Build the "REMI Minor" (version 2)
(Reinvented Electronic Musical Instrument)

a DIY Project by M.J. Bauer

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

Preview -- 'REMI Minor' (mk2) Variant 

After implementing the local user interface code (for 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 the plan is to be able to select a preset using the handset, the local UI becomes kind of redundant, assuming that the command-line interface (CLI) has functions to configure the device and to set up user options. This was the primary motivation to develop a second version of the REMI.

Version 2, also known as the "REMI Minor" will simplify construction by omitting the LCD screen, keypad and audio high-pass filter. Many other features are optional, so that the REMI Minor may be constructed in stages, adding more features as desired.

A single-digit 7-segment LED display may be added which will normally show the selected preset. The LED digit will also show a "self-test" code on startup if a fault is detected. An optional push-button on the front panel will allow stepping through the 8 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 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. 

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 via the MIDI-USB adapter, 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. These are readily available for charging battery-powered devices via a USB cable. 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.

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 LED digit display board (schematic TBD) connects to the 'UEXT' header (10-pin DIL) already fitted on the Olimex board for I/O expansion. The LED board will use 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 'PRESET' 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 v2 Controller Module schematic (PDF)

Download Olimex PIC32MX340 manual (PDF)

The picture below shows the recommended layout for the mk2 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, coloured grey in the wiring diagram.

remi mk2 board layout

Right-click on image to download full-size copy.

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 ...___

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Last revised: April 2017

Go back to REMI part 1