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

Thinking about the tuner more now, specifically the display.  Currently using the string of 8 LEDs on the prototype demo board, with PWM blending so that at most 2 LEDs are lit, 1 LED per half step.  I initially thought that having it so that "on the note" = single LED lit would be most intuitive, but I find myself playing it so that "on the note" = two LEDs lit to roughly the same brighness (in between LEDs).  I think I prefer this because PWM gives linear intensity change with value change, whereas our eyes respond to luminance logarithmically.  So it's easier to light two LEDs to the same brightness than it is to try to get only one to light.

There are some nice, bright, inexpensive, diffused, wide viewing angle bi-color LEDs (red & green) at mouser for 25 cents (and of course out of stock):

http://www.mouser.com/ProductDetail/Kingbright/WP59SURKCGKW/?qs=sGAEpiMZZMs4quMj8r4lmiKK%2fK5xPADvQwO9brGXxbQ%3d

It would be interesting to employ staggered rows and multi-colors.  Might be able to get away with only 12 LEDs this way.  In between notes could be red & green = muddy yellow.  On the note would be two adjacent LEDs showing either red or green.  I'll make an order and do some experiments.

"It would be interesting to employ staggered rows and multi-colors.  Might be able to get away with only 12 LEDs this way.  In between notes could be red & green = muddy yellow.  On the note would be two adjacent LEDs showing either red or green.  I'll make an order and do some experiments." - Dewster

I would be really interested in the results from these experiments -

One thing to keep in mind though (well - only if you are designing this with a view to other users - as in, production) is that peoples perception of colour varies hugely - there is quite a large percentage of the population with some degree of colour blindness -  many who, whilst not clinically colour blind (as in, they can tell red/green and are allowed to drive PSV's) cannot differentiate hues well.

For this reason I think your 2 LED arrangement is good - One can combine this with colour variation, as this would cater for most levels of visual ability - But I think going to a single bi-colour LED would have less universal usefulness.

"...peoples perception of colour varies hugely..."  - FredM

Very good point.  Do you think red/green is a particularly bad combination with all the "green weak" males out there?  I like it because red+green ~= yellow.

For a bit of clarification, I'm thinking of trying something like this:

    1       3      5       7       9     11

    O      O      O      O      O      O

O      O      O      O      O      O

0       2      4       6      8      10

Where the bottom row would be wired green|red and the top row wired red|green (left|right).  Pitch increases from left to right like a piano or guitar.  Say 0|1 being illuminated equally (red) corresponds to the note A.  1|2 being illuminated equally (green) would then correspond to A#, and so on.

A 0|1 (red)

B 2|3 (red)

C 3|4 (green)

D 5|6 (green)

E 7|8 (green)

F 8|9 (red)

G 10|11 (red)

Red would always correspond to a "forward slash", green to a "backslash".  So even completely colorblind people could see those shapes (though not sure if that's sufficiently intuitive to easily play, even if you can see the colors).

The position directly between notes would be a single LED lit yellow (red+green, and more or less given the PWM width and analog/digital noise).

I kind of like it starting on A because this gives symmetric lighting shapes with Am and C scales.

I am not a musician - or at least, I am illiterate regarding the score's etc - so my opinions on this should be taken with a big pinch of salt..

I really like your scheme - To me, it combines everything needed and caters for possible problems with sightedness. I like Red/Green for the same reasons you do, but it does have this problem of being less suitable for green insensitive people (and unusuaslly, I am not talking about the environment ;-)

I think your scheme is suitable for even those with quite severe colour perception problems, as the changes in luminocity will give the additional required visual clues.

Personally, I could not use a visual tuner.. Oh, I could see the colours - But I would find (do find) looking at anything while playing is far too distracting - I most often play with my eyes closed (in fact, I dont really even get into listening to music deeply unless my eyes are closed - even often play my keyboards with my eyes closed, LOL )

I think its a dicipline thing - I have none! ;-) I get distracted so damn easily - I would be looking at the two LEDs and seeing if I could vibrato to some "flicker fusion" frequency, LOL.

Fred.

"I really like your scheme..."  - FredM

Thanks for your thoughts!  Now to order the LEDs.  I must say that the parts selectors at mouser, DigiKey, et al can be both good and bad - lots of parameters aren't filtered correctly, numbers aren't considered to be in numerical order, parts aren't in the category you expect, DIPs are sometimes shown as SMT, etc. which makes for some time consuming and often fruitless searches.

