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Simple MIDI Music Keyboard BY TIM BLYTHMAN This MIDI Keyboard is a follow-up to our 64-key MIDI Matrix. It is similarly flexible and offers a way to easily make music, although it can be repurposed for many other uses. W hile MIDI Matrix panels are popular for being a compact way of controlling and interfacing to MIDI equipment, a linear keyboard arrangement like a piano is more ‘standard’ and, for many people, quite intuitive. This is a modular add-on to the MIDI hardware we introduced in April and May this year (siliconchip.com.au/ Series/363). Like the MIDI Matrix, it doesn’t have to be used strictly for MIDI or musical purposes. The MIDI Matrix was designed to be used with an Arduino Leonardo board, as the Leonardo can easily provide a native USB MIDI interface through the versatile Arduino MIDI libraries. We also demonstrated a few different program sketches that can run on the Leonardo to give various features, and showed some ways to interface with software on both a PC and an Android smartphone. At the same time, we presented an Arduino shield that lets you interface the hardware to a great range of MIDI equipment using standard DIN connectors. The Keyboard is intended to replace the Matrix as part of a larger construction, as presented in the earlier parts of this series. Refer to those articles, particularly the first part, to understand how the Matrix (and now Keyboard) can be used. At a minimum, you need an Arduino Leonardo board and some jumper wires to turn the Keyboard presented here into a minimal MIDI Encoder. 92  Silicon Chip The Matrix The original Matrix is basically just an array of pushbuttons that the Leonardo can scan to receive user input. In our MIDI software, each keypress is converted to a musical note. Each row or column of the Matrix is wired to a digital pin on the Leonardo. By using the time-honoured technique of scanning each row in turn, individual button presses can be detected. In our version of the software, the rows are connected to pins configured as inputs with weak pull-ups. Initially, all column pins are set to a high impedance input mode too. Each column is configured as an output in turn, and driven low. If any button connected to that column is pressed, its corresponding row pin is pulled down through the switch contacts. By scanning the columns in turn, we can detect individual button presses. While this system is simple, it cannot detect multiple simultaneous keypresses; for this, each switch needs to be fitted with a diode to prevent ambiguous closures propagating through the Matrix. Our Matrix omits these diodes in favour of simplicity and compactness, and this linear Keyboard is the same in that respect. The new Keyboard We considered a linear keyboard for our original design but could not work out a way of making it both compact and functional. We have now formulated a modular design, so a useful Keyboard can be built that is still compact, or it can be expanded to 64 keys, resulting in a device that’s over a metre long! But it still only needs 16 wires to connect it to the Arduino. The basic unit of the Keyboard is a single PCB with eight keys. Each key is wired to the same row contact as the others and also to one of the eight column contacts. A single Keyboard module is identical to one row of the Matrix. Fig.1 shows the circuit. CON1 is wired to the columns, with each terminal on CON1 wired to one side of each Our prototype uses three of these PCBs, as a keyboard made from a full set of eight PCBs would be well over a metre wide. We’ve retained the CON1 and CON2 pads on some of the boards to demonstrate and test the different options. In practice, only one set is needed; note that connecting to CON3 and CON4 is equivalent. Australia’s electronics magazine siliconchip.com.au Fig.1: this is the simple circuit of a single PCB with eight switches. The offset between CON4 and CON6 is what makes it easily expandable up to eight PCBs and 64 buttons. of the tactile switches, S1-S8. Position 1 of CON2 is connected to the other side of switches S1-S8. At each end of the Keyboard module PCB are connectors CON3-CON6, which can be used to daisy-chain subsequent PCBs to expand the Keyboard. These are eight-way surface-mount pads spaced 2.54mm apart. CON3 and CON5 (on the top side of the PCB) are wired in the same order as, and in parallel with CON1. Thus, the column signals can pass between the PCBs by joining their adjacent CON3 and CON5. These are wired as a parallel bus. Similarly, on the back of the PCB, CON4 on one PCB connects with CON6 on the next. CON4 is wired the same as CON2, but the clever part is how we have wired CON6. Pin 1 of CON6 is wired to pin 2 on CON4, and so forth, all offset by one position. Say we wired up an array of eight of these modules, numbering them 1-8 from left