Question about Open Theremin V3

Posted: 9/19/2021 6:49:13 AM

Joined: 9/19/2021

I have bought an Open theremin V3 kit. And before I placed the potentiometers, I checked their resistance... Each one had 1.5k except one, which had 100k. So I don't know where should I put it.
Now I soldered it into the "timbre" place.
Timbre works, and everything seems to work, but I am not sure if the pitch control is working fine, I feel like if it doesn't change much (but it does). I haven't used any theremin before so maybe I am just not used to it.

It is worth mentioning that it is not sounding good yet, there quite a lot of noise in the background, but I am trying to improve my grounding (and finding the most stable psu I can) and thinking of replacing the Arduino as it might not be an original one.

It is sounding exactly the same as the first video in this post. There's a second video in that post, but I didn't hear that in my theremin.

Thanks in advance

Posted: 9/19/2021 10:24:20 AM

From: Occitanie

Joined: 3/4/2012


It is strange that you received 1.5K and 100K pots in your kit. They are suposed to be 10K which is the typical value for such application. If you look at the schematic, the 4 pots are used in the same way, having different values is also surprising.

The timbre pot should have significant impact on pitch (if I put my right hand at at given spot and turn this pot, then the variation of pitch is arround the octave).

Voltage on pot's cursor should move lineary for 0 to 5V as you turn it. Use the dedicated test points to measure it (free solder pads near upper right connector that goes to the arduino)

As there is some room for pot value variation, it could hapen that you configuation somewhat works fine. But in any doubt maybe consider replacement with proper value (10K)

I understand that you bought a genuine kit. Maybe liaise with Urs Gaudenz (contact at open theremin) for some support or appropriate parts.

If you have same sound as in it means that your amplification device is highly cliping.

Posted: 9/19/2021 2:27:11 PM

From: Colmar, France

Joined: 12/31/2007

First: On a v3, the timbre pot should not affect pitch, at least not with the latest (v3.1) software. The number of wave table entries being equal for all wave tables and the pitch iteration step being independent of the timbre setting make pitch jumps of nearly one octave just impossible. Changing the timbre means just changing the base address of the wave table, as one can see in the open source code.
Second: Before interpreting whatever, we should know how @Somanos measured the potentiometers. These are used as voltage dividers in the OT circuit, not as rheostats. Thus, the nominal resistance has to be measured between the two extreme pins of the three and be independent of the wiper‘s position.
Finally: Knowing well the atmega 328p MCU used in the Arduino uno (see its data sheet), one should understand that for the relatively coarse quantization on the hi-impedance analog input pins, the absolute value of the potentiometer’s resistance is not important and that almost every value between 100 Ohm and 500kOhm would work well.

Posted: 9/19/2021 6:23:28 PM

From: Occitanie

Joined: 3/4/2012

On @Thierry first coment: 

The timbre pot should have significant impact on pitch (if I put my right hand at at given spot and turn this pot, then the variation of pitch is arround the octave).

Excuse my typo error... Of course, one should read "The pitch pot should have significant impact on pitch..."

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