Build the "REMI Minor" (mk2)
(Reinvented Electronic Musical Instrument)

a DIY Project by M.J. Bauer

This page is a continuation of an earlier article. See part 1 here.


REMI 'Minor' (mk2) Variant 

After implementing the local user interface code (for the REMI 'major' LCD screen & keypad), it dawned on me that most of the functions provided by the UI would probably not be needed very often, once the user has configured the device with their preferred options. Thereafter, the only frequently used function would be to change the instrument "preset". Since selection of a Preset can be done using the handset, the local UI becomes kind of redundant, now that the command-line interface (CLI) has functions to configure the device and to set up user options.

The REMI 'minor' simplifies construction by omitting the front-panel LCD screen and keypad. All parts shown on the REMI 'minor' schematic will fit into the prototyping area on the Olimex board. Many features are optional, so that your REMI may be constructed in stages, adding more features as desired. Of course, you can "cherry pick" options from both the Major and Minor build variants to customize your REMI. The firmware adapts itself automatically to support any build variant.

A single-digit 7-segment LED display may be fitted on the front panel which will normally show the selected preset number. On startup/reset, the LED digit will show a "self-test" code if a fault is detected. An optional push-button on the front panel will allow stepping through the 8 (or maybe 10) available presets.

Another option is to fit a MIDI-USB adapter inside the Controller Module which will allow the REMI to be connected to a USB port on your computer. The REMI can be powered either from the USB port, or from the DC inlet jack using an external 5V DC regulated power supply.

A classic MIDI OUT (5-pin DIN) connector may be fitted as well as, or instead of the MIDI USB connector, but if you prefer the classic MIDI IN connector, the MIDI circuit from the REMI 'Major' may be built instead of fitting the MIDI-USB adapter inside the case. If you have a keyboard instrument or MIDI controller with classic MIDI OUT connector, you can then plug it directly into the REMI for use as a sound module.

An EWI handset is not essential to creating music with the REMI. A planned new software feature will allow REMI's built-in sound synthesizer to be controlled by a computer or MIDI keyboard instead of being controlled by the handset. This capability should appeal in particular to makers who want to experiment with new sound patches, using a MIDI sequencer application to drive the REMI synth.

 

REMI v2 Controller Module sketch
REMI mkII Controller Module concept sketch

REMI mkII enclosure LH end view

REMI mkII encosure LH side view

Controller Module Design & Construction

The REMI version 2 Controller Module is based on the PIC32-MX340 proto board from Olimex priced at 19.95
(= US$22 = UKú18 = AU$28, approx.) plus postage.

A 5V DC supply rail is required, but the Olimex MX340 board doesn’t have a 5V regulator... only 3.3V. The recommended solution is to modify the board so that it is powered from a plug-pack PSU with regulated 5V DC output. Carefully remove the bridge rectifier (G2) and, in its place, solder a Schottky diode (1N5819) from the DC input jack centre-pin contact to the input of the LM1117 3.3V LDO regulator (VR1). Solder a wire link from the DC jack outer ring contact to GND. Fit a compatible plug to the cable on the PSU.

Alternatively, if you plan to fit a MIDI-USB adapter inside the REMI enclosure, the module can be powered always from the USB connector. Going with this option, there is no need for the DC inlet jack, so it may be removed from the board to prevent accidental connection of a PSU with the wrong output voltage.


Olimex PIC32-MX340 Prototyping Board -- size 100 x 80 mm.
(Pictures of an assembled board will be shown in a future update.)

I also recommend that the ICSP (PIC programming port) connector be replaced with a right-angle type so that it is possible to attach a programming tool (PICkit-3, etc) without opening up the enclosure. The ICSP connector is best removed by desoldering one pin at a time. When all pins are removed, clean remaining solder from the pad holes using solder-wick and a flux pen. 

For the enclosure, I chose a plastic box having dimensions of about 160 x 100 x 55 mm. The concept sketch (above) shows the planned layout. The Olimex board is mounted on the bottom (back) of the box using 10mm standoffs. Cutouts on the left side give access to 3 connectors - DC IN (5V), ICSP header and "RS232" serial port.

An optional front-panel LED digit display board connects to the 'UEXT' header (10-pin DIL) already fitted on the Olimex board for I/O expansion. The LED display board uses the 'UEXT' SPI2 signals to interface with the PIC32. A single IC (74HC595 - SI/PO shift register/latch) is all that is needed to drive the 7-segment LED from the SPI port. The front-panel 'SELECT' button will hijack the SPI 'MISO' signal for input to the PIC32 when there is no other slave device selected. (There are no other SPI slave devices anyway.)

REMI 'Minor' (Rev.B) schematic (PDF)

LED Digit Display board schematic (PDF)

Download Olimex PIC32MX340 manual (PDF)

The picture below shows the recommended layout for the REMI Minor controller board. Wiring should be done with very fine hookup wire. Suitable multi-strand wire in 6 colours can be obtained from 6-core flat telephone cable of the type used with modular (RJ12) crimp connectors. Wire guage is 28AWG (7 x 0.12mm strands). Multi-strand wire is a bit more difficult to solder neatly than solid-core wire, but it is more flexible and hence less likely to break when wiggled about during construction. Short links and most of the GND wiring are done using bare 28AWG tinned copper wire.

REMI_minor_Board_Layout_revA

Note: The board layout shown is Rev.A, which is obsolete! The picture is intended
for illustrative purposes only. A revised layout will be posted in a future update.

MIDI-USB Adapter

The REMI v2 prototype will use a low-cost MIDI-USB adapter cable obtained from an online supplier, AliExpress. The only construction challenge was getting the circuit board out of the sealed casing. I used a hacksaw to cut around the edges, then prized the casing apart with a flat-blade screwdriver.

MIDI-USB intact

The plugs will be cut off leaving short lengths of cable to wire the adapter into the REMI controller module.

Note in the picture below that a few components have been omitted from the PCB, e.g. U1, D1 and R5. 
I'm guessing U1 was an opto-coupler for the MIDI input. 
The MIDI Specification 1.0 calls for optical isolation in the MIDI input to avoid earth loops which can cause induced noise in audio circuits. The screen of the USB cable was suspiciously unconnected, perhaps to achieve the same effect as the opto-isolator, i.e. to avoid a potential earth loop? (Petty cost-cutting in the extreme!)

MIDI-USB board

The flip side of the MIDI-USB circuit board has 3 LEDs labelled 'PWR', 'IN' and 'OUT'. The LEDs can be made visible through holes in the REMI front panel.

... to be continued ...___

But don't wait for me to complete a REMI Minor prototype before you start to build yours. The Rev.A (Major) prototype has already verified the hardware design and firmware functionality. If you need help to source suitable components, or with construction and testing, just send me an email.



Send comments, suggestions and inquiries to:

Last revised: 17 May 2017

Go back to REMI part 1

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