The OpenTheremin is a stand alone Theremin with built in oscillators. Through the control voltage outputs (CV) an existing modular system can be connected and controlled easily. However the design of the OpenTheremin version 4 allows the electronic circuit of the OpenTheremin to be built into a modular Theremin system itself. This page describes a possible arrangement of such a built.
The dimensions of the OpenTheremin cirucit fit with the Eurorack format. However with the antenna and signal connectors there is some tinkering needed. The circuit does not feature a standard Eurorack connector yet and so we will have to be creative there.
Here is how it can be done.
1. The modular case
For this build I decided to go with the FRAPS TOOLS PLUS modular case. The case 42HP case is made from extruded aluminium with two wooden sides. The case would not need to be metal and in fact a wooden case may make the isolation of the antenna wire even easier.
2. The power supply
The OpenTheremin circuit needs to be powered with a stable 5V DC (max 100mA). So any rack power supply that also offers 5V should be fine. I decided to build one that can be powered through a simple USB-C connection. For this I used a cheap symetric +/-15V step-up converter module followed by two linear regulators.
As a commercial alternative I want to recommend the psu II – 1hp power supply solution by our friends at seismic industries. This super slim USB-C power module provides +/- 12V and 5V you need for the Theremin.
My DIY power module was then wired up to a USB-C break-out board that screws to the case.
3. Antenna connectors
For the modular theremin I choose a common TNC connector to mount the antennas to the wooden sides. The aluminium tube connects to the outside of the connector as it does not actually need the co-axial connection. It just makes for a solid and easily removable connection.
The nut on the TNC connector had to be drilled slightly bigger to fit the 6mm aluminium tube. The tube was then just glued in with epoxy and the electrical connection made with a bit of conductive silver glue.
A stiff wire connects the antenna terminals with the screw terminals on the theremin board. The wire is fixed with plastic wire mounts to prevent the antenna-wire from touching the metallic case.
3. Signal Beak-Out Board.
Next the output jacks need to be wired up. All the signals can be found on soldering pads at the edge of the board.I made a small front pate with holes for the jacks and labelled them accordingly. The gate pad is quite close to the edge and I had to be careful for it not to touch with the rack frame.
I then screwed everything in and I think it is looking great. The audio signal can be used in combination with the control voltages and the gate.
4. Choosing Modules to combine with the Theremin
Finally the most creative part is choosing Eurorack modules to combine with the OpenTheremin.
To emulate the sound and functionality using the control voltages a VCO (Voltage Controlled Oscillator) and a VCA (Voltage Controlled Amplifier) are needed. I build and experimented with the modules from MRG Synthesizers based in the UK.
For a wider variety of sounds and the possibilities to experiment with gate and envelopes I also put it the new Behringer Brains module. This module offers I wide variety of sound generators and has several CV inputs to control pitch, volume and parameters of the sound. The trigger input can be connected to the OpenTheremin gate to play percussion sounds. The Behringer Brains is a derivate from the famous Mutable Instruments Plaits and is based on its open source code. Unfortunately Behringer decided to not make their code available openly.
A small mixer in the setup proofed to be very handy when combining the different audio sources and to adjust the final volume. The perfect fit for me was the simple ABC Six Channel Mixer from Bastl Instruments. I had the chance to meet Václav and Ondřej form Bastl Instruments several times and they are just great.
So this is it. Time to patch all up and have fun. OpenTheremin V4 boards are available at gaudishop.ch
MIDI implementation for OpenTheremin V4 is here! While the Open.Theremin has it's own sound generator and can be played as stand alone instrument the MIDI software allows you to connect your theremin to any MIDI device or various MIDI synthesizers installed on you computer. This largely extend the functionality and is just great fun to play.
The following MIDI implementation and instructions were developed by Vincent Dhamelincourt and published in a friendly forked GitHub repository.Vincent is an experienced electronics and firmware engineer from Lunel, France and long term member of the OpenTheremin community. *** A great thank you! ***
Why MIDI on a theremin?
