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Theremin...

F

Fred Bartoli

Jan 1, 1970
0
My little daughter wants to 'do' some electronics with me during these
christmas vacation. :)))
As all young kids she's got totally unrealistic ideas, but when I told
her about the theremin, she got enthusiastic and I feel this might be
the right project: she's learning piano and music, has an amazing ear
for her age and is very interested in all aspects of music. On my side,
it's simple enough, but not simplistic, and can lead to excellent
results if carefully done. Oh, and there's no chance for her friends to
have one :)

Now this has to be completed in about one week and she's young so we've
not much time to test and try...
Anyone having pointers to some you actually build and that is working well?
Any hint welcomed.
 
J

Joerg

Jan 1, 1970
0
Fred said:
My little daughter wants to 'do' some electronics with me during these
christmas vacation. :)))
As all young kids she's got totally unrealistic ideas, but when I told
her about the theremin, she got enthusiastic and I feel this might be
the right project: she's learning piano and music, has an amazing ear
for her age and is very interested in all aspects of music. On my side,
it's simple enough, but not simplistic, and can lead to excellent
results if carefully done. Oh, and there's no chance for her friends to
have one :)

Now this has to be completed in about one week and she's young so we've
not much time to test and try...
Anyone having pointers to some you actually build and that is working well?
Any hint welcomed.

This one was just featured in IEEE Spectrum and you can buy it as a kit:

http://www.moogmusic.com/theremin/?section=product&product_id=70

Here is the article:

http://spectrum.ieee.org/geek-life/hands-on/if-you-build-it-santa-will-come/0

Looks like the only thing the author didn't like was the not so exact
fit of the oak enclosure parts. $359 is probably a bit steep for "doing
some electronics" but it does look like a fancy instrument. Not
something your wife would want out of the living room immediately. And
since the Euro is pretty high and if you daughter promises not to do
anything stupid or dangerous for at least six months, maybe Santa Claus
has a heart ;-)

Not sure what the French authorities would think about the EMI these
things generate, I think they are quite strict over there.
 
E

Ecnerwal

Jan 1, 1970
0
Greegor said:

Well, I can report that the University of Glasgow digital version (from
that page) both uses a VCO that's more or less unobtainable these days,
and the circuit was not functional when a high school student tried to
toss it together for a project - but I can't swear that he got it all
right - on the other hand, I couldn't find where he got it wrong,
either, and we did find at least a couple of errors in the published
schematic (which is still the same, as I recall, but I don't recall what
the errors were at present. I might have that info in an old email, but
given the VCO, it's mostly irrelevant.) I'm far from being a great
designer, but I am a pretty good technician and troubleshooter.

Many, most or perhaps all ( I can't claim to have tried them all, or
even throughly investigated them all) of those schematics are almost
unbuildable as is, because they are old, and depend on parts which are
no longer available (or not very easily.) Substitute enough
sort-of-similar new parts and unstated assumptions in the original
circuit go awry.

As such, that page is hardly the resource of "Things we've actually
built" that Fred is looking for, unless you folks have actually built
them. I sometimes suspect that the people who make the things for sale
seed the net with bad schematics to keep themselves in business, but I
have no proof of that, and file it mostly under paranoid delusion. But I
have been keeping an eye out for something "easily buildable with
obtainable parts" for the next student that wants to give it a whirl -
and I haven't seen it yet.
 
M

Martin Brown

Jan 1, 1970
0
Fred said:
My little daughter wants to 'do' some electronics with me during these
christmas vacation. :)))

How little?
An electronic organ stylophone type thing might be a lot easier.

You could do a simple - just the frequency one with a single antennae
and rely on the local oscillators entraining into lockstep as a mute.
As all young kids she's got totally unrealistic ideas, but when I told
her about the theremin, she got enthusiastic and I feel this might be
the right project: she's learning piano and music, has an amazing ear
for her age and is very interested in all aspects of music. On my side,
it's simple enough, but not simplistic, and can lead to excellent
results if carefully done. Oh, and there's no chance for her friends to
have one :)

Now this has to be completed in about one week and she's young so we've
not much time to test and try...

I think you are hopelessly optimistic too. I have just built a
relatively simple one for a Christmas science lecture.

Tracking down and getting the right parts will take longer than that.
Almost all the once common tuned IF coils are no longer in manufacture
and require careful tracking down to find suitable ones. If you still
have some in a junk box then fine.
Anyone having pointers to some you actually build and that is working well?
Any hint welcomed.

