Processing realtime EEG data from the OpenBCI system requires software running on a computer. For the EEGSynth project we do the rapid application development using the platforms that we are most familiar with, i.e. standard laptops and the FieldTrip toolbox, which is based on MATLAB. However, in the end we want to implement as much as possible using affordable and open hardware and software. Hence we opted for the Raspberry Pi, a credit card–sized single-board computer. It runs Linux, which makes it easy to use standard programming platforms and interfaces such as Python and Redis to implement the software stack.
In the first EEGSynth studio performance you can see Stephen in the middle, operating the MATLAB-based GUI for the EMG/EEG processing, and Jean-Louis at the back operating the synthesizer. The goal of the technological development is to put Jean-Louis completely in control and to make the interface of the EEG synthesizer as similar as his other modular synthesizer modules. Hence the need for fitting the Raspberry Pi into a Eurorack synthesizer case.
Here you can see some photo’s from the construction of the front panel.
The front plate has holes for the various interface ports to interface with the Raspberry Pi. For a sturdy mount I glued a section of L-profile rails to the front plate.
After mounting the Raspberry Pi, I connected the HDMI and audio port with a short cable to the front panel.
Here you can see the Raspberry Pi in the Eurorack case, next to the power supply.
I am working on fitting a Raspberry Pi as a Eurorack module in the EEGSynth. A previous post shows the completed case. Besides the Raspberry Pi which needs 5V, it will also hold a CV/Gate controller and some other modules for interfacing between the digital and analog parts of the synthesizer. The operational amplifiers and some other ICs on those modules require a symmetric positive and negative rails.
As I am not planing any critical parts that require an very stable voltage (such as a VCO) in my enclosure, I decided not to go for an expensive linear power supply, but rather construct one myself on the basis of two 12V switching power supply that I salvaged from some old wall-warts that once served some external 3.5 inch USB hard disks. The AC-DC converters have the 220V side isolated from the 12V side. This allows to connect the positive DC rails of one to the negative DC rails of the other, resulting in +12V and -12V from either converter relative to the common ground.
The Raspberry Pi requires quite a bit of current compared to most synthesizer modules, hence converting the 12V into 5V with a voltage regulator such as the L7805 would not be very efficient. Therefore I also added a 5V 2A AC-DC converter.
For safety I made a small plexiglass enclosure using the laser cutter at the Techlab Nijmegen. All electronics, except for the connector, switch and three LEDs are completely enclosed behind the aluminum front panel.
The connector with the synthesizer modules consists of a flat cable, wired according to the A-100 system bus.
The LEDs show the status for each of the three voltage levels. Surprising is to see that it takes a good 10 seconds for the capacitors of the +12V and -12V to completely drain when switching the power off.
I constructed a Eurorack case as enclosure for the synthesizer modules that I am working on. The basis is a single 86 HP rails cut into half and two end-plates from Clicks & Clocks. Using the laser cutter that is available at the Techlab in Nijmegen I made an exactly fitting box from some nice pieces of 5 mm multiplex.