My New Year gift to TW: A new theremin circuit

Oh yes, very important hint, dewster, thank you!

The small 6V voltage regulator can not provide enough current for the LM386 and would get ways too hot before breaking down.

Thus, the supply for the LM386 has to be taken directly from the 9V battery!

Dear Thereminists:

I built the circuit. I tried to MPF102, BF256 and BF245 transistors and the circuit has not worked. It's a bit difficult to find a JFET with V (gs) off = -1.5 / -3V and Idss = 2 / 6 mA, at least in my country. I am currently trying to get the J113. Is it possible to publish a list of replacements 2N5484 that are easily available and that work?

best regards

The J113 should work. But where is the problem to get the 2N5484? Although it is out of production, there are lots of sellers on eBay who have still big stocks.

btw.: nice sounding device, thank you thierry.

i built it with first with 2N5484 but also had some 2N5457 and 2N5458 and they both work good. got them from www.musikding.de but they are about triple the price of a j113. 

The 2N5457 and 2N5458 MAY work, depending on individual tolerances, but there is no warranty that they'll work since these are operated below their specifications. That's why I don't recommend these. If someone is experienced and knows how to handle a stuttering oscillator or ugly wave forms, they might naturally try out whatever FET they want. :-)

Do I have to change VCC from 6V to 3.3V if using J113? I'll receive my J113 order in 4 hours. I also ordered the BF256B by mistake instead of the BF256A. I have all of the exact parts except for the 2N5484.

Thank you so much for this, Thierry, and to everyone who tried to build the design. To me, this is the simplest, beautiful sounding theremin circuit with a very long proximity range that I could find that has proven reproducibility. I'm a beginner in EE and attempted my own designs using RC oscillators like the basic relaxation oscillator and Wien bridge oscillator, but I could only change the frequency substantially when my hand is an inch away from the antenna. I am very excited to build this and share the results.

The J113 supports a maximal Drain-Source Voltage of 40V. You should keep the VCC=6V.

Thanks Thierry! I'm having a problem with the mixer.

Both fixed and variable oscillators work fine at around 440kHz, and connecting the antenna to the other oscillator gives 420kHz.

I don't know why I am getting no output whatsoever from the mixer. Is there another simple mixer circuit I could use?

You don't need another mixer. If the fixed oscillator remains at 440kHz while the variable one goes down to 420kHz, the beat frequency will be 20kHz which is almost inaudibly high and will be damped by the low pass filter in the mixer. You will have to re-tune the fixed oscillator with the variable capacitor down to ~420kHz, too, to get an audible beat frequency. The active mixer acts at the same time as a 100Hz high pass filter, a 3kHz low pass filter and amplifier. Thus, as soon the beat frequency will be within the 100Hz to 3kHz range, you should get an audio signal at the output. 

You can also try to trim both oscillators temporarily for a 1kHz beat frequency without connecting an antenna (i.e. 440kHz and 439kHz) and then check the signals at the gate, source and drain pins of the mixer get with an oscilloscope, and post pictures here. I'm sure that we'll get it up and running.

"You don't need another mixer. If the fixed oscillator remains at 440kHz while the variable one goes down to 420kHz, the beat frequency will be 20kHz which is almost inaudibly high and will be damped by the low pass filter in the mixer. You will have to re-tune the fixed oscillator with the variable capacitor down to ~420kHz, too, to get an audible beat frequency. The active mixer acts at the same time as a 100Hz high pass filter, a 3kHz low pass filter and amplifier. Thus, as soon the beat frequency will be within the 100Hz to 3kHz range, you should get an audio signal at the output.  You can also try to trim both oscillators temporarily for a 1kHz beat frequency without connecting an antenna (i.e. 440kHz and 439kHz) and then check the signals at the gate, source and drain pins of the mixer get with an oscilloscope, and post pictures here. I'm sure that we'll get it up and running."

Sorry for the late reply. Last Friday, we were finishing our autonomous quadcopter project that we built from scratch. We wanted to use theremins as human proximity capacitive sensors so it would follow humans around while making sound, but could not get a decent range with our RC oscillators. Basically, the output of the theremin is sent to a high pass filter, and only when the theremin outputs at the cutoff frequency (human at a certain distance) will the microcontroller detect amplitude over a threshold.

Below is the pitch-only Thierrymin I breadboarded hastily, the top being the voltage regulator and mixer, and the bottom the fixed and then variable oscillator (with the trimmer cap and antenna removed temporarily):

The capacitor that connects the speaker with the mixer above is placed incorrectly. But after correcting it, we still could not get output from the mixer. The oscilloscope should have read even an ultrasonic output, but each of the JFET pins is not outputting anything.

We simply used a simple RC oscillator in our project, and I just want to finish this Thierrymin to give as a Christmas gift!

EDIT: When we showed our TA your oscillators he was really impressed.

EDIT2: Oops, I forgot to say that the 8k2 resistor in the mixer in the picture was misplaced--it was connected to G instead of S. We corrected it, but still no output.