Building a Theremin

Hello guys!
I have started building a Theremin and I´m a little bit lost with the antennas... :( they have to be a special type? some characteristics in particular? do you know where i can buy the real ones?
if you can help me or give some information, it would be awesome!
thanks!

The needed size and thus the capacitance of the antenna depends mainly on your circuit design and the desired playing range. So let us know which theremin you are building and then we'll come out with detailed info.

With the same antenna you may for example get 3 non-linear octaves on a circuit design whose pitch oscillators have a free-run frequency of 460kHz or 6 linear octaves on a 285kHz design with a 40mH linearization coil.

So tell us more, please!

HI!
I have read some information about theremin and it said that the theremin has to be played in a range of 200kHz more or less. so may be the second option you have given me it´s the best one!

After the antenna, I want to put a F-V converter and then a function generator. the problem is I´m not sure how to get the difference between the two signals. I have read also something about a mixer which gives the addition and subtracion of the two input signals. do you think it is correct??

thank you for everything!

You are talking about using voltage control circuitry here.. You may find this link helpful..
Smirnov circuits (http://asmir.theremin.ru/tsensors_sch.htm) - These circuits give a voltage output proportional to sensed capacitance, and work well - they are far simpler than using a heterodyning scheme where one must do a F-V .. (Analogue Theremin Sensor #3 is what you need - it works extremely well.. Alternatively you could combine the circuits in #2 to build a conventional heterodyning Theremin.. This consists of pitch oscillator [Q1 and components on the transmitter cct] and reciever cct containing local oscillator [Q2 and components] and heterodyning mixer [C7,C8,R12,D1] .. it is D1 which does the mixing in the same way as on the EtherWave.

These circuits produce a highly non-linear output - you will need to follow the voltage with a non-linear (probably double exponential) converter circuit before you drive the linear VCO (function generator) and have a different non linear function for volume control, to convert the relationship to log (or use a log VCA).

BUT - What worries me, is the questions you are asking! - If you are having problems obtaining or devising a heterodyning mixer, then you are a million miles away from being able to design and build a playable Theremin employing voltage control techniques..

Is this a project which has been landed on you by some ignorant lecturer? If it is, then simply use the Smirnov designs (link above) and dont worry about real-world issues like linearity and thermal drift.. If you are wanting to design/build a good Theremin, there is a long hard road ahead - but it is worthwhile if you want to learn electronics and are really willing to study and master some difficult stuff..

..More.. [i]"I have read also something about a mixer which gives the addition and subtracion of the two input signals. do you think it is correct??"[/i]

see: TW waveforms Thread (http://www.thereminworld.com/forum.asp?cmd=p&T=3850&F=3) for full details of what the heterodyning mixer does.

[i]I have read some information about theremin and it said that the theremin has to be played in a range of 200kHz more or less. so may be the second option you have given me it´s the best one![/i]

That was only an example. The "real" response of a pitch antenna depends on multiple factors, not only on its shape, but on the oscillators design (RC or LC), presence of an adequate linearisation coil (calculated in function of the static antenna capacitance vs. circuit board and environment and the dynamic capacitance vs. the players hand and his body in series with the capacitance between player and ground) and on the ratio between the oscillators and the antennas capacitance.

Although I'm not an English native speaker I got the impression after reading your "I have read some information about theremin" that you aren't at all aware of the complexity of creating a theremin design which requires very good electronic, mathematical and musical skills.

If you don't have these I would rather suggest that you build first a working theremin from a kit and start later with modifying some of its parts or parameters in order to acquire a feeling for the complexity of this subject.

FredM! in the circuits that you passed me, in the second one, the sensor will give a signal proportional to the capacitance value isn´t it? I don´t know which is the voltage that I need to use and also how to calculate the value of the lineralization coils. some of the values of the capacitors aren´t specified. they are in pF?
I do really need your help.
thank for everything

[i]>>"FredM! in the circuits that you passed me, in the second one, the sensor will give a signal proportional to the capacitance value isn´t it?"[/i]

By the "second one" I assume you are referring to Smirnov's "Analog Sensor #2".. This is in 2 parts - The 'sensor' which will output a frequency which changes as a function of sensed capacitance (but the frequency change will be quite a tiny percentage of the total frequency - for example, frequency may vary from 485kHz to 490kHz) - And the 'Reciever' which has a reference oscillator and mixes this with the 'sensor' frequency to produce a 'difference frequency' - If this reference oscillator is tuned to 490kHz, then the sensor frequency above (485kHz to 490kHz) will produce an audio difference frequency from the reciever circuit, which will go from 5kHz to 0Hz.

