My New Year gift to TW: A new theremin circuit

Posted: 4/10/2018 11:55:33 AM

Joined: 4/2/2018

I am not very active but dislike lonely threads. 

I appreciate your intervention, thank you

Home PCB making is a good thing to learn. I would never recommend construction of a high frequency analog project like this on one of those white proto boards due to unwanted capacitance coupling between components.

I understand that. I know that building a theremin is better done on a PCB than on a breadboard. In fact, since I have very little experience in soldering but I have used breadboards a lot in the past, I was thinking about trying to make this circuit on a breadboard first, before go further to solder it. Besides, in this thread and on the internet there is a lot of evidence that on a breadboard it can work as well.

In theremin design an ugly sound vs a beautiful sound are only separated by a thin margin of knowledge. 

Now that is exactly my point. What I got is not even close to ugly sound, nor is the noise I'm getting in any way affected by my hand. The circuit as it is now on my desk is very far from being a theremin. Since Thierry's idea, undoubtedly, does work, I must assume something in my implementation has gone wrong.
But is that really the use of a breadboard? Wouldn't that only affect the quality of the sound, instead of the working of the whole circuit? Also, if it's the breadboard, by disassembling the circuit and putting it together again in different rails I should hear some difference, or, if I'm very lucky it should even get better. This behaviour, though, I'm absolutely not seeing.

Posted: 4/10/2018 12:42:02 PM

From: Colmar, France

Joined: 12/31/2007

Just a few quick remarks (not much free time actually):

5V is not enough supply voltage, stabilized 6V are needed, that's why there is a 7806 voltage regulator in the schematic. With the original 5484 FETs which have been explicitly chosen for their extremely low pinch-off voltage of -1.5 to -3V, the circuit is known to work from 6V on. You decided to use incompatible FETs with a pinch-off voltage which can even go beyond -6V. How can you expect an oscillator to work if the required source-gate cut-off voltage of these inappropriate components exceeds already the available power supply voltage? 

Besides all that, a quick check with your oscilloscope would have shown you that the oscillators do not work this way because of too high quiescent current due to a too low Vgs.

C44 has to be identical to C14 and both have to be 3.3pF. If not sure about their marking, take your capacitance meter and check them.

Thus, build the power supply section as required by the design, buy the needed FETs, and then write back, because then there are other issues to fix like the circuit layout: In Theremin circuits, each 1/100 of a pF matters. Breadboards are a no-no. And the antenna connection has to be placed far away from all other wiring, that's why on my PCB design, it is in a corner. With the antenna wire close to the audio cable, and the antenna not mounted vertically above the PCB, it can simply not work.

Posted: 4/10/2018 12:55:46 PM

From: Northern NJ, USA

Joined: 2/17/2012

Leave every other row clear on the breadboard, and set the breadboard on a plastic box - both of these will reduce parasitic capacitance.  If the capacitor is small and only has a '3' on it, it most likely is 3pF.  What inductors are you using?  I've had poor experience with the types wound on ferrite bobbins.  Also, get your hands on a scope, if only for a long enough to debug things, without it you're flying blind.

Posted: 10/15/2018 7:55:04 PM

From: Portland, Oregon

Joined: 10/15/2018

Hi Thierry, I realize I'm late to the party on this thread, but I'm excited to build this circuit.  Full disclosure, I am a beginner and have very limited knowledge in circuits/electronics and how they work. This project will be a great learning experience :-)

1) For my variable capacitor, I am using the Sprague-Goodman GYA22000 (2.2-22pF).  I was expecting it to have two leads, but it has three (two rotors and one stator).  How do I wire this? Do I leave one of the rotors floating/unconnected?

2) I saw the comment "The audio out has line level and high impedance and thus can not drive a speaker directly.  It is intended to be connected to the inner wire of a shielded cable (shield connected to GND) which transports the signal towards an external amplifier."
I'm trying to make sense of "inner wire," do you mean a single wire that is shielded (serious question, I know it may sound silly)?  

Could an "external amplifier" be, for example, a guitar amplifier?  If so, would I just connect the shielded audio/inner wire to, say, a female 1/4 inch connector, and then simply use a standard instrument cable to hook up to the guitar amp?

Thanks so much for your help.

Posted: 10/15/2018 8:11:02 PM

From: Colmar, France

Joined: 12/31/2007

1.) yes, leave one rotor pin unconnected.
2.) all instrument audio cables for guitars etc have one single inner conductor and a grounded shield around.

A guitar amplifier will basically work but not sound very well. Guitar and bass amplifiers are optimized to be fed from these magnetic pickups and thus have a relatively high input impedance, be somewhat too sensitive (risk of unwanted overdrive and distortion when being fed from a theremin) and will have a bent frequency response to compensate the inductive effects of the pickups. So, there is a risk that the theremin will sound muffled and distorted. Better use a keyboard amplifier. These can deal with line level signals at the input and have a flat frequency response. I’m using a Bose L1 compact PA system, not only for performing on the Theremin but also for testing Theremin and related circuits.

Posted: 10/24/2018 9:11:43 PM

From: Portland, Oregon

Joined: 10/15/2018

Great, thank you very much!  A couple more things:

1) I'm looking for a suitable veroboard, and per your instructions, I am avoiding the strip boards and boards with square holes/dots.  However, as someone new to electronics, I was quite surprised at the wide variety of boards available on  I noticed that many of the boards have round holes/dots, but the dots are enclosed inside square pads (see link below):

What are these square pads, and do I need them?  Is there a specific board/product you can recommend?

2) Back to the audio plan is to create a wooden enclosure for the circuit, with the audio out consisting of a single conductor wire with grounded shield, soldered to a mono female 3.5mm jack.  The jack will be mounted to the wooden enclosure, so that I can plug a male mono 3.5 mm cable into it to transport the signal to the auxiliary input jack of an external speaker (In my case, the Bose Soundwave IV speaker).  Since my speaker is stereo, I think I'd also have to get a mono to stereo adapter.  

Is this approach possible, or am I trying to "drive the speaker directly," which you said will not work due to high impedance?

I think I understand your keyboard amplifier suggestion, but after attempt to build/tweak/perfect this circuit, I'd like to give it as a gift and want to avoid having to also provide an amplifier :-)

I feel silly asking some of these questions here, but please be assured that I am here to learn and greatly appreciate all of the knowledge in this forum.  Thanks!  

Posted: 11/24/2018 3:45:59 PM

From: Stockport, England

Joined: 11/24/2018

Hi Thierry, I would just like to say thank you for designing and publishing your straightforward-to-follow theremin circuit. I have built it on prototyping matrix board in a layout that closely follows your pcb layout. My son and I had great fun building it together and we now just need to mount it all in an old cigar box  Once again, thanks.

You must be logged in to post a reply. Please log in or register for a new account.