I built the Mini Theremin circuit described in the UK magazine EPE
May/June 2008. They reprinted a design from Silicon Chip magazine at:
www.siliconchip.com.au

I had to bodge a 2nd IF from the ones I could get. The optocoupler is
rare in the uk. Some of the bits I used came from Oz. Largely because
they were cheaper there than locally sourced bits in small quantities!
(and I wanted to use exactly the same part for the equalising coil)

The design works more or less as advertised. I'd recommend powering the
main output amplifier off the supply rail but apart from that it is OK.
You probably do not have a hope in hell of getting it to work without
the detailed alignement instructions. There are a lot of things to trim
before it behaves at all. The hand wound equalising coil is only 300t
but must be minimum self capacitance and is tricky to get right.

If you don't mind about non-linear frequency to hand movement tuning
then you could ignore the resonant equaliser.

Typically I got the tricky frequency side to work first time, but had
real problems tuning the amplitude control side (where the right IF
component was no longer available). Frequency side uses an MC1496
balanced mixer and a load of 445kHz IF transformers as the pullable and
reference oscillators. Ceramic resonators resist being pulled off
frequency too well and have almost totally displaced these IF cans from
the market. You have to trawl round radioham sites to find one with the
right internals and pinout.

Someone here very helpfully pointed me at a UK supplier with a stock of
obsolete Toko IF transformers that fitted the bill:

http://www.jabdog.com/toko-ez.htm

You need a fair amount of tweaking to get them all to behave
simultaneously without interfering in the audio band. The board layout
of the project leaves a bit to be desired but works. There are still 7
knobs to play with on the front panel of this *simple* design!

You are probably best off ignoring the built in tuning test circuit as
the trailing leads mess up the tuning! A bodge on a bit of veroboard on
short leads works much better.

One other comment is that Australian towel rails at 15mm seem a bit
weak. UK I had to use 18mm towel rail supports and pack them to take
15mm chrome tube as the two antennae.

Regards,
Martin Brown
 
M

markp

Jan 1, 1970
0
Fred Bartoli said:
My little daughter wants to 'do' some electronics with me during these
christmas vacation. :)))
As all young kids she's got totally unrealistic ideas, but when I told her
about the theremin, she got enthusiastic and I feel this might be the
right project: she's learning piano and music, has an amazing ear for her
age and is very interested in all aspects of music. On my side, it's
simple enough, but not simplistic, and can lead to excellent results if
carefully done. Oh, and there's no chance for her friends to have one :)

Now this has to be completed in about one week and she's young so we've
not much time to test and try...
Anyone having pointers to some you actually build and that is working
well?
Any hint welcomed.

I'd be tempted to do something much simpler and much more digital, like a
set of infra-red LEDs and receivers time multiplexed to create an x-y scan.
This doesn't sound like a quick project to me, there may be many hours spent
with your daughter gazing over your shoulder while to try hard not to swear
at the box!

Mark.
 
M

Martin Brown

Jan 1, 1970
0
Ecnerwal said:
Many, most or perhaps all ( I can't claim to have tried them all, or
even throughly investigated them all) of those schematics are almost
unbuildable as is, because they are old, and depend on parts which are
no longer available (or not very easily.) Substitute enough
sort-of-similar new parts and unstated assumptions in the original
circuit go awry.

Tell me about it. But the only real trouble I had building the EPE one
from May/June 2008 was finding the right IF coils (one I had to bodge)
and the daft trailing leads for the test set up circuit. Their design
isn't perfect but it works OK on the pcb as laid out.

EPE (UK hobbyist magazine) will sell you the article and the PCB though
I copied the part that I hadn't got from a public library.

http://www.epemag.wimborne.co.uk/0508.htm

The equaliser coil is a PITA to get right. But it is easy enough to have
something where waving your hands makes the right sorts of noises.
As such, that page is hardly the resource of "Things we've actually
built" that Fred is looking for, unless you folks have actually built
them. I sometimes suspect that the people who make the things for sale
seed the net with bad schematics to keep themselves in business, but I
have no proof of that, and file it mostly under paranoid delusion. But I
have been keeping an eye out for something "easily buildable with
obtainable parts" for the next student that wants to give it a whirl -
and I haven't seen it yet.

I suggest you obtain the IF coils ASAP. They are hard to find now and
getting harder by the day. I guess you could use ceramic resonators for
the reference LO on the frequency side and the edge filter for
amplitude. That still means you need two IF transformers to make one.

Regards,
Martin Brown
 
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