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

"The varnish brand is Luxens. Clear satin finish. Dries completely in 2 hours.
It contains acetone and n-butl acetate.
It is said to have a good adherence on wood, metals and PVC.
It can be cleaned with acetone
It's also said to have a low toxicity."  - André

They don't sell that brand here in the US?  I've got some spray matte clear enamel from Rust-oleum that has acetone and n-butyl actetate in it.  May give this experiment a go on my end too.

Here are the results.
No significant effect on inductance value.

I will make all remaining coils with varnish and shrink tube.

"Here are the results.  No significant effect on inductance value."  - André

My feeling is that the PVC former is the main dielectric the windings "see" due to its bulk and location within the electric field, so the varnish and heatshrink tubing contribute little to this.  I'll try something a bit different here, and try to measure any change in free air resonance of a couple of my coils (which are headed out the door shortly).  I may also check to see if start-up drift is affected.

Thanks for taking the time to perform this "basic experiment" André, I do appreciate it!

The Varnished Truth

I performed the same experiment as André, but I measured free air resonance instead of inductance, hoping to detect any changes in self capacitance.  Test subject was an unwitting 1.7mH coil that was accidentally wound with double coat magnet wire, so no loss if it got messed up.  Varnish was "Rust-oleum Matte Clear Enamel" in a spray can.  It contains acetone, liquefied petroleum gas, n-butyl acetate, dimethyl carbonate, petroleum distillates and aromatic hydrocarbons.  I think I may opt for the "satin" version on my next coil.  Drove one end of the coil with an FY6900 function generator, 3V sine wave.  Sensed with a test lead placed a few inches away from the other end of the coil that went to my scope, with 256 sample averaging to reduce the noise.  Coil was on top of two (count 'em) empty plastic boxes to minimize stray environmental capacitance.

Free air resonance:
- 2.554 MHz bare
- 2.557 MHz with varnish
- 2.546 MHz with heatshrink
- 2.546 MHz with varnish & heatshrink

These numbers are so close and so dependent on the setup that I'm reluctant to squint too hard to see any trends, or draw any conclusions.  But I do think varnishing is a very likely a good move as it further immobilizes the turns, and so probably combats thermal expansion and therefore electrical drift.

I had a feeling something like this would crop up and render home-made coil dope a non-issue - often you just have to wait long enough for it to do so.  Anyway, yet another experiment i should have performed years ago.

Glad if I could help !