My Theremin "Guitar" Project

So this is my project I'm planning to do.

Its a double instrument, a keyar and theremin.

These are my blueprints including the schematic for the theremin i found online

http://www.majhost.com/gallery/alian20029/My-Website/keytar_theremin_blueprint.jpg

I've never built a theremin before or ever even seen one in person. so my question is with the antenna.

i plan to slide my hand up and down the neck like a guitar to change the pitch of the theremin. Will it work like i want it to at that angle and length? i was planning on just using a normal radio antenna. I dont really know how the whole antenna thing works, does it create a "bubble" around the tip where the radius changes pitch in all directions around it? if thats the case would the angle of the antenna matter? and how does the length of the antenna affect it? would it be better to have a longer antenna hidden inside the keytar body parallel to the guitar neck (the body will be wood, the spaces for the keyboard and circuitry will be made into the wood with a router)

also, if im reading the schematic, wouldi be right in saying that the variable resistor would control the volume?

answers and tips are apreciated :)

Hello Skipmonkey!

Neat blueprint - its a shame you probably arent local, I could do with someone having woodworking skills in exchange for electronics help (which you badly need - just as I badly need a woodworker)..

You have absolutely no understanding of electronics, as demonstrated by your question [i]"wouldi be right in saying that the variable resistor would control the volume?"[/i] .. Sorry to be so blunt, but, of all the electronic projects you could undertake, a Theremin is probably the one which requires the greatest understanding of electronics.. and you dont have even the basics... We all must start somewhere, and we all develop our understanding as we go along.. But the Theremin is the wrong circuit to start with.. Particularly if you want to build it into a 'tar!

I am not going to go over explaining Theremin electronics fully - there are many sources you can get this information from.. but lets look at your schematic..

There are 2 4093 schmidt gates, these are configured as CRUDE oscillators - If the input is low, the output is high - In this state, the capacitor connected to the input charges (in the case of one gate, via the potentiometer.. in the case of the other gate, via a fixed resistor..

When the voltage on the capacitor is charged to the +Ve threshold of the gate, its output goes low, and the capacitor starts to discharge through the same resistor or potentiometer it charged through.. This process continues.. one gets a triangle wavefor across the capacitor, and a square wave from the gate output.. The frequency is determined by the size (value) of the resistor and capacitor.

In your circuit, the potentiometer sets the frequency of one of these oscillators. This circuit needs (at least) another resistor in series with the potentiometer to have any chance of working.. Also, the values for the capacitor, potentiometer, and series resistor are critical and must be calculated based on the frequency span of the other oscillator.

The other oscillator has a frequency determined by the resistor, capacitor, AND capacitance of the "antenna". Forget the word "Antenna" - This circuit is a CAPACITANCE sensor, and your "antenna" is just a plate of a capacitor, with your hand changing this 'plate's capacitance to ground. The frequency of this oscillator will vary as your hand's position varies..

Both oscillators run at high frequency (>100kHz) and should be tuned to run at the same frequency when your hand is away from the plate. The oscillators are CRUDELY mixed by the EXOR gate, the following capacitior is wrong.. it could even destroy this gate.. you need a resistor of at least 1k before connecting to this capacitor.. this resistor and capacitor should form a low-pass filter tuned to no more than about 8kHz.. This filter gets rid of high frequency sum signals, and you end up with audio difference signals IF everything is tuned ok.. The following 4020 is redundant - And, in fact, if you replace the 4070 with a 4013 you end up with a much better chance (still not a good chance) of getting something to work.. (check my other postings for how to use the 4013).

[i]"i plan to slide my hand up and down the neck like a guitar to change the pitch of the theremin. Will it work like i want it to at that angle and length? i was planning on just using a normal radio antenna. I dont really know how the whole antenna thing works, does it create a "bubble" around the tip where the radius changes pitch in all directions around it?[/i]

The only thing which will affect the pitch, is how much capacitance is seen on the 'plate' by the oscillator. This will be primarily determined by the DISTANCE of the hand from the plate.. There are no "bubbles" there is no "tip" and there is NO MAJIK! - Despite what you may read elsewhere! - There is ONLY CAPACITANCE - Simple, Down to earth (LOL) capacitance!

Advice:

Read a lot more about Theremin circuitry, Build a Th

in that case would you suggest buying a kit? for example from harrison instruments?

What (Musically) do you want to do?

Even with a good kit, incorperating a Theremin into a 'tar type instrument is a MAJOR challenge.. Your body acts as a large capacitance which WILL be sensed - It is hard enough to play a standard Theremin, where it (and the sensing field) is stationary and onc can stand absolutely still and control the pitch by hand movement..

If you want to actually PLAY the instrument with even rudimentary pitch control (and here I am talking about getting to within 3 semitones of the note you want, not about cents, LOL) you will not achieve this with a mobile instrument of the type you are proposing, without a LOT of sophisticated electronics to compensate for changing background capacitances.

If you just want a pitch-bend function, or a effects generator, then this is probably achievable quite simply.

I dont know the Harrison kits (one of the family of designs that, for personal preference reasons possibly not based on any good reasons, I do not like the look of).. So I cannot comment on them.