In order to more intelligently select coils, I've been simulating the AFE again and thought I would share some thoughts here.  A picture from the simulation spreadsheet:

tank_mode=0 is how my AFE is currently operating, where the tank inductor is in series with the tank capacitor, and the oscillator drive looks like a voltage source.  The oscillator sense point is different than the drive point, the sense voltage lags the drive voltage by 90 degrees at resonance.

tank_mode=1 is how most Theremins operate, where the tank inductor is in parallel with the tank capacitor, and the oscillator drive looks like a current source.  The oscillator sense point is the same as the drive point, so these voltages are in phase at resonance.


TANK_MODE=0

Pros: Tank acts like a low pass filter so tuning is completely non-critical and fixed value components can be employed.  Antenna voltage swing is easily adjusted via the drive resistor.  Resonance condition is quite clear if the tank and equalization inductances are roughly the same value.  Resonance always coincides with maximum antenna voltage swing.

Cons: For highest sensitivity the tank capacitance must be minimized, so the tank inductance must be rather large.  Operates at a lower frequencies, which could be an issue with audio heterodyning.  

Other: Quadrature drive is more complex, but resonance is clearly defined.


TANK_MODE=1

Pros: Tank capacitance can be quite large without negatively impacting sensitivity, so the tank inductance can be made quite small.  Operates at higher frequencies, which is generally good for audio heterodyning.

Cons: Tank acts like a bandpass filter, producing two resonance peaks (tank LC / EQ & antenna|hand LC) that interact and require critical tuning, so one or more variable components must be employed.  If mistuned the maximum antenna voltage swing will not be realized.

Other: In-phase drive is simple, but resonance is not as clearly defined.

If all you have access to is small value air core coils, I believe you're pretty much forced into using mode 1.  But mode 1 generally has three interacting tuned circuits: tank LC, EQ|antenna LC, fixed oscillator LC - which can make it a bear to tune.  And without test equipment of some sort you won't know if you have maximum antenna voltage swing.

The simple resonance of mode 0 is a huge plus IMO because the interacting tuned circuit count is reduced to two, and it always gives you maximum antenna voltage swing.

Sorry, my intention with this post was not to rag on mode 1, I mainly wanted to point out the strengths and weaknesses of both approaches.  And I haven't made an analog Theremin with mode 0 so there could landmines out there I'm unaware of. 

Been thinking about what to use as a Theremin enclosure.  I'm kind of partial to the looks of the SubScope with the controls on top of a long bar.  A couple of weeks ago while at the local Home Despot shopping for hurricane supplies I ran across 4" and 5" square vinyl fence posts:

http://www.homedepot.com/h_d1/N-5yc1v/R-202084744/h_d2/ProductDisplay?catalogId=10053#.UK5QS-915n8

http://www.homedepot.com/h_d1/N-5yc1vZc3mq/R-202084745/h_d2/ProductDisplay?catalogId=10053#.UK5R5u915n-

There are a variety of caps that look interesting, with the simple internal and external pyramidal shapes the least garish:

http://www.hooverfence.com/vinyl_fence/postcaps/standard-post-caps-lmt.htm

http://www.usavinyl.com/capupgrades.html

The ball cap might be interesting to mount antennas on (or coat the inside with sticky aluminum tape and use the ball itself for the volume antenna - bad idea?).

A cap for horsing around ("... adds a equestrian feeling to any Theremin").

The post material is available in a variety of wood grains, stucco look, and colors too.

"The ball cap might be interesting to mount antennas on (or coat the inside with sticky aluminum tape and use the ball itself for the volume antenna - bad idea?)." - Dewster

LOL ;-) I think its a great idea - but thereminists seem to like their loops.. I have a desire to avoid the conventional loop.. just too damn difficult to get them made so that they look / are good (IMO, coat-hanger wire types on low cost theremins and expensive PAiA clones are the worst of all worlds)

To me, a flat or spherical volume antenna seems perfect - I have one with 4 "antennas" using aluminium adhesive tape stuck on the inside of a frizbee - even that looks better IMO than a bent coat-hanger !

But - when it comes to artistic flair and good taste, well, I am probably the worst person to comment.. Some people who have seen my constructions / modifications / prototypes, and particularly the colours I choose - well - lets just say they "advise" me to let someone else take care of that aspect..

Fred.

I like a loop. I can snap my fingers in and out of the hole for staccato punctuation without worrying about hitting it.

(But I would gladly give it up for an xyz sensor. For what I do, two extra dimensions of control over the sound would more than compensate. Some impact-proofing measure might be advisable though. I get a little excited at times.)

Not the horse head though. Neigh, I say to that. Unless I was doing a Mafia gig.

"Not the horse head though. Neigh, I say to that. Unless I was doing a Mafia gig."  - GordonC

How about the horse head mounted on the pitch end, nose up in the air? ;)

Maybe use the ears for extra articulation.  Put LEDs in the eyes for a demonic effect.

Make a travel case for the whole thing that looks like a feedbag.