This Clever Credit Card-Sized DJ Console Aims to Have You Cutting and Mixing on the Move!

There is a huge crossover between the audio world and that of electronic engineering.

Since before the dawn of the first synthesizer, we've turned to electronic engineering to enable musicians to enhance their expression of art.

These days, the game is strong.

The advancement of capabilities found within the ICs used within even the most basic of kit can result in a huge range of potential synthesized noises, damned impressive DSP chains, and crate-loads of curious controller contraptions, designed to allow your dexterous digits to get the drums kicking when mixing and mashing up the function.

Despite the rather portable nature of a number of recent interfaces and controllers on the market, gadgetmies has decided to take it upon himself to delve into the design of a complete DJ controller / mixing setup, barely bigger than a couple of credit cards — that serves as a trio of USB-MIDI physical capacitive-sensing interfaces.

Clever capacitive control elements

This trifecta of circuit boards boasts all the basic elements needed in order to provide a solid state control surface for your disc jockeying needs, with nary a moving part in sight!

The magic of the mix happens thanks to a multitude of different capacitive touch elements, formed from copper shapes, on the front faces of the PCB boards.

If we take a look at one of the "turntable" boards, we can see a central "circular" capacitive slider, two discrete capacitive touch buttons, down at the bottom right of the board, and finally, a linear capacitive slider on the boards right hand side.

It's a very slick design — almost a perfect application demo for what the capacitive sensing technology is capable of, even when it's being implemented on a relatively older part — the Microchip ATmega32U4, one of the more well-known ATmega parts that contains a native USB interface included inside.

Despite the simplistic looking layout, reading in all those capacitive sensors is a sure-fire way to eat up all your I/O in short order. We can see from the schematic above that there is barely a pin left unmapped.

It's also really nice to see a schematic that pays very close attention to the manufacturers recommended application notes. See the well compensated supply rails, with adequate decoupling where required — that's the sort of thing that some will overlook in haste, and can cause all sorts of "ghost in the machine" debugging woes, even in a simple design.

When it comes to a capacitive sensing application, the need for such a clean set of rails becomes almost a critical design feature.

Mix it up my DJ!

If we mooch on over to the mixer, we can see a similar design, just with a some changes that adapt the console layout and positions of the capacitive elements, to more closely match the typical layout of a console mixer.

Again, we find this set of sliders and buttons being driven by yet another ATmega32U4, tasked with reading in the values returned by the sliders and buttons, along with some basic user feedback in the form of a few discrete LEDs, at the lower corners of the board.

As with is the case of the turntable, the mixer also pretty much maxes out the available I/O on the ATmega32U4, and follows the same, well defined power filtering stages on the supply rails.

It might not be generating audio, but even the control surface interfaces used to command synths into life will certainly benefit from such a well implemented design.

Take a note of this set of notches...

If we take a look at both boards with a little more detail, we can see a design trick that we've seen before — a USB connector, formed from little more than the PCB itself, and some clever milling paths.

Shown below, we can see the brains of the turntable PCB, and the five-finger contact set, that will neatly slip into the device end of a micro-USB cable.

This is a trick we've seen in previous projects, such as the Fluff M0, from user @de∫hipu, where the cost of the USB connector is quite literally dropped from the BoM, with the contact fingers of the connector replicated in the copper and fiberglass of the FR4 stack.

Seen below is how the system looks with the board tab inserted into the cable connector, rather than the cable connector being conventionally inserted into the normally fitted board receptacle.

While this isn't the most robust solution — you'd likely not want it on a , and while can be subject to some wear and tear over time, it can save you some cost if you are planning to rapidly iterate through a few designs in short time — as one might expect to do when working with capacitive sensing designs!

A miniature, mobile mixing platform!

With the one's and two's assembled and, and in conjunction with the populated mixer console all feeding data into the host computer, it is the task of the DAW software running on the host machine to deal with the DSP that comes with manipulating audio.

Although the ATmega32U4 used here is certainly proving its worth as a versatile USB-MIDI building block — offering everything needed for both of the designs shown here to function as intended — heavy DSP operations can be a challenge for even the beefiest of Cortex-M chips.

And, while there is certainly scope within the capabilities of the ATmega32U4 to handle itself as a USB audio device, it's still an 8-bit MCU... We're going to want a more powerful chip to churn through anything more complex than chip tunes!

So while we're waiting to see a mini-mix from gadgetmies to rock out to, we suspect they might already be imagining a second revision - we could see the mixing console being somewhat upgraded to host a more powerful device, perhaps even with the power needed to host the audio playback and manipulation routines that would come from the emulation of scratching and mixing two separate audio files!

For those who are itching to get mixing, get your DJ fixings over on the CC MIDI project GitHub over here.

We'll be keeping an eye on gadgetmies to drop their first mixtape from this micro MIDI instrument console over on mixcloud, but feel free to keep tabs on them too — on Twitter, for more timely updates on this trio of tiny touch interface tools.