Volume antenna on minimum Theremin

Hello, I'm new to this whole theremin scene and actually this is my very first posting. I'm sorry if such thread is already there but I couldn't find it with a quick search. Tell me about it if there's one.

I study electronics in Finland and it's time to do our graduation projects. I chose to do a Theremin because I have seen many videos about the instrument and they really raised my curiosity. I decided to construct the Minimun Theremin by Harrison Instruments (http://www.harrisoninstruments.com/101/101_schematic.html) but I noticed that it has no volume antenna. I was now wondering if one could be added to this schematic and how it could be accomplished. I have read through some other schematics that include the volume antenna but I can't understand how I should add those to minimum theremin schematic.

Has anyone added a volume antenna on this schematic or does someone know how it could be done? Maybe even have a ready schematic of this volume antenna which could be easily added to the minimum theremin schematic.

Help would be greatly appreciated.

P.S. Thanks for the great site with loads of information about this great instrument.

Adding just a volume antenna is not all.

You will need an oscillator which is de-tuned by the increasing capacity when approaching your hand to the antenna, a discriminator circuit which transforms this frequency change into a voltage change, and finally a VCA circuit which uses this voltage change to increase or decrease the output signal of the pitch mixer.

Thank you for your answers but I have some questions about the details.

The de-tuning oscillator: Can I use one that is identical to the pitch antenna oscillator and does this de-tuning oscillator also need it's own local oscillator or no other oscillator at all?

The discriminator circuit: I can't quite understand how one can change the frequency change to voltage change via discriminator circuit, at least not with the ones I'm familiar with.

VCA circuit: I couldn't find any information about this circuit with quick googling around. What does VCA stand for?

Schematics to the circuits would be greatly appreciated. Unfortunately I'm in great hurry with this project and there is no time to plan every circuit from the scratch.

P.S. I'd like to credit you, Thierry, in my project documentation so could you give me your full name?

You may take a similar oscillator for the volume antenna, but you should design it for a different frequency to minimize side effects on the pitch circuit.

The simpliest thing would perhaps be to take a 455 kHz-LC-Oscillator. It is easy to build a Meissner-oscillator with a standard adjustable lc-based 455 kHz if-filter. After this a 455 kHz ceramic-filter (easy available since its a standard if-filter) and a rectifier. The more the oscillator goes away from the initial 455 kHz, the less voltage you will find behind the ceramic filter / rectifier.

[b]edit:[/b] I have only now seen that your circuit is made with CMOS inverters and RC-oscillators. This will also work. If you have difficulties in calculating the correct values for the R's and C just let me know![b]/edit[/b]

You may select two different behaviours with such a circuitry:
a)If the Oscillator has initially exactly 455 kHz when far away from the volume antenna, the dicriminator output voltage will be high and only decrease when you approach your hand, so Volume will decrease when approaching.

b)If the Oscillator has initially a slightly higher frequency than the 455 kHz when far away from the volume antenna, the dicriminator output voltage will be low and only increase when you approach your hand, so Volume will increase when approaching.

A VCA is a (V)oltage (C)ontrolled (A)mplifier whose amplification factor depends on a control voltage. An integrated circuit such as the LM13600 will do this job.

There is no secret around my full name: Thierry Frenkel

But there is no obligation to give me credits, I'm already glad to help you.

And (as usual): Please excuse my bad English, my native languages are French and German

I'd definitely like to do the one with low volume when hand is close to the loop. About the volume antenna: Does it have to be a loop? Would a square plate antenna work?

I'm off to do some schematics now and I'll let you see them after I've got something ready.

By the way, does those oscillators give which wave form? My simulation and testing shows square, is that right? And what defines the frequency of the CMOS inverters and RC oscillator? I've tried to differ the R and C values of U2 inverters but the simulation shows that the frequency stays the same (little less than 6 MHz). How is the frequency dependant on R and C values?

The VCA you proposed uses +- 9 V, I'm searching for a one that would use 5V only. Well, let the google be merciful. :P

Thanks again

A plate antenna will also do its job. No obligation for a loop.

The oscillator frequency depends mostly on C1 and R2 (see variable pitch osc.), and on the internal delay of the CMOS inverters which is strongly dependend on the supply voltage.

Since there are lots of tolerances, start with C1=15pF an R2=15kO to come into the 455kHz range, connect a frequency counter and see what happens.

In every case the volume osc. needs a possibility to adjust it, such as the fixed pitch osc. So, no need to draw schematics, just:
a) copy the fixed pitch osc. including all of its adjusting circuitry [b]not[/b] using the unused U2D, U2E and U2F, since their supply voltage is adjusted for pitch trimming. Take a separate 6xInverter chip. You nee three for the osc. and we'll find also something for the others ;-)

b) modify C9' = 15pF or a trimmer capacitor 2 to 20pF
c) modify R12'= 4,7kO
d) connect the volume antenna in a similar way as the pitch antenna.
e) connect a frequency counter at its output and play around with it until you can get up to 460kHz. and pull them down to 455 or less (doesn't yet matter) when approaching your hand to the volume antenna.

If that works we'll see for the filter/discriminator stage.

I'm starting to understand how those oscillators work and how the Voltage supply affects the delays of inverter. Here's the schematic, anything wrong?
http://img409.imageshack.us/img409/8867/volumeoscillatorbd8.png

I think we can move on to the next stages now. So we use those three unused inverters in filter/rectifier?

And about the VCA, could I use any Operatinal amplifier maybe something like this one
http://www.ortodoxism.ro/datasheets/rohm/ba7655a.pdf ?
Or maybe one of these: LM124-LM224-LM324? It would make the circuit much simplier if I could use only positive 5 volts as powersupply.

EDIT: I decided to make a power source so I could plug the Theremin into 230 Volts and not use the 9 V battery as a power source. To determine the values of a power source I'd like to know how many milliamps does Theremin need at max. Any guesses?

@Kultti: Your circuit design is ok, out of D1, which must not be in place since we will not mix this oscillator signal with another.

As I told already before, you may not use the unused inverters of the other 4069 (U2) since its supply voltage is adjustable for pitch trimming. You will need another 4069 (U3) to have the opportunity to adjust its supply voltage independendly for volume trimming.

A "normal" opamp like the x24 series will not give a VCA functionality. So, out of that given that you insist on unipolar supply voltage, we will have to invent another circuit to attenuate the audio signal. Let me think about this until tomorrow. Its already late (or early) here in France. Perhaps we could use a FET transistor as voltage controlled impedance.

Any unstabilized external power supply with 9V and 100mA (standard, cheap and ready available) will do the job.

Unipolar supply voltage is not insisted. I think than I could easily get that negative voltage if I took a transformer with two secondary windings, eh?

I really don't want that this project gives you any unnecessary head ache. :) How about that LM13600, it's maximum ratings tell that maximum supply voltage is +-18 volts, I think that means we can't use it with 5 to 12 volts?

Haven't seen you around lately, Thierry. I just wondered if I could have your e-mail address so I could ask you if something urgent comes up with the project.

Anyways I'm hoping that you're still on with this project and hopefully I see you soon.