Mycroft Mark II - July 2020

Originally published at:

In our last Mark II update, we gave a sneak peak of the new daughter board we’ve been working on. After some further internal revisions we are keen to get the communities keen critical eyes on the details.

After the failure of the original hardware specification, we have been building prototypes using all off-the-shelf parts. The benefit of this approach is that each component is better supported and tested, meaning we can focus on integration.

Over the last few months our prototypes have used the Raspberry Pi as the primary single board computer, with a ReSpeaker mic array, Adafruit amplifier, and a range of other components for power and IO. This enabled us to continue strengthening the software stack on a known piece of hardware. During this time we have continued to talk with hardware vendors to explore new possibilities, however none have been able to provide us the confidence that would match the hardware we are already using at a reasonable price point.

Simon Jester 201

This brings us to the SJ201 daughterboard. The part number SJ201 is derived from Mike’s “Simon Jester” alias in “The Moon is a Harsh Mistress”. This is a single board that interfaces directly to a Raspberry Pi 4 through the 40-pin GPIO. It provides a high-quality speaker and microphone array, as well as LEDs and hardware inputs for non-voice interaction and feedback.

The SJ201 brings together a range of off-the-shelf components onto a single PCB, helping to simplify and shrink the enclosure design. It also reduces the cost per unit whilst providing the premium experience expected of a fully-featured audio front-end.

Outside of the Mark II, the SJ201 can also be used as a USB microphone array with barge-in support. However if powered solely by USB it can only be used as a mic array due to the limited power that can be supplied to the amplifier.

Mark II Form Factor

After carefully considering many different configurations we decided to mount the SJ201 to the Raspberry Pi GPIO header at a 90 degree angle. This allows the microphone array, LEDs and buttons to be positioned at the top of the enclosure, whilst maintaining the simplicity of a direct connection to the main board. As well as allowing the Raspberry Pi to cool efficiently and to be in an optimal orientation to connect to the display via DSI.

This change means that a landscape orientation of the display is the most likely going forward. Below is a block diagram of the new form factor. A block diagram shows the positioning of internal components and how they connect to each other. Keep in mind this is only the internal components and does not have an external enclosure, it is not how the end device will look and does not show the camera module placement. This would be positioned centrally above the screen like a laptop.

We know this alters the original industrial design considerably, and we will share more on that design in future updates. In the meantime we would appreciate your feedback on the new form factor. How might a landscape orientation change your use or placement of the device?

How you can help

We are keen to hear from you, particularly the hardware heads and electrical engineers in the community.

In a weeks time, we will be ordering the first batch of SJ201 manufactured prototypes. These will be units for engineering validation, ensuring the design works as expected, and meets our specifications.

It is always possible (even likely) that with new electrical designs there will be some flaw that we haven’t accounted for. This is why we go through the process of engineering validation, to find and fix these flaws. The earlier we catch them, the faster we can move to production and the better the product we can ship to everyone.

Like our previous designs, everything is open and freely available on Github. We would love to hear your feedback on the current design, particularly if you have experience in electrical engineering and design.

What do you think of the current PCB design?

Do you have a favorite PCB design checklist or guidelines?

How well does the SJ201 meet those?

See all the details and comment directly on Github or join us in the Community Chat, Forums if you have any feedback.

Further experiments in openness

One final thing, you already know that we are big fans of open source. We try to be as open and transparent as possible about everything we do. So in a new experiment in openness, we’ve started recording our bi-weekly internal development sync meetings and posting them to YouTube. The first is already available. If you’re interested in following the everyday details of Mycroft development you can now see exactly what we’re working on and where our focus is.


As someone else here already mentioned I also wonder if it would make more sense to switch the positions of the Pi and the SJ201. Right now the Pi is blocked from all sides by the daugtherboard, the “stand” and the screen making it hard for a proper air flow. But this is just an assumption on first glance. I must admit I had to lie to find the new setting very sexy, so to say, the chunkyness is hard to overlook. It would also be nice if you could give some more information on what the big stand is for. It has quite a volume. Also I wouldn’t see a problem in turning the whole Pi/Sj201/screen bundle by 90 degrees which would lead to the more aesthetic upright alignment of everything and would provide an open airflow to the top. Considering the height of the stand it might be possible to position the connectors high enough to be used when they face down. Cables could then be led to the back and wouldn’t stick out of the side.

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Hi there, check out Derick’s response to a similar question here:

Essentially, we ideally want the mic array on the top or front of the device. Though it will be interesting to test what performance is like if the mic array was on the side. I think it would be significantly worse, particularly from a distance and different angles, but could be worth testing.

In terms of the “stand” in the block diagram, this is just a placeholder and doesn’t represent the final enclosure design. My understanding is that the two squares on the front are placeholders for speakers. With the change in form factor, Derick’s rethinking what that might all look like.

By the “connectors” do you mean the connectors shown in green on the block diagram, or the usb ports, or something else?