I got an Open.Theremin.v3 about 3 months ago. It was a very easy kit to assemble, basically you just solder the pots, button and pins on the shield (you might want to use a fine point iron), then just put on the arduino, load the code and it is ready. You might want to have the pins rest on the arduino while you solder them on the shield, as this will prevent them from acquiring a tilt and needing to be bent to fit.
You will also want to find some aluminium pipes (4mm inner diameter) in order to make some decent antennas. Wire will do, although the pipes seem to give it better linearity, and also the do not vibrate for too much time if you accidentally hit the theremin.
You will notice that this circuit is quite susceptible to changes in temperature. If you calibrate early in the morning, you will notice the need to turn the pitch pot throughout the day to adjust for the differences in zero beat.
Other than that, the theremin is very responsive, extremely linear and can be set to be easy to play, if you have an at least half-decent sense of pitch (I normally play the classical guitar, so my ear is not as trained as much as, say, a violinist 's). I was able to play recognizable melodies in a week and now can play passably (as in "not too out of key") Rachmaninoff 's Vocalise. I also tried a Moog EW Standard in a shop, and I found it a bit more difficult to play, probably like the V3 on the high register.
The function of the register pot seems a bit nebulous at first. Reading the code reveals that the pot gives a value by which the pitch signal is divided. In the highest register you get the greatest resolution and noise (birdies on high frequency, likely due to unfiltered audio). The range is about 7 octaves.
The first register is 4 octaves.
Changing the register while holding a note will effectively transpose the note an octave up or down. Zero beat will also move. If you adjust zero beat to the distance to your body on high register, then turn the register lower, you will notice that zero beat moves forward. This is an inherent side-effect of how the code is written. Also, as you lower the register, you lose the ability to play the highest octaves, while the lower remain intact. However, the register pot truly adjusts sensetivity, as it manipulates how much of the instrument 's resolution is required to change pitch.
I personally find it easier to play on the lowest register, as it is easier to hold a steady pitch, and I do not need extended range.
I am also experimenting with reproducing the theremin voice of Clara Rockmore. Thankfully, someone had once posted an image of that waveform through an oscilloscope. Recreating that with wavetable synthesis is quite easy. Here is a wavetable I use:
https://pastebin.com/n3j3Nu79
So, all in all, you get a theremin that is tweakable, fun to use, and, as far as I 'm concerned, a true musical instrument.