to right, with CON3 and CON4 wired to CON5 and CON6 respectively. Connecting to CON1 & CON2 on the first module, we would have the equivalent of a full 8x8 Matrix only with the keys in a single row. Fig.2 shows how the ‘rows’ are mapped back to CON2 on the first PCB. CON1, CON3 and CON5 are all simply wired in parallel and are not modified by this system. Other configurations If you look closely at the PCB, you can see that the little tab where CON1 and CON2 jut out is scored for removal. This lets you remove these tabs on all but one module. In fact, since CON3 and CON4 are wired identically to CON1 and CON2, you can even remove the tab from all boards and simply take the matrix connections from CON3 and CON4 Fig.2: this shows how multiple 8-button Keyboard PCBs are joined so that the Arduino can tell which key has been pressed. Each PCB along the chain offsets where the connection is ultimately made at CON2, allowing for up to 64 keys to be sensed. siliconchip.com.au Australia’s electronics magazine August 2021  93 of the leftmost board instead. If you don’t mind remapping the pins in software (or changing how they are wired back to the Leonardo board), the CON1 and CON2 connections do not have to be made on the first board. You could even take these connections from the middle of the array. We’ve designed the PCB to use large 12mm tactile switches, as these have a much nicer feel with a larger finger surface. You might find that some smaller switches can be made to fit by bending their leads, although we haven’t tried that. Since there is less space for routing on this PCB than the Matrix, it lacks the option to fit illuminated switches that the Matrix had. Hardware Like the Matrix, the Keyboard we are presenting has quite a basic design, so that you can customise it to your requirements. The switches are placed on 20mm centres, with four M3 mounting holes provided on each PCB. Nominally, the mounting holes will be on 40mm centres, although this depends on the accurate assembly of adjacent boards. The PCBs are 20mm high, not counting the tab for CON1 and CON2; 28mm with the tab in place. We strongly recommend mounting the Keyboard to a sound backing so that the PCBs do not flex when the keys are pressed. The connections for CON3-CON6 will not provide much mechanical strength as they are effectively surface-mounting pads, and are only bonded to the PCB superficially. Construction Most people will want to build a Keyboard with multiple PCBs laid out as a continuous strip, so we will describe what is needed to achieve this. The Keyboard is built on a PCB coded 23101213 that measures 158 x 28mm. Use the PCB overlay diagram, What about the black keys? You might be thinking that pianos actually have two rows of keys, white and black, and you would be right. Also, there are seven white keys per octave, not eight. We have kept this as a linear array of eight keys to make it simple and applicable to a wide range of applications. We plan to produce a 12-key PCB at a later date which has the keys staggered and grouped like a piano. In the meantime, if you’re keen to use this board like a proper piano, you could build it in two rows, with the top row offset horizontally 6mm from the bottom row and with gaps in the keys at the top to give the proper configuration. Both rows could be wired up in series (assuming they contain no more than 8 PCBs total). The software could be modified relatively easily to remap the two rows of keys into the correct sequence so that it can act as a keyboard piano. The restriction of only one keypress being detected at a time would remain, though. Our planned future piano keyboard PCB would remove that restriction. Fig.3, as a guide to fitting the components. Plan and lay out the modules before commencing construction. To keep things compact, the connections between the boards are a little tight, and it will be easier to join them before fitting other components. If you want a different layout, just about any method of wiring CON3 & CON5 and CON4 & CON6 respectively will work. You might even like to use header sockets on one and header pins on the other to allow the units to be unplugged, although this will not achieve a tight spacing. To start the PCB assembly, snap off any CON1/CON2 header tabs that are not needed. Do this by scoring along the line with a sharp knife to cut the copper traces, then carefully flex the PCB with pliers to make a clean break. You might like to clean up the rough edge. As well as our usual warnings about avoiding the inhalation of PCB dust (eg, by working outside and wearing a mask), take care not to file away the traces which run close to the edge of the PCB, especially at the back. Each PCB is 158mm long, meaning that there is 2mm of spare space for a joiner if the key spacing is to be kept even. We used cut-down double-row pin headers. The