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