To know more what you can do with a MIDI on a theremin see the following document. All the tips here can be applied to Open Theremin. MIDI-and-Theremin1.0.pdf
Connecting your OpenTheremin V4 to MIDI
There are two possible ways to connect your OpenTheremin via MIDI. You can go through the USB cable to your computer and use a serial to midi router or you can solder a true MIDI connector the OpenTheremin board.
MIDI synthesizer on the computer
To control a MIDI synthesizer on your computer the OpenTheremin with MIDI software is sending MIDI at 115200 baud through USB serial.
You can also use the OpenTheremin MIDI implemented to communicate with a real MIDI device.
With a MIDI port the OpenTheremin can be connected directly to any MIDI compatible synthesizer or to a computer through a USB MIDI interface.
To do so add a female 180° five-pin DIN connector to the pads indicated with VCC, TX and GND. Use 220 Ohm resistors on the pins VCC and TX as shown here:
For the OpenTheremin Version 4.5 with break-out connector use a Flat Ribbon IDC wire cable with 10pins and 1.27mm spacing (link).
The pinout of the cable is as follows. The red wire is wire no 1.
Connect the DIN connector to the wires 2 (VCC), 6 (RX) and 10 (GND) as follows:
Set the baud rate to 31250 by changing Serial.begin(115200); to Serial.begin(31250); in the code (see instructions below).
Download and install the MIDI code for OpenTheremin V4 from the following git repository and carefully read the instruction below. I recommend to first setup and test the theremin as described in the instruction that came with the theremin (using built in sound generator) and then move to MIDI.
Open the File "Open_Theremin_V4.ino" in Software/Open_Theremin_V4/
In "Application.cpp", take care of selecting MIDI mode that corresponds to your situation (put "//" in front off inadequate line - MIDI through serial is selected by default here): Serial.begin(115200); // Baudrate for midi to serial. Use a serial to midi router.
//Serial.begin(31250); // Baudrate for real midi. Used with a DIN connector.
The MIDI open theremin generates NOTE ON/OFF messages and Continuous Controler changes (MIDI CC) depending on settings and hands positions next to antennas.
NOTE ON/OFF:
In MIDI standard NOTE ON/OFF messages have a NOTE NUMBER and a VELOCITY.
Let's consider a Fade-in / Pitch Variation / Fade-out sequence (I use right handed convention):
Fade-In When left hand moves away from VOLUME ANTENNA (LOOP) and volume crosses a settable threshold (Volume trigger), a NOTE ON is generated. VELOCITY depends on how fast left hand is moving. Right hand's position next to PITCH ANTENNA (ROD) determines the starting NOTE NUMBER.
Pitch variation When right hand moves next to PITCH ANTENNA (ROD), PITCH BEND messages are generated (if activated) to reach exact pitch as long as pitch bend range will do. Beyond, a new NOTE ON followed by a NOTE OFF for the previous note are generated if legato mode is activated. Pitch bend range can be configured (1, 2, 4, 5, 7, 12, 24, 48 Semitones) to align with synth's maximum capabilities.
Fade-Out When left hand moves close to VOLUME ANTENNA (LOOP) and volume goes under Volume trigger threshold, a NOTE OFF is generated to mute the playing note.
MIDI Continuous Controllers (MIDI CCs):
It is possible to assign independent MIDI CCs to the PITCH ANTENNA (ROD) and to the VOLUME ANTENNA (LOOP).
Configuration:
There is two calibration mode. Best use a speaker connected to the theremin to verify calibration and timbre variation.
Normal calibration of antennas: press button for 5 seconds -> Runs normal calibration of antennas (LED blinks until finished). Audio processing from antennas to output jack, including volume and pitch pots, LEDs and button functions, is exactly the same as in open theremin V4. You can play the Audio and the MIDI side by side.
