Forbidden Planet "Krell" Display – MIDI CC Controller – Part 2

By Kevin on April 4, 2025

This revisits my Forbidden Planet "Krell" Display – MIDI CC Controller using my Forbidden Planet "Krell" Display PCB with a Waveshare RP2040 to create more of a "all in one" device.

Warning! I strongly recommend using old or second hand equipment for your experiments. I am not responsible for any damage to expensive instruments!

If you are new to Arduino, see the Getting Started pages.

Parts list

Updated Forbidden Planet "Krell" Display (see below).
Completed Forbidden Planet "Krell" Display PCB Build
Waveshare Zero format device (I'm using an RP2040)
4x 6mm brass spacers
4x 20mm nylon spacers

PCB

This requires a built of the Forbidden Planet "Krell" Display PCB with the following:

2 potentiometers
MIDI IN and OUT

I've used potentiometers that are their own knob, as they only poke through the casing by around 5mm or so.

If it you are able to get longer shaft pots, then that would probably be worthwhile.

Updated 3D Printed Case

This requires the following from the Krell Display 3D Printed Case:

1x Frame
2x Inserts (with potentiometer holes)
1x EuroRack support

This requires the following options in the OpenSCAD code:

show_frame = 1;
show_quadframe = 0;
show_insert = 1;
show_support = 0;
show_quadsupport = 0;
show_eurorack = 0;
show_eurorack_support = 1;

alg_pot1 = 1;
alg_pot2 = 1;
alg_cv   = 0;

The frame does not really take into account the PCB at present, but I've reached the "good enough I want to do something else" stage, so I've just added a couple of small cut-outs (using a hacksaw) for the two MIDI sockets, and am content that the components stick out a bit from the back.

This cutout has to be 10.5mm from the end, 6mm wide, and 5mm deep.

At some point I might go back and design a deeper frame that has the cut-outs included and some kind of snap-on back to make it a self-contained box.

But for now, this is left as an exercise for, well, anyone else 🙂

Construction

I've used four brass 6mm spacers to screw into the mounting holes in the frame. Then the PCB can be inserted, taking care to squeeze in the 3D printed support around the LEDs and pots, and fixed with 20mm spacers which will also act as "legs".

The Code

I've used a Waveshare Zero RP2040 and Circuitpython for this build. This is a combination of some of the test code used for the Forbidden Planet "Krell" Display PCB but with added MIDI.

The code supports both Serial and USB MIDI.

I wanted an equivalent of the Arduino map() and constrain() functions and didn't immediate spot them in Circuitpython so wrote my own:

def algmap(val, minin, maxin, minout, maxout):
    if (val < minin):
        val = minin
    if (val > maxin):
        val = maxin
    return minout + (((val - minin) * (maxout - minout)) / (maxin - minin))

This allows me to map the analog read values (0 to 65535) down to MIDI CC values (0 to 127) whilst also allowing for some inaccuracies (I've treated anything below 256 as zero for example):

alg1cc = int(algmap(alg1_in.value,256,65530,0,127))

I've used the Adafruit MIDI library, which I'm still not really a fan of, but I wanted to include MIDI THRU functionality to allow the controller to sit inline with an existing MIDI stream. But it doesn't seem to work very well.

I was already only updating the LEDs/MIDI CC if the pot values had changed, to cut down on the number of Neopixel writes required.

I experimented with changing the scheduling of the analog reads and MIDI but that didn't seem to help very much. In the end I made sure that all MIDI messages queued up in the system would be read at the same time before going back to checking the pots.

    msg = midiuart.receive()
    while (msg is not None):
        if (not isinstance(msg, MIDIUnknownEvent)):
            midiuart.send(msg)
        msg = midiuart.receive()

It will do for now. Moving forward, I might try the Winterbloom SmolMIDI library. If that still doesn't give me some useful performance then I might have to switch over to Arduino C.

Find it on GitHub here.

Closing Thoughts

The MIDI throughput is disappointing, but then I've never really gotten on with the Adafruit MIDI library. I use it as USB MIDI on Circuitpython is so easy, so will need to do something about that.

I'm still deciding on the PCB-sized supports too. The original seemed to have nicer diffusion of the LEDs, but that could have been the difference between 5mm SMT neopixels and these THT APA106s which seem more directional in the first place.

And I really ought to finish the 3D printed case properly too.

So this is "that will do" for now, but I ought to come back and finish it off properly at some point.

Kevin