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

Posted: 10/30/2024 8:42:49 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

"Interesting articles.Amazing what people do in their kitchen !They can't be married ! About thermal drift, I made an experiment.I heated the coils with a hair dryer for several minutes. I only noticed a drift of one or two digits.From 0.970 mH to 0.971 mH and from 1.925 mH to 1,927 mH." -- André

Yes, I've done tests too from freezer to room temperature, and saw no drift with the air-cores.  Ferrites on the other hand drift a lot.  I turn the meter on for a while before testing to hopefully let it stabilize, who knows how stable these inexpensive things are.  I know the capacitance measurement of the meter I have drifts a lot, I assume due to the ferrite reference coil inside it.

Posted: 10/30/2024 8:49:02 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

"After using option "7. Factory Reset", the out-of-bounds values were gone and also the "ER!" message. Once it was working I used Quartus to program the FPGA configuration that I compiled and it still was working as with the original FPGA configuration."  -- gerbold

I thought that was probably the issue.

"When monitoring the pitch drive output of the AFE, I can see that the frequency raises up to 1.6 MHz, then breaks down to zero and rises again. I assume this is the effect of my poor inductance with the ferrite ring core. The volume does look pretty much the same."

If you have some small pF caps laying around, you might be able to get it to lock with your low Q inductors by plugging caps into the C6 socket on the AFE board.  Maybe start with 10pF or so and stop at 100pF.  The goal is ~3Vpp or so at the "C divider" test point if you have a scope.  You won't need to do this with André's high Q coils.

Posted: 10/30/2024 8:54:43 PM
André

From: 30 km south of Paris (France)

Joined: 12/23/2022

Today I tried your suggestion of applying just a solvent on the coil.
I used acetone which does react with PVC.
It doesn't work because acetone does not penetrate between the wire turns !

I tried something else. I just coated the coil with a clear varnish in spray.
It's drying now. I'll tell you the result tomorrow when it's fully dry.

Posted: 10/31/2024 6:46:43 AM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

Thank you for experimenting and reporting your results André!

Posted: 10/31/2024 1:37:32 PM
André

From: 30 km south of Paris (France)

Joined: 12/23/2022

Fully dry. It's almost invisible and very solid. I tried to scratch the coil with my nails, perpendicularily to the wire. Impossible to damage it.
I even wonder if it is necessary to put a shrink tube ! 

Posted: 10/31/2024 2:05:21 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

"Fully dry. It's almost invisible and very solid. I tried to scratch the coil with my nails, perpendicularily to the wire. Impossible to damage it.  I even wonder if it is necessary to put a shrink tube !"  - André

I've seen Tesla coil videos where they use epoxy applied with an old credit card here.

If everything is that secure then heatshrink will add another layer of dielectric, raising the self capacitance and likely lowering the self resonance - though all things being equal, making the coils more physically robust is obviously a good move.  My educated guess is that the dielectric between windings near to each other is likely more important than the dielectric from the windings to the environment.  Of course the coil former itself has a higher dielectric constant than air, as does the insulation on the wire - these things pretty much have to be there for our types of coils but their influences are directionally incorrect.  This is why you see critical RF coils which are wound in the air and made of uninsulated wire or tubing.

Probably no need to be alarmed by it all though - the time is ripe for experimentation!  I'd be interested in a before and after inductance measurements of (using the exact same coil for all measurements): bare coil vs. varnished coil, bare vs. heatshrink, and bare vs. varnish + heatshrink - but only if that might work out for you, and only if you feel like doing it.  The measurement setup has to be fairly physically repeatable though if the results are to be meaningful.  If you are using a similar meter to mine, even small changes to the coil positioning can be seen because it operates at resonance like a Theremin.  I do measurements with the meter and coil both on top of an empty plastic box.

What spray varnish are you using?

Posted: 10/31/2024 8:24:16 PM
André

From: 30 km south of Paris (France)

Joined: 12/23/2022

Yes, I can do that this week-end on one of the coils.
If there is no significant change, I'll do a double insulation (varnish + shrink tube) on all of the 8 remaining ones.

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.

You must be logged in to post a reply. Please log in or register for a new account.