The Jaycar / Silicon Chip kits are good - For an application like this their simplest / cheapest should be fine.. They are 2 antenna Theremins, but one can adapt to bypass the volume control circuit.. You could even use the volume antenna instead to give a voltage for something else..

Certainly, playing with the SC/Jaycar kit would give you a lot of knowledge you need before you do anything else!

well as you can see in my blue print it will also have the midi keyboard so the theremin is there just as a neat sound effect to go along with the keyboard to change things up a bit.

i have no intention of being able to play any specific notes.

you should be ok then.. ;-)

i suggest you get a ring modulator, and tap the audio from your keyboard - mix this with what comes out of the Theremin in the ring mod.. This way you will get a much more musical and interesting effect.. As a sound effect, the Theremin really (in my opinion) is extremely limited and most often does nothing to enhance the performance.. but when used whth a ring-mod, can give some extremely interesting effects even if not played musically.

Hi skipmonkey,

If you've searched for previous theremin guitar threads you'll know I'm something of a detractor. But, a little while ago Fred posted something that I can't find just at the moment - a description of an "antenna" that has caused me to reconsider my position.

It is made of a thin film of metal mounted on a board for rigidity, thin at one end and thick at the other so that - by keeping your hand (or finger) a fixed distance away from the antenna the theremin's pitch can be varied by moving your hand along the length of it. Fred had intended the idea to be used on a tabletop device, but I see know reason why a guitar neck mounted antenna would not work equally well.

Note that I said "at a fixed distance" - the antenna is far more sensitive to movement towards and away from it than along the length of it. Perhaps a clear plastic guide rail to rest your finger on whilst playing would help.

Also it is difficult to shape the antenna so that notes are equally spaced. (I note that you are not interested in playing it melodically. As an experimental thereminist I would suggest that having predictable spacing is still a good idea.)

I would add to Fred's ring-mod suggestion that you also can't go far wrong with an echo. :-)

(Fred - you might like to check out the result of my most recent ring-mod experiment, this time with the ring-mod being fed the dry theremin signal and the same signal pitch-shifted by 25 cents, to produce pitch-related beats in the sound. "The New Consonance" - currently the featured video on my youtube channel (http://www.youtube.com/user/GordonCharlton).)

If you really are dead set on making a theremin-guitar thing, here's a possibility you could play with.

First build a "real theremin" ... Jaycar, Moog, PAIA to get the feeling of what it does and how it works.

Next build your "guitar" neck with a metal bar or plate at the "nut" with a wire running down through the neck and out to a phone jack. Put a metal plate into the back of the body and run a wire from that to the sleeve part of the phone jack. Connect a guitar cord from the guitar to the pitch antenna connection of your theremin.

Now making it actually work is where the challenge is. Possibly it won't work at all because you add too much "hand capacitance" no matter where you put your left hand. But if it works at all, you might be able to tune it for a reasonable use by changing spacing or size of the metal plate, connecting or not connecting the sleeve of the guitar cord to theremin GND, or maybe you would have to run the connection out of the headstock into the theremin.

Don

[i]"If you've searched for previous theremin guitar threads you'll know I'm something of a detractor. But, a little while ago Fred posted something that I can't find just at the moment - a description of an "antenna" that has caused me to reconsider my position." - Gordon C[/i]

Here (http://www.thereminworld.com/forum.asp?cmd=p&T=4204&F=1) is a link to the above..

However, the 'shaped antenna' design has major problems when used in applcations having high background capacitance.. And this is the case with all instruments where the field must cater for other grounded or metal objects nearbye.. such as portable 'tar type instruments.. It can be ok where it is mounted on a good dielectric (like plastic) in free space - but beyond that, its application is extremely limited - and complex electronics is required..

Not wishing to hijack this thread, I will only briefly mention my problems with Epsilon. This was a sophisticated digital system using haped antennas.. there were 4 antennas placed side-by-side, adjacent pairs were shaped so the wide end of one lined up with the thin end of the other.. The capacitances were differentially processed so as one moved ones hand over the antenna, capacitance of one increased while the other decreased.. From this, total capacitance could be deduced and one could derive the horizontal position and the distance of the hand from the antenna. Background capacitances caused the actual changes due to hand position to be miniscule (far too tiny to directly be used in conventional Theremin circuitry) so was digitally processed to 'multiply' the differences and compensate for the background.

Playing this instrument was impossible - pitch and volume control on the same surface (not to mention the forwards/backwards tone axis I also included) is, I now believe, unworkable.. both from a technical and player perspective.

Without similar digital processing, I do not think it possible to incorperate the 'shaped antenna' scheme into a portable instrument - and one needs to do this to compensate for unwanted capacitances and capacitance changes.

At this time, I do not think an instrument like this is worth the bother.. Far easier to incorperate a ribbon controller.

Gordon,
"The New Consonance" is absolutely wonderful - wonderful sounds.. I would never have guessed at how you created them! I would never have guessed about the Ring-mod being used in such a way -

I played this directly through the Tannoys and never watched the Vid until I had played it twice.. Vid is great, But I prefer your music without it!