not an arpeggiator :-P
I am not that great at building cases, here are my v1 and v2 prototypes.
videos are easier than reading
how to install arduino midi library
I wanted to play chords using the Arturia BeatStep Pro sequencer. This sits in the MIDI chain between the sequencer and the synth and uses the incoming MIDI note to stack a chord.*
1 Switch: Choose between Major and Minor modes. Major mode has a dominant 7th rather than major 7th chord, I haven't used it much cause major scales are basic.
4 Pots:
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Key Selector: Choose the key, other chords are mapped to scale for diatonic chords (I hope/think)
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Chord Selector: Starting with the root note, add more notes to create a more complex sound. I tried to group the less complex sounds at low values and the more extreme sounds at high values. So, in Minor mode you can turn up to go from one note to a power chord to a minor chord to a minor 7, 9th, 11th, 13th.. with a couple more variations.
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Inversion Mode: at zero the root note is at the bottom. At 100% the root note is at the top of the chord. Variations exist in between.
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Strum Rate: When I first made this I just wanted to play simple block chords for techno-y sounding stabs on the DX. By accident I realized that the strummed chord can sound really nice and give some movement to a beat. Rhythms like like Legowelt or Flying Lotus... so at one extreme the chord plays all at once. At the other extreme the notes are plucked out one at a time slowly. Yes, like an arpeggiator. but no repeating or syncing cause I have a BeatStep that can keep time and play arpeggios :-)
Version 2 has all the features of Version 1, and these additions:
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a Legato Switch changes the way MIDI noteoff messages are sent, allowing the strummed chord to ring out from a trigger or be cut short.
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a Bypass Switch, turn off the effect and this just becomes a 1:2 MIDI splitter. More useful than turning everything down as in V1.
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a Second MIDI Channel. I am using two DIN/MIDI plugs driven from an Uno's Tx port and it works ok. The Master MIDI channel is the same as the recieving channel and does the 'strumming' chord effect. The second channel is used to transmit block chords with the same notes as the Master chord, but with its own unique settings. It works nicely with my (newish) Behringer Crave in arp mode, giving me a 'free time' chord flourish from the DX and accompanying in time Arpeggios from the Crave. If you dont want the second channel stuff just don't connect the hardware and it should be fine, maybe tie the 'chord size' analog pin to ground :)
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Second Inversion Mode: as above but for the second MIDI Channel, and with notes shifted 2 octaves cause I like it more.
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Chord Size: increase the number of notes in the block chord by adding octaves above and below.
Version 1 GPIOs per the .ino sketch:
Version 2 is similar with pot and switch IOs listed at top of the .ino file.
Prototype using a Pro Trinket (same pinouts as UNO) before I made it diatonic:
* Akai and Oberheim used to make boxes that did this sort of thing and this functionality is now built into heaps of DAWs, possible some synths. my Yamaha DX doesn't