: 3/28/2024 9:18:12 PM

    From: Northern NJ, USA

    Joined: 2/17/2012

    A Tale Of Three Coils

    Giving my coil design program something to do:

    Coil on the right is the usual 1 mH / 10.6 DCR single layer solenoid on 38 mm dia PVC plumbing in the D-Lev kit.
    Coil on the left is a 1 mH / 4.4 DCR Brooks on a PETG printed bobbin.
    Coil in the middle is a 1 mH / 10.4 DCR 16 donut choke with 15.6 turns per donut, on a PETG form with similar dimensions to the solenoid.

    All wound with Remington AWG 30 single coat wire.


    1. Free air resonance for the solenoid is 3.14 MHz, for the choke 2.46 MHz.  As expected, the Brooks coil is a total dud for Theremin field coil use - I couldn't even find its free air resonance.

    2. The choke clearly has somewhat lower Q than the solenoid, with the kit pitch plate of ~12 pF I measure Q=50 for the choke and Q=75 for the solenoid.  This is a really rough measurement that doesn't take into account 50 Ohm drive droop at resonance, the real Qs are almost certainly higher.  But the lower free air resonance is consistent with lower Q.

    3. I have some AWG 32 double coat that might improve the choke Q by reducing the proximity effect for a similar volume side choke.

    4. More donuts with fewer turns would also likely improve the choke Q.

    5. The Brooks is trivial to wind, and the choke is vastly easier to wind than the solenoid.  For the Brooks and choke you just blindly scramble it in.

    6. The coil design program really nailed all three coils in terms of mH and DCR - so yay! for that.

    7. Next I'll try more donuts with fewer turns each of the double coat wire and a 2 mH target.

    8. The groove going from one donut to the next needs to be spiral, and the OD of the coil form should probably be a bit higher than the windings to catch the wire in the groove.

    9. I should also do drift testing with the choke.  It has nail polish "dope" on it to keep the windings secure.

    Posted: 4/3/2024 7:33:10 PM

    From: Northern NJ, USA

    Joined: 2/17/2012

    Bobbin' For Apples

    Another coil former with an M12.5/1.25 pitch thread to join the 2 halves:

    Two of them 3D printed and wound with 180 turns of 32 AWG 2 coat to give 1 mH and 7.8 Ohms DCR:

    The inner winding diameter is 16mm, the outer is 40mm.  The gap between the faces is 1mm, or about 4 wires wide.  I'm seeing a Q of around 25 for these when connected to the kit pitch plate.  Just way too much in the way of DCR increase via proximity effect, and the self C probably isn't too hot either, though of course better than the Brooks variant (anything is better than the Brooks).  Placing them on top of one anther increases the inductance but lowers the Q, some spacing gives a trade-off here.  With no magnetic coupling the L is obviously doubled but the Q is essentially unchanged.

    Next step I think is litz wire, though I'm not incredibly hopeful.  I believe single layer solenoids are the epitome for Theremins - it's no mystery as to why Theremin himself chose them.

    Posted: 4/13/2024 12:52:42 PM
    Yngvox Moogsteen

    From: The Middle

    Joined: 9/23/2021

    Any plans on demonstrating the D-Lev in the Midwest?  I would love to be able to see it or maybe even try it out. 

    Posted: 4/14/2024 12:20:19 PM

    From: Northern NJ, USA

    Joined: 2/17/2012

    "Any plans on demonstrating the D-Lev in the Midwest?  I would love to be able to see it or maybe even try it out."  - Yngvox Moogsteen

    So sorry, no, I never get out to that part of the country.

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