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Fade 4 Build Guide

Fade 4 is a performance MIDI and CV controller with 10cm faders and housed in a robust aluminum enclosure. It is available as a DIY kit if you prefer to solder it yourself. This Build Guide will walk you through the assembly process!

Fade 4 is constructed with two separate PCBs, interconnected with a short 10 pin IDC cable. The Jack PCB has the CV and MIDI jacks (presoldered), while the Main PCB has the microcontroller, op-amps, USB port, and all other surface mount components presoldered.

The kit contains 3 bags of parts - #1 Components bag, #2 Hardware Bag, #3 Fader Bag. We'll focus on the Components bag first.

We'll start with the 2 x 5 pin headers. One is soldered on the Jack PCB and one on the Main PCB. Make sure these are placed on the "footprint side", meaning the side where there is printing.

Now that we're warmup up, we'll move to soldering the 7-Segment LED displays. There are 4 displays, and they need to be soldered in the correct orientation, on the opposite side as the 2x5 pin headers. Pay attention to the diagram on the footprint.

These need to be soldered completely flush to the board - any skew in how they sit will prevent the case from assembling correctly. A trick I use is to solder 2 opposite pins to start, and then inspect for positioning errors. If you are skewed at this point, you can reheat one of those pins with your iron, while moving the display with your other hand.

These pins should be clipped down with side cutters so there is no risk of a short.

Next up we will be soldering the 2 pushbuttons. These buttons has a flat edge which should line up with the flat part on the footprint outline.

At this point we will actually install the standoffs, since these can be tricky to place with the faders in position.

There are 6 Female to Female standoffs and 3 Male to Female standoffs. The F-F standoffs are placed on the fader-side of the Main PCB. The 3 M-F standoffs are placed on the component-side of the main PCB, near the USB port edge.

The 3 standoffs near the bottom (opposite side of the USB port) are held in place with 3 M3 screws, inserted component-side.

A little trick I picked up from the Plinky workshops I've been hosting - if you add a little more solder to the USB port you can make it more robust and hopefully last longer.

We're now ready to solder the Faders. The faders have 3 pins (plus 2 mechanical pins), while the footprint has 6 pins. This is just so dual-gang versions of this pot can be used. The extra 3 pins are not connected to anything and you can ignore them.

Just like with the displays, it's very important that these are installed straight in all dimensions. I like to solder the two big pins first and then assess placement. The pins may have bent during transport, if the faders are not inserting cleanly, just bend the pins back into position with a pair of needle nose pliers.

Once positioning is correct, go ahead and solder the remaining 3 pins on each fader

Congratulations, the soldering is complete! Now we just assemble the board into the case.

The board will place easily into the enclosure, and the USB port should slide into position.

6 M3 screws can be use to fasten the board to the case face. Tighten these as much as you can.

Now we can install the jack board. The jacks should go into their holes as far as they can, and the slotted PCB will let you tighten this board down.

The 10 pin IDC cable can now be installed to connect the jack board to the main board. The end without a strain relief connects to the jack board. Each board has a printed indicator for the red stripe. It does not matter how the cable folds over, as long as the red stripe is lined up with the red strip text on both boards.

Set the Slide switches to the MIDI Cable Type you prefer to use. Up for MIDI B, and Down for MIDI A. Each switch controls the polarity of the cable, so either can be used.

Install the bottom cover, install your fader caps, stick on your rubber feet (if you want) and you're done!


Technical Description

Fade 4 runs on an Atmel SAMD21G microcontroller running the Feather M0 Bootloader. This allows Fade 4 to be developed on using the Arduino programming environment, and make use of Arduino libraries.

The SAMD21G handles all USB communication while using it's internal ADCs to read the positions of the faders. Each fader is connected between 0V and 3.3V, with the wiper providing a voltage between 0V and 3.3V. An opamp is used to multiply this voltage by 1.5x to provide 0 to 5V to the CV outputs. The fader position is sampled through Responsive Analog Read to smooth out any jumpiness in the measurement.

MIDI is handled using the Forty Seven Effects MIDI Library, making sending MIDI and USB MIDI the simplest part of this device.

The 7 segment LED displays are of the common cathode type, meaning the 7 LEDs in the character have their anodes broken out to individual pins, while the cathodes are grouped together into one pin. I use a 2N3904 transistor connected to this cathode pin to enable the digit, allowing the 4 digits to share character pins, as long as each has its own transistor and cathode pin.

These displays need to be enabled one at a time, and using persistence of vision it appears that the 4 digits are on all the time. The timing of these characters is handled by TC3, triggering an interrupt to turn the current LED charactor off, reconfigure the pins, and turn the next one on.

A second timer, TC0, is used to generate a PWM signal on the 4 cathode transistor pins. This PWM signal can be changed to adjust the brightness of the display from 0 to 100%.

I posted a blog post a while back about the physical challenges of designing this product, and I'm really happy with being able to 3d print my case prototypes before release.


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