MIDI configuration:
REGISTER pot becomes "SELECT PARAMETER" TIMBRE pot becomes "PARAMETER VALUE"
To change the settings, select a Parameter from the list below and move "Parameter's Value" to change corresponding setting. While you rotate the pots, the LED toggles (OFF/PINK) every steps to give you some angular feedback before going back to PLAY/MUTE Status.
Register: 4 positions as in original Open Theremin V3
Timbre: 8 positions as in original Open Theremin V3
Channel: 16 positions (channel 1 to 16)
Rod antenna mode: 4 positions (Legato off/Pitch Bend off, Legato off/Pitch Bend on, Legato on/Pitch Bend off, Legato on/Pitch Bend on)
Pitch bend range: 5 positions (1, 2, 4, 5, 7, 12, 24, 48 Semitones). For classical glissando and in order to have same note on audio and MIDI, use exactly same pitch bend range on your synth. Maximum setting possible is recomended.
Volume trigger / Velocity sensitivity (how fast moves the volume loop's hand): 128 positions (0 to 127)
Default configuration is: Register = Lowest Register, Timbre = 1st Waveform, Channel = MIDI Channel 1, Rod antenna mode = Legato on/Pitch Bend on, Pitch bend range = 2 Semitones, Volume trigger = 0, Rod antenna MIDI CC = None, Loop antenna MIDI CC = 7-Volume.
MUTE BUTTON: Sends ALL NOTE OFF on selected channel and stay in mute until it is pushed again.
AUDIO: Audio processing from antennas to output jack, including volume and pitch pots, LEDs and button functions, is exactly the same as in open theremin V4. You can play the Audio and control MIDI devices side by side.
CV GATE: CV GATE processing from antennas to output jack is exactly the same as in open theremin V4. You can control your analog devices and MIDI devices side by side.
CALIBRATION: This device runs normal calibration of antennas after pushing button for a few second as per initial project
Results: As long as you are inside pitch bend range you can play with the same linearity as with device's sound (same pitch is audible on stable notes without vibrato). On 24 semitones wide pitch bend range capable monophonic SW synth (tested with Gforce's Minimonsta), the effect is rather good: We play on 4 octaves just like a normal theremin. 12 semitones capable synth are also good but sometime note transition can be heard.
SEE VIDEO:
HINTS and NOTES:
What can I do to get a theremin like glissando? Activate picth bend and set pitch bend range of the theremin with a high value (12 semitones or 24 semitones). Set pitch bend range of the synth with the same value.
If I do not trigger with the volume hand it also seems to trigger a new tone with the pitch antenna. Guess this is how MIDI works. When legato mode is activated, if you trigger a note (with volume loop) and go in one direction (with pitch antenna) a new note will be triggered at the limit of pitch bend range.
Legato mode is used as a workaround for a limitation of MIDI (max 24 semitones pitch bend). Maybe some synth can perform pitch bend on more that 2 octaves but none of mine does...
Tweakable parameters (in application.cpp): Changing this to your taste may require some test and trial.
"#define VELOCITY_SENS 9" -> How easy it is to reach highest velocity (127). Something betwen 5 and 12.
"#define PLAYER_ACCURACY 0.2" -> Pitch accuracy of player. Tolerance on note center for changing notes when playing legato. From 0 (very accurate players) to 0.5 (may generate note toggling).
Percussive Sounds The volume trigger can be configured so as we have some volume at note attack on percussive sounds. The volume trigger setting is also used to set sensitivity for velocity (how fast left hand is moving when note is triggered). Volume trigger = 127 (Maximum) won't generate any NOTE ON. It can be used to generate MIDI CC only.
Manipulation of MIDI CCs Manipulation of "Rod antenna MIDI CC" and "Loop antenna MIDI CC" is not error proof. MIDI newbies should be advised to change their value in MUTE mode.
Added and removed compare to Open Theremin V4. Serial communication implemented for program monitoring purpose was removed (Particularly during calibration). If you need to monitor calibration for antenna problem fixing, please use original master branch.
