EuroRack 6HP MCU Experimenter Module

I’ve been following the work of HAGIWO for some time and always been really impressed with the breadth and range of modules they produce. I have noticed some patterns in their microcontroller based modules and so wondered if it was possible to build some kind of generic template PCB that would allow me to do some experimenting whilst reducing the number of links and connections required between components compared to using protoboard.

This is the result.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

These are the key materials for the main concepts used in this project:

• HAWIGO’s Repository of DIY EuroRack modules.
• Doepfer EuroRack Construction Details.
• Doepfer EuroRack Technical Details.
• HAGIWO’s own generic PCB used for a dual EG. 

If you are new to electronics, see the Getting Started pages.

The Circuit

I wanted to include the following elements:

• Microcontroller interface – ideally an Arduino Nano or a Raspberry Pi Pico.
• PWM audio output stage.
• Gate/Trigger input and output stage.
• CV input and output stage.
• EuroRack compatible power connector.

I wanted suitable protection on the various stages, so have essentially correlated some of the circuits put up by HAGIWO and pulled out a range of elements into my schematic. I’ve been looking through the designs for the following:

• XIAO RP2040 VCO.
• AD9833 LFO/VCO Re-build.
• Euclidean Rhythm Sequencer.
• 2OSC Mozzi VCO.
• Mozzi FM.
• Additive VCO.
• Chord VCO.
• Overflow folding VCO (XIAO).
• General PCB from the Arduino Dual Envelope Generator.

Although the basic template for many of HAGIWO’s designs are very similar, often specific component values might be tweaked, so I wanted to ensure there was nothing too prescriptive in the design here to prevent tailoring.

In the end I used some specific component values from certain circuits in the schematic itself, but really they are just an example starting point. The values required will depend on the expected input and output levels and whether converting to/from 5V or 3V3 operation.

For reference, common EuroRack voltages are (from the Doepfer Technical Specification):

• Audio signals: 10Vpp i.e. -5V to +5V.
• Modulating Control Voltages: 5Vpp i.e. -2.5V to +2.5V (LFOs); 8Vpp i.e. 0V to +8V (EGs).
• Trigger/Gate: 0 to 5V, although up to 12V should be accepted.
• Pitch Control Voltages: Typically 1V/octave.

Key design principles for the control board:

• Will allow for a 5V Arduino Nano or a 3V3 Raspberry Pi Pico, with the circuits VCC taken from the power output of the microcontroller (so 5V or 3V3 respectively).
• Powered from the EuroRack +12V either into VIN of the Arduino Nano or via a L7805 generating 5V into VSYS of a Raspberry Pi Pico.
• Anticipating that not everything will fit on a single board, there is a header interface between the microcontroller and everything else.
• The connections to each of the circuit stages is via headers or blank solder pads, allowing patching across to any microcontroller pins as required.
• There is no support for controls, pots or jacks on the board, it is assumed those will be added to a panel and patched across to the control board using headers.
• Some mounting holes will be provided.
• Only the components and circuit blocks required for a specific build will be used. If using an Arduino Nano, this includes being able to omit the L7805 and associated power circuitry as it can be powered from +12V into VIN.

A future post of usage notes will go into the detail of each circuit element a little more.

EuroRack Power

I was pretty keen to include some protection for the EuroRack power side of things, following some of the advice from here: https://www.youtube.com/watch?v=cBGyEBQnIws.

The problem is that although it is possible to protect the 10-pin version of the EuroRack power connector from reverse connection, it is a lot harder to do with the 16-pin version.

Pinout from the Doepfler Technical Specification (note -12V is “pin 1” and usually denoted by the stripe on the cable – but not always!):

For the 10-pin version we essentially just have to cope with +12V and -12V being the wrong way round. With the 16-pin version it isn’t that simple and there are several problems if a cable is plugged in the wrong way round:

• The CV/Gate signals will overlap with -12V and GND.
• The 5V connection will overlap with GND.
• The +12V connection will overlap with GND.
• As there are three sets of GND pins, typically all connected together, the +12V and +5V of the power supply and any other modules in the system will be connected together!

That last one is very likely to cause damage to any module or power supply plugged into the rest of the rack. The worst-case scenario is that it could damage the PSU and ALL other modules in the rack… none are going to like receiving 12V when 5V is expected… err…

As this is a PCB designed for DIY experimentation, I wanted to ensure that the chances of a serious problem with any other kit it might be hooked up to are minimised.

But I should repeat at this point: do NOT use this module alongside any other expensive equipment or rack or power supply. It is meant for DIY experiments only and should only be connected to other DIY or otherwise sacrificial equipment.

Still, I’ve only “wired up” the following of the EuroRack power support connector:

In particular note that I’m only using one of the GND connections – the others are left not connected or linked at all, and especially not to each other. This means that should a cable to plugged in the wrong way round, there is nothing on my PCB that will bridge +5V and +12V via GND; it also means I can include some basic protection against +12V and GND being the wrong way round.

I’m also planning to use shrouded connectors, but of course that won’t guard against incorrectly wired cables or other weirdness.

I was in two minds about attempting to use the +5V rail, but as I couldn’t find any advice online about how to protect that, I just opted for the use of a L7805 and the +12V rail, which I was planning to use anyway as it seemed more “universal” than expecting a 5V too.

The final power related part of the schematic is thus:

PCB Design

I’ve deliberately designed this onto two boards – a MCU board and an IO board, with headers between them. The power circuit is part of the MCU board. The idea is that all of the components on the MCU board are on the underside; all the components on the IO board are on the top side; that they will link together; and that the power connector will be on the underside and all IO connections (jacks, pots, etc) on the top.

I’ve also overlapped the footprints of the Nano and Pico and linked up GND and VCC (5V for the Nano and 3V3 for the Pico), so either could be used.

I’ve deliberately kept the size of the PCBs below 30x100mm, but will get these built as two individual designs. But keeping it within 6HP and 100mm has made for a very packed board!

In terms of silkscreen labels, each section of the schematic has a specific numbering scheme – for example, all components associated with the first PWM circuit are numbered between 100-199. Similarly for all the other sections.

But this is not a PCB that is meant to be populated without referral to the schematic and some bespoke design on a per-board basis.

Closing Thoughts

This is not a board to be soldered up and just used. It is a template, meant to remove the need for some connections, with some commonly used “circuit patterns” allowing easy connection to a microcontroller and EuroRack power supply.

But I’m quite pleased with how I’ve done my best to minimise the chances of this board causing a problem with other DIY modules in a rack.

But for this more than any other boards – the usual warning applies. Do not use with expensive equipment. Test thoroughly, double check cable wiring, and do not use alongside other expensive modules in an expensive rack of kit.

In a future article, I’ll go through some notes on how I expect to be using it.

Kevin