[i]>>"I don´t know which is the voltage that I need to use and also how to calculate the value of the lineralization coils. some of the values of the capacitors aren´t specified. they are in pF?
I do really need your help.
thank for everything"[/i]

Voltage is not critical - anything from 5V to 12V should work - 9V from a battery is probably your best starting point.

Analog Theremin Sensor #2: This (as shown in the block diagram) gives an audio output when the signal from the sensor circuit is combined with the reference oscillator in the Receiver circuit.. This is a conventional Theremin – It DOES NOT output a control voltage to drive a VCO, it produces audio by heterodyning two HF signals (mixer consists of C7,C8,R12,D1 in the Receiver circuit.
If this is what you wanted, there are simpler circuits – This one is designed to allow the ‘sensor’ part to be located a distance away from the receiver.. But, having said that – There is no reason really not to use this circuit – it is robust and has no FETs to blow up..

Capacitor values are a bit obscure –
Sensor Circuit: C1=100n, C2=1.2n, C3=10n,C4=1.2n,C6=15n (n=nF. 1nf=1000pF)
C5=100u (u=uF, 1000nf=1uF)
Receiver Circuit: C1=680p,C7=22p,C8=22p
C2=10n,C3=100n,C4=100n,C5=1.2n,C6=10n,C9=3.9n,C11=1n
C10=47uF Non-polarised Electrolytic.

[i]>>"I need to use and also how to calculate the value of the lineralization coils."[/i]

Smirnov gives all the required information - Note, the coils determine oscillation frequency.

From Smirnov's (http://asmir.theremin.ru/tsensors_sch.htm) page:
[b]"It is important to match frequencies of sensor oscilator and receiver oscilator. They should have similar initial values. The relation of the frequency of receiver oscilator and it's inductance L1 will be as following: 6.8 mH (milli Henri) will give you approximetely 130 khz, 1.8 mH - 230 khz, 0,5 mH - 370 khz, 0.44 mH - 500 khz, 0.27 mH - 800 khz, 0.082 mH - 1 mhz."[/b]

You originally stated:
[i]"After the antenna, I want to put a F-V converter and then a function generator."[/i]

If you still plan to do that, you would take the Reciever output to the F-V....
But why go to all that extra complexity, when "analog Theremin Sensor#3" is far simpler and gives a voltage out directly ?

Here is what I think you need to do before anything else...

Work out WHAT YOU ARE REALLY WANTING TO DO, and let us know! - What do you want... Do you want a Theremin which gives audio out, do you specifically want to drive a function generator (VCO)..? Or do you have a project specification (for college / whatever) that you need to follow?

If you want a basic Theremin, there are far easier routes to achieve this than the direction you seem to be persuing..

If you need a voltage to drive a function generator, then Smirnov's #3 is a good path - BUT there are other routes which are simpler - in particular, there are simple 'digital' oscillators and mixers well suited to driving F-V converter IC's... And these do not require coils.

Are you wanting something you can REALLY play, or just something which works well enough for you to get a good exam result?

I cannot really help you anymore unless I have more information about what you want - and why!

Hmm ... so you aren't a real Russian unless you drink vodka and build theremins! LOL

Seriously, though, that was a very interesting site and Smirnov has some excellent ideas there.

Don

[i]"Seriously, though, that was a very interesting site and Smirnov has some excellent ideas there."[/i]

Hi Don -
Yes - Nothing astoundingly 'new', but the circuits are simple and work well - The oscillator circuits are extremely simple, stable and reliable.. They are almost identical to the oscillators I most often use - I based my oscillator design on an oscillator which was part of an inductive joystick (Dinosour Electronics - 1970's.. Later became Flightlink Electronics.. who I worked for .. Their joysticks have been made for 30+ years using this oscillator) - It is quite a remarkable oscillator because it works almost regardless of what one does to it, and is almost impossibly efficient.

The cleverest circuit (in my opinion) is #3 - The way that L2 couples the antenna (via W2) back to the oscillator, so that the antenna forms part of both the bandpass filter and the oscillator - Ie, changes of capacitance cause the oscillator frequency to change AND cause the BPF frequency to change - results in a discriminator which has high sensitivity but is not as critical to set up as the usual BPF scheme (as used in most volume circuits).

I only came across the Smirnov (http://asmir.theremin.ru/tsensors_sch.htm) site recently, and it came as something of a shock to see circuits so similar to what I have been secretly working on (down to virtually the same oscillator) .. Another reminder to me that "There is nothing new under the sun"..

BTW - This Smirnov site, despite being .ru and despite comments elsewhere on TW about the "danger" of .ru sites, has not caused me any problems - no nasty pop-ups or anything which gives me any bother.