turning old piano into midi keyboard (keyboard matrix)

[floating-water]

I've come across articles on this and I've tried to understand it but I'm struggling to understand how to implement it with the gear I have. I'm looking to use these keys with a synth i'm building through midi to cv I guess.

I understand I need a multiplexer and an arduino or microcontroller and that the keys are usually organised in columns and rows. The pics are the keyboard im working with, its 61 keys and seems to have two ribbon connections coming off it, both with 8 output pins coming off of them which seems easy enough? I wrote out a table in a way I think I understand it but how on earth do I go about implementing this?













Is there a Vcc and ground coming out of the ribbon cables? If so that changes the number of outputs? How do I know which is columns and rows?

Appreciate any help
Thx

[PHOBoS]

For a keyboard matrix you will have to send a voltage in sequence to the rows and for every row scan all the columns.
send a voltage to row 1, read the voltages from column 1 to 8
send a voltage to row 2, read the voltages from column 1 to 8
send a voltage to row 3, read the voltages from column 1 to 8
etc,..

note that unless the keyboard itself already has diodes or other components you can swap rows/columns. So you could also
send a voltage to the columns and read the rows.

what you need to watch out for is that if you don't send a voltage to a row it has to be unconnected or high impedance.
So don't just send high an low signals as this can lead to shorts. A mux would be an easy way to do this though you could
also use transistors or even diodes. There is also things like ghosting and blocking that you have to keep in mind but you
should find that out along the way. Here's something to start with https://deskthority.net/wiki/Rollover,_blocking_and_ghosting

[floating-water]

Yes my guy Phobos. Is there importance in the actual voltage reading? I don't even own a multi-meter yet, in theory could I attach each pin to breadboard for row, for column and say use an led to get a response off the columns?

[PHOBoS]

well if you use a microcontroller then the voltage needs to be compatable for that which is usually 5V for an Arduino.
I did notice some diodes on the PCB so you will have to check how those are connected determine what to use as rows/columns.

Not sure what you want to do with LEDs. I guess you could test turning them on with keys but you have to keep the current low
with those membrane switches so I would recommend to use transistors or other buffers to drive the LEDs.

[Grumble]

To clarify what Phobos wrote.

[floating-water]

ok i broke my eyes and followed the traces on the board:

found 7 groups of 8 keys and 1 group of 5 -> connected to the bottom ribbon cable and to the keys

and 5 groups of 8 and 3 groups of 7 -> connected to the top ribbon cable and to the diodes

so judging by the numbers the table i wrote must be correct?

in that diagram above, the diodes face the rows and in this keyboard, the diodes face the top ribbon cable so the top is the rows? does that mean they are pull up?

and what would be an implementation you would recommend, shift register?

[floating-water]

so id put the 8 rows into digital pins on atmega(arduino), the 8 columns go into shift register inputs

the clock, latch and data from the register goes into the arduino digi pins

besides modifying the amount of rows/columns in this code is there anything else I need modify?

[PHOBoS]

I'll have a closer look at the code later but here is how you'd have to wire it up using the pin declarations given in the code.
I don't know what order the rows columns are in on the PCB but that's something you probably have already figured out.

Note that because of the diodes you don't have to worry anymore about possible shorts so in this case in can be driven
directly with a shiftregister instead of a mux.


hmm I just noticed that pins 8 and 9 are used twice
edit: updated

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[floating-water]

ah ye that will be me bein a dumbass, i got the base code from an instructable and started making changes quickly so it be (?):

[PHOBoS]

some things I've noticed sofar
- the number of columns and rows should probably be set to 7, not 8 as it starts from 0
- the initial note value is set to 31 which is a G (octave 0) so its value depends on the layout of the keyboard and
what you want the lowest octave to be. You could add switches or something else later to make this adjustable.
- the function 'scanColumn(int colNum)' can be simplified since you only have 8 columns.
- there is some inversion going on, not sure yet what's up with that. Do you have a link to the instructable ?