Serial port is used to send MIDI messages now.
Support request about MIDI can be directed directly to Vincent here.
LICENSE:
Original project written by Urs Gaudenz, GaudiLabs, 2022 GNU license. This Project inherits this 2022 GNU License. Check LICENSE file for more information All text above must be included in any redistribution
The software of the OpenTheremin V4 can be easily updated to new versions that may unlock new features. The OpenTheremin is compatible with the Arduino UNO standard and all software is open source. Follow the instructions below to load new software to your OpenTheremin.
Install Arduino IDE
To upload software to the OpenTheremin get the latest version of the popular Arduino IDE programming environment. Download and install here: http://arduino.cc/en/Main/Software
Download the latest OpenTheremin V4 software from the public GitHub repository. To download all the files you need, click on the green "Code" button and choose "Download ZIP" here:
Control Voltage and Gate signals (abbreviation CV/gate) are analog methods of controlling synthesizers, filters, drum machines and other similar equipment with external signals. The OpenTheremin V4 generates two control voltage signals form the pitch and volume antennas. Additionally a gate signal can be generated to control effects or envelopes. While the OpenTheremin has an integrated tone generator (Audio output) these signals can be used to expand your setup and enrich your thremin sound by control any equipment with CV and gate inputs.
Coralie Ehinger has developed a unique way of combining the Theremin instrument with modular synthesizers while preserving the classical tone of the instrument. See here for here videos.
Control Voltage
The control voltage signals (CV) of the OpenTheremin V4 are accessible via the 3.5mm jack marked CV. The jack provides both the pitch and the volume CV signal through the two channels. To access the two signals best use a stereo to 2x mono splitter cable.
Splitter cable 'stereo to 2x mono'.
The pitch signal is on the red (right) plug/socket, the volume signal on the black/white (left) plug/socket. Be careful to get a true 'stereo to 2x mono' cable as most splitter cables on the market are '1x stereo to 2x stereo' instead. OpenTheremin connected to a Waldorf 2-pole filter through CV.
By default the OpenTheremin V4 is set to output logarithmic control voltage with 1V/Octave. This is a common standard as used by synthesizers such as Moog and Roland. The characteristic can be changed to linear control voltage with 819Hz/V as used by Korg or Yamaha by changing a software flag. Just change CV_LOG=1 to CV_LOG=0 "build.h" file and re-upload.
For the volume most synthesizers "exponentiate" the volume control voltage themselves, thus OpenTheremin send the "raw" volume signal to CV.
Gate
The gate signal can be tapped by a splitter cable via the OpenTheremin Audio output. The black/white (left) channel is the mono audio signal, the red (right) channel is the gate output. The gate triggers at a fixed level of the volume antenna. To activate the gate set the trigger levels by change the following lines in the code and re-upload:
In the file "build.h":
// Set the trigger levels for the Gate signal (0 to 127, 128=off) #define GATE_ON 20 // That's the level which will drive the Gate high when volume increases from lower #define GATE_OFF 16 // That's the level which will drive the Gate low when volume decreases from higher
The code for the control voltage and gate was contributed by "Theremingenieur" Thierry Frenkel. For the gate he developed a special impedance sense algorithm to prevent shorts when plugging in mono jacks. Kudos and a big thank you to Thierry.
MIDI implementation for OpenTheremin V4 is here! While the Open.Theremin has it's own sound generator and can be played as stand alone instrument the MIDI software allows you to connect your theremin to any MIDI device or various MIDI synthesizers installed on you computer. This largely extend the functionality and is just great fun to play.
Control Voltage and Gate signals (abbreviation CV/gate) are analog methods of controlling synthesizers, filters, drum machines and other similar equipment with external signals. The OpenTheremin V4 generates two control voltage signals form the pitch and volume antennas. Additionally a gate signal can be generated to control effects or envelopes. While the OpenTheremin has an integrated tone generator (Audio output) these signals can be used to expand your setup and enrich your thremin sound by control any 