plastic spacers are very close to 2mm deep, giving the necessary spacing. Start by cutting down the headers to be used for joiners. This is fiddly but necessary, as there is no more than 8mm between adjacent switch bodies on neighbouring PCBs, and typical pin headers are around 11mm tall. You can halve the number of cuts by shifting the pins in the plastic. Place the PCB on a hard flat surface and rest the 2x8 pin header in the CON1/CON2 holes. Push the plastic down firmly with a flat edge that fits between the pins. A steel ruler is ideal for this. This will move the pins such that only 1.6mm (the PCB thickness) of each pin is proud. Now reverse the 2x8 pin header, and use the depth of the PCB as a jig to cut 1.6mm from the other side of the pins. The pin stubs may fly off at speed, so wear safety goggles and aim the header while cutting so that they will fly away from you. See the photos opposite that show what the header should look like after being trimmed and then attached to the PCBs. Soldering these headers is a little tricky as they are not a snug fit. Treat them like a surface-mounted part, applying flux paste to the pads before soldering. We recommend securing the parts during soldering with hightemperature tape (eg, Kapton) so they don’t move around. Tack the ends in place and check that the pins do not foul the tactile switch footprints. You might even like to test-fit the switches to confirm clearances. Solder the remaining pins, and be generous with the flux. It will help the solder to form clean beads that Fig.3: there’s not much to get wrong during assembly, although we recommend fitting the PCB joiners first, as the tactile switches will make access difficult when soldering them. The buttons should snap into place, so soldering them is easy. 94  Silicon Chip Australia’s electronics magazine siliconchip.com.au Parts List – Full 64-key Keyboard 8 Keyboard Modules 7 2x8 male pin headers, trimmed in height (CON3-CON6) 1 2x8 pin header (male or female to suit Leonardo connections, CON1 and CON2) mounting hardware to suit usage (M3 tapped spacers, screws etc) Keyboard Module 1 double-sided Keyboard PCB coded 23101213, 158 x 28mm 8 12mm tactile switches [eg, Diptronics DTS-21N-V or Jaycar SP0608/SP0609, Altronics S1135 + S1138] sit where they need to. Flip the board over and complete the headers on the back of the PCB. Remove any excess flux using a flux cleaner, and test the exposed CON3CON6 pads for continuity between the ends of the strip. As you can see from Fig.2, CON3 is wired straight through to CON5. But CON4 will be offset relative to CON6 (unless you have the full complement of eight PCBs), so check that each pad on CON4 is connected to one and only one pad on CON6. It’s best to do this now, as it can be quite fiddly to rework these connections with the tactile switches in place. Fit the switches next. They should snap neatly into place; just check that they are sitting flush before soldering. Finally, solder headers for CON1 and CON2. We used female headers to match the cables we had made up for the Matrix, but you can use whatever works for your arrangement, even soldering wires directly to the PCB. these are CON3 to CON5 on the front of the PCB. If none of the keys on a PCB work, then it may be a problem with the CON4 to CON6 row connections on the back of the PCB. The cut-down header pins (shown above) measure around 7mm tall so that they will fit between the end switches on adjacent PCBs (shown below). The plastic part is 2mm tall, so uniform board spacing is achieved too. Conclusion Like the Matrix, the Keyboard is designed to work with our MIDI hardware and software. But we think that readers will find other uses, especially in cases where many buttons need to be connected to a microcontroller. SC With the set of Keyboard PCBs wired up to our MIDI shield, we have a linear array of buttons that you can play like a piano. But keep in mind that by default, unlike a piano, multiple keys cannot be played at the same time. Hooking it up We tested our unit with the MIDI_ ENCODER sketch. If you haven’t done so already, we recommend reading the earlier parts of this series of articles, as they describe the software in more detail. Since the Keyboard is effectively equivalent to a Matrix fitted with non-illuminated switches, you can transfer many of the ideas relating to the Matrix to the Keyboard. As with the Matrix, wire CON1 of the Keyboard to CON2 of the MIDI shield (or corresponding Leonardo pins) and CON2 of the Keyboard to CON1 on the MIDI shield, connecting pin 1 to pin 1. Check that all the buttons work as expected, using the key notifications that appear on the Arduino Serial Monitor. If you find that some keys on a PCB don’t work (but not all), check the column connections for continuity; siliconchip.com.au Australia’s electronics magazine August 2021  95