[floating-water]

ah okok was curious how to set the first note. just gonna keep editing and pasting the code so i can keep track if ok

[PHOBoS]

thanks for the link, I'll check the instructable.

[floating-water]

right right ok fascinating. is it just the exclamations that initiate the inversion? so if a board didn't have diodes you could invert the readings through the code to prevent ghosting?

[PHOBoS]

no, you can't prevent the ghosting in code that way but could use it if the diodes were reversed.

There is still something else that is inverted:

[floating-water]

ah lol ok. one thing are u sure the number of columns need to be 7 each, on his he labels them 6x11? edit: i realise it probably just depends on what i number them later in the rowValue?

[PHOBoS]

[floating-water]

beautiful, ill update once ive tried it im sure theyll be issues, hope this thread helps someone and thanks for ur help Dr Phobos

[floating-water]

If im building the actual synthesizer into this keyboard space, how is the keyboard usually connected to everything else?

I see in the Juno 106 diagram, it has a CPU master & slave so Im assuming thats like, in this case, an arduino midi out and ardunio midi to cv circuit in one? How does that get simplified, as opposed to building a MIDI out circuit with MIDI jack from the keyboard then a MIDI to CV with MIDI jack to the rest of the circuits

[Grumble]

It is not surprising that the Juno 106 uses two processors, I mean it was designed about 35 years ago, the processors in those days usually had a 1MHz clock frequency (compare this to the 16MHz clock of an Arduino Nano)
So they used the one processor for key scanning and conversion to midi and sending midi to the second processor and to the circular 5 pin din connector, while the other used for sound generation. Which it does remarkable well!
My guess is that if you use clever programming, you could do with one (fast) processor like a teensy or blue pill stm32 or use an Arduino Nano for keyboard scanning and converting it to midi and use a teensy with audio shield for midi input and audio generation

[floating-water]

Lol right ye obviously shouldn't have compared that CPU. I completley forgot the Juno used DCO's, what does that even mean were they just controlled digitally or actually digital oscillators

I'm going down the cem3340 route for the vco and keeping everything analogue where possible so bar the audio generation, handling both the scan matrix and MIDI to CV you think would be possible with a teensy? How complex are we talking? Also how does the 'out' of the keyboard meet the 'in' of the midi to cv part without a midi cable in between

[floating-water]

Think I'll work on the circuits separately, learn the fundamentals first but I'm still baffled as to how the circuits get connected to each other without a MIDI cable in the middle. I came across a small reddit bit on this:

https://www.reddit.com/r/synthdiy/comments/bf9a2a/midikeyboard_matrix_to_cvgate/

he/she talks about shift registers after the keyboard matrix could someone help me understand that?

[Grumble]

For my keyboard (look left ) I used a shift register to detect the keys pressed, I can do that because my keyboard has just 32 keys which is 4x a byte so I use a 32 bit word uint32_t like this:

[floating-water]

Caught your youtube channel, your synth sounds p.h.a.t and I love your compositional style you could score an ominous, quirky british TV show with your setup I reckon. How one earth you build a vocoder?

Anyway ye so you absolutely lost me at 32 keys; so whats happening here. Do you have your keyboard being scanned by a cpu/arduino first or is this void of all that? How does CV get generated? Am I asking the wrong questions

[Grumble]

A few years ago I bought a midi keyboard designed for an old sound card which used a 25 pin D-connector for parallel printer but also for midi.
When I opened the keyboard I noticed that there were two almost identical boards with each about 32 keys.
I used one of these boards for my midi keyboard, so this answers one of your questions: It just gives midi out and I use that for my synth if I need a midi keyboard and no audio whatsoever.
(For clarity I added a schematic to this post).
You can see that I added a 74LS259 which is a 8 bit addressable latch, used to make one of eight bits high so I do not need a bit-mask for conversion but just add 1 to the previous state.

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[floating-water]

When I tried compiling the sketch it gave an error:

'scanColumn' was not declared in this scope

on this bit: