My New Year gift to TW: A new theremin circuit

"hello engineers and other therepathics" - Dani

LOL ;-) .. A newly identified type of personality disorder? TPD .. Therepathic Personality Disorder!

Hey, I wonder if its grounds for disability allowance...  ;-)

Someone asked "I might build this on a breadboard, is that an adequate spacing?" and got the reply "If you say breadboard, does that mean one of these white solder free ones where 5 or more contact holes are always connected together? A such one wouldn't work, too much parasitic capacitance"

Thereminists are KNOWN for their contrariness - that reply sounded like a challenge to me, so I built it on breadboard - AND IT WORKED!

Theremin Defies Common Sense

For anyone in the UK interested in building it...

I used Maplin RF Chokes for the inductors (WH47B) - at a very reasonable 0.79 GBP each

The trimmer was a Maplin WL70M, costing 1.09 GBP

To be fair to Thierry...

I *was* careful with my layout, using wide spacing wherever possible.

My friend and I are studying AV Technology and we're trying to make one of these for an electronic project. We've just made it on a breadboard for the moment (despite the parasitic capacitance issue) and it seems to be working, though it is very high frequency, and requires your hand to be quite close to change frequency. Also the closer your hand is, the lower the frequency gets. We'd really like to get it to work better. Could anyone give us some help or advice? We'd really appreciate it. 

Update: I've gotten it working much better, by putting two more inductors in parallel with the others. Two need to be apart from each other and two need to be pushed together. Left oscillator and right oscillator respectively.  Any explanations? 

By putting inductors in parallel, you decrease the overall inductance which raises the oscillator's free-run frequencies which would make the circuit (when correctly built) too sensitive and thus not so well playable.

When you encounter a reversed pitch field as you describe in the previous posting, that means that the oscillators are not correctly adjusted and that the fixed oscillator is running on a too low frequency compared to the variable oscillator. To correct this, you will make sure several things:

a) Build the circuit on a circuit board (no need to have a printed circuit board, a veroboard style with copper dots not stripes!!! will do) following my recommended PCB layout. Apparently you are a victim of the parasitic capacitances of the breadboard.

b) Use a pitch antenna of exactly the indicated length and diameter

c) Adjust the variable capacitor called "COMP" and the antenna length precisely following my tuning instructions in the article

Theremins are sensitive devices. A tenth of a picoFarad in capacitance may mess up everything. And a picoFarad is a trillionth of a Farad (standard unit for electrical capacitance).

Hi! I'm having trouble finding the 2N5484 transistor or equivalent here in Slovenia, so could you maybe suggest an easy to find alternative?

Every NJFET with a Up between -1.5 and -3V and a Idss between 2 and 6mA should do the job. 

oh dear. I spent 20min searching through datasheets and am just completely lost. I could not find any Up and ldss values anywhere (maybe they have different names?). Could you please give me an example? I'm sorry but I'm a complete beginner.

Up is the so called pinch-off voltage. That means the Gate-Source voltage where no more Drain current flows. It is generally listed under the "Off characteristics" in the data sheets and it as sometimes also called Ugs(off) or Vgs(off).

Idss is the Drain saturation current which flows at Ugs=0V. It is generally listed under the "On characteristics" in the data sheets and it may also be called Ids(on).

You may try to replace the 2N5484 by the BF256A. By the way: Both types can be found and ordered online on eBay...

thank you for the explanation :) So would the BF256B be acceptable too?

I know they can be ordered from ebay but I'm trying to buy everything at a local store to avoid shipping fees and waiting time