Last updated Sat Aug 22 02:52:47 PDT 2009

This is just a quick rundown on a board I call "Fracuino". I did the schematic entry about 3 weeks ago, the board layout over a few days after that and then ordered a dozen prototypes to try it out. This page is based on the v1.0 layout and will hopefully evolve if this doesn't turn out to be a really stupid idea.

Fracuino is my name for yet another physical form-factor compatible with the Arduino open-source electronics prototyping platform.

My complaint with the existing modules is that the form-factors are mostly aimed at breadboarding or designs that are more or less "cobbled toegether". Note that this is a rather broad generalization and feel free to take quiet issue with my opinion if you like. What I wanted was a board with things useful for electronic music that could be built in to one-off modules in the standard Synth form-factors ( Frac / Euro / MOTM ).

What I came up with was a board that has the following major features

  1. *Duino compatible CPU ( Atmel mega8/168/328 ) with bootloader.
  2. FTDI TTL Serial cable connector. ( TTL-232-R ).
  3. MIDI in/out connectors and buffers.
  4. Single row connector of some flavor to access all I/O pins across a single edge of the board.
  5. Standard Synth power connectors.
  6. Onboard voltage regulator to supply special user desired voltage as needed.

Why these choices ( in order ).

  1. This one is a no brainer. I didn't choose a larger CPU like the 644P as its just so bulky and didn't fit this concept. Maybe Fracuino2?
  2. The TTL-232-R cable is cheap ( $20 ) and readily available and more importantly usable across multiple devices. No soldering any SMD parts and it only costs about 25cents to add a TTL serial port to a board so cost per board is negligible.
  3. Another no brainer I'd think. If MIDI isn't needed for an application, quite a few parts can just be left out. I think #4 should still make it a useful form-factor.
  4. The concept I had in my head was to put a right angle female connector down the edge of the board so whatever you are controlling can just be plugged in the back on perf board ( picture below ). Boards could also be stacked with common cheap connectors, soldered on at right angles. Heck, depending on what you are doing you might be able to just kinda solder it randomly onto the back of the board.
  5. Another no brainer. Doepfer and MOTM-4Pin power connectors. As 5vdc is needed, there is an optional regulator to derive it from the MOTM +15 rail. I know there is a new 6-pin MOTM standard. I haven't switched to it yet and I figure that 4pin is more common at this stage. Should be trivial to make a Doepfer to MOTM-6 cable if you really hate that 7805 regulator and have MOTM-6 available.
  6. This may seem like an odd thing, but there is space on the board for an additional arbitrary 78xx regulator. The idea for this form-factor came about while working on a project. I decided to make the board a little more generic so it was usable for more then just the one hairbrained idea.

The Fracuino concept isn't intended to be a fully programmable module that you are constantly making code changes to. This is *not* a PSIM-like board! It is intended as a shim to control and abuse circuits where once the code is done you want to bolt it in the rack and never see it again except by the front panel. This is a major reason why I chose the TTL cable vs onboard USB. While the various boards available on, or the breadboard compatible units like the Boarduino and Sanguino are great, I didn't think the cost or form-factors quite worked for a permanent installation in a modular synthesizer. Its also pretty trivial to add an Atmel CPU and the cheap TTL socket and minor support circuitry to whatever breadboard you are working on to make anything *Duino-like however I find that trivial doesn't mean its easy or not tedious. My hope is that this board will make certain tasks easier and faster to accomplish.

Anyhow, some pictures which may or may not make my idea/concept seem clear.


The bare board. Regulators at the top. Power connectors upper left. MIDI lower left. CPU in the middle. Lots of pins across the back. The pins across the back are wired in 30 rows. I found some right angle female .100" 20pin connectors at Jameco and chose them. They arn't quite a perfect fit due to connector body plastic thickness but a very little bit of sanding made them work fine. I will probably add a couple blank rows of pins to the next board rev. Also I need to add a little space between the MIDI connectors. They work fine with plastic jacks, but Jameco mixed in a few with metal faces and they are slightly wider.

Here is the first board, stuffed. Using Doepfer power so no 7805 installed at IC1, however I do have a 7808 installed [IC2] for +USER voltage at pin 6 at the back. The jumpers [JP2] next to the serial connector [SV1] are to disconnect the TXD/RXD lines from the MIDI circuitry during programming as both the MIDI libraries and the Bootloader need access to the USART. The jumpers [SV10] to the right of the CPU are to disconnect AGND/AREF/AVCC from the default and do something else with them. I figure most people will put in wire jumpers. Same for JP1 up next to the doepfer power connector, adding this jumper connects the two MOTM ground leads toegether. If you are running a full Star ground, GNDA goes to only one pin of the MOTM connector. also quite possibly fully ignorable.

Here's a piece of stripboard plugged in to the back. The Fracuino board is 4"x3" so it gets a little deep pretty quickly but for the things I have in mind is should provide more then enough proto area. I can also think of a few ways to use a deeper proto board with this however you'd need to run wires to the MIDI jacks although to be honest I'll probably use chassis mount jacks anyway as I'm not really a fan of the board mount ones. Originally I had thought to bring some traces to the "front" from the connector. Honestly I just ran out of room. I also think that odds are good you'd be bringing some analog signal up and probably want to shield that as it ran past the CPU.

That should about cover it. Oh yeah, the schematic pdf is here. Pretty much looks like every other board of this type out there. As of this writing I have a few boards if anyone is interested in being a guinea pig. So far I have found no outright errors on the board. The MIDI-IN opto circuitry works and the two transistor buffer on the MIDI-OUT seems to be fine also. As mentioned above I plan to tweak the rear connector layout a little bit and also the spacing on the MIDI connectors. I'd also love to shrink the board some. Very easy to do if I leave off a few things ( SV10 comes to mind ) but at this point I don't know what will be useful or not...

Tom Arnold /