My prototype designs for MT32-pi breakout boards

[lausvi]

Hi!

I made some prototype PCB designs, in planning to build a housing for the mt32-pi. I am just a beginner, but I thought this would be a great project to design something small and simple.

My idea is to run a 40-pin GPIO cable from the Raspberry Pi, and on a small front panel board to break it out to the required pin headers (display, DAC, IO board). The front panel PCB will to the side of the LCD, making a low-but-wide front panel, bit like the original MT32's front panel.

I got my first PCBway prototype boards from the mail today, and in the gallery linked below, you can see my first revision. There are some issues to be revised: mainly that 1) the encoder's button doesn't work, 2) LCD-panels mount holes do not match the front panel's holes, 3) the 3,5mm jacks on the IO board are on the wrong side of the board (I had to draw the footprint manually, so I must have put it wrong way round... d'oh!) and that 4) the layout around the GPIO connector is a bit crowded. But most importantly IT WORKS and it's starting to look much nicer than first rev on a breadboard with wires everywhere :)

I have ordered a GY-PCM5102 DAC but haven't received it yet so the I/O board is not yet populated. The idea there is to have a small PCB just for the rear panel; the midi in (5-pin DIN), aux in and main out (3,5mm) are located there. The aux in is mixed with the MT32-pi's output (a bit like in Roland SC-55 and probably other midi modules).

Here are some photos: https://postimg.cc/gallery/FQfkMJF :)

I'll need to do more tweaking on the design, but when I am happy with it, I'll surely share the KiCad files!

[dwhinham]

...I am just a beginner...

Wow, great job! I can see how the 16-char size fits well for this design. I'll get onto that pretty soon; I'm looking at OLED/LCD-related code at the moment. 🙂

Congrats on the first boards turning out alright and mostly working - it usually takes a few goes to get it perfect, but it looks like you have a great working starting-point to iterate on.

I have a wiki page about the GY-PCM5102 now with some hints on setting its jumpers up properly in case you hadn't seen it. The Aux-In idea is useful, many syntheiszer modules have this feature, SC-55 as you said and my MU2000 both have this.

I'm sure people will appreciate the KiCad files when you're happy to share them.
Thanks for posting!

[lausvi]

Ok, time for an update!

I wanted to have a proper case for my MT32pi, and finally got around making something substantial! I remembered a piece of audio gear I had lying around; a Pioneer audio timer (ie. one of those mains switch-boxes where you could set a timer, an outlet would come on and then a compatible audio device (like a tape deck with timer controls) would start). I got this for free from an audio-dealer friend who was going to toss it as it had been sitting on the shelf for years and "nobody buys these", I was told. I did felt some remorse repurposing the case, but the fact is that I had no use for the switch as it was. It also was not in pefect condition and it had couple of faults (bad contact on the mains switch, issues with the mode selector). Nothing that could not have been fixed. Anyway, I went on and started figuring out how to fit everything in...

The display window was much bigger than the original display, and just about perfect size. I was able to cut the internal display fram a bit larger to fit the LCD in there. It currently has the 2x16 LCD in. The 2x20 could be fitted, and I might do it later. Power is supplied by an external 5v PSU with a barrel-jack plug (the socket fitted straight into the hole that was the mains cable gland!). I repurposed the original mains switch on the rear, looks nice. I was able to get an encoder that fitted the original knob, too (its just the volume knob + button now). There are two LEDs on the front, red one is now being used for power on. The other green one would be great as a MIDI message -indicator, if we can have that functionality in the future ;)

The main PCB I designed for the previous post was used as a break-out board. The smaller IO-board on the rear houses the MIDI-interface plus the audio section (GY-PCM5102 DAC, plus an optional analog mixer).

Using various screws, standoffs, rubber feet and double-sided tape I was able to mount everything inside the case pretty neatly. I used two of the front panel buttons (with the custom board to hold the switches). The rest will probably be left unused. I left the switch-bank on the end for now, if I decide to use them later. I might try to print some larger stickers and cover the holes. Also I'll try to scrape away some of the texts on the front panel.

The vertical button-holes on the left could be used to house something fun like a LED-VU-meters, but nah, I don't think I'll bother.

I didn't have my powertools at hand so the rear panel is still somewhat work-in-progress. The MIDI in is at the moment wired into an external connector, but there is room on the rear and I should be able to get them neatly there later on. Also I might add some vent-holes under the Pi, although it seems to work fine even with the case closed.

Gallery here with more pics of the inside.