Half Deck - Mk 5

This is my fifth keyboard design. This repo will be updated as I get around to expanding the data here.

This is a split keyboard in two halves. One half has a microcontroller that implements USB and communicates with the other half using a two-wire bus protocol known as I2C.

The other half has an IO expander component that is connected to the keyboard matrix and that can be queried over I2C.

A TRRS (Tip-Ring-Ring-Sleeve) cable is used to connect the two halves; this is a 4-wire cable that carries the power and I2C signals to the IO expander.

Firmware

You can find example firmware for an atmega32u4 based Feather board here: https://github.com/wez/qmk_firmware/tree/flutterby-rebased/keyboards/halfdeck

Before you begin

The PCBs are double sided so that the same design can be used to build either the RHS or the LHS of the keyboard. Make sure you read through this doc and understand where you're going to place the components before you cut or solder anything, as it can be difficult to undo those actions.

If you are using a plate with your keyboard (eg: the top acrylic plate with the key switch holes), keep in mind that you can't solder in any of the switches until you have the plate as the switches effectively bolt the plate to the PCB. If your plate material is thicker than ~1.6mm then it won't clip into the small lugs on the switches and you may need to add some M3 spacers to raise the plate up to the appropriate height; depending on how you flush you solder your diodes that may not be required.

The labels for the controller and IO multiplexer are biased towards right handed users, or rather, assume that the touchpad be located on the RHS. If you want to place it on the LHS you will need to swap the multiplexer and the controller. This should be straightforward, but will require attaching those components upside-down. Pay careful attention to the labels on the pins to ensure that you have things placed correctly!

Assembly - Left Hand

The top of the PCB faces up for the Left Hand. You'll know you've got it facing the correct direction because it has a graphic showing a left hand in the bottom left corner, as well as some version information down there.

You will place the key switches on this side of the board for the left hand but solder them in on the other side.

If you are right handed and also want to connect a touch pad, you will connect the SX1509 on this half.

If you are left handed and also want to connect a touch pad, you will connect the controller and the touch pad on this half.

I2C Jumper configuration

On the top edge of the PCB there is a little notch where the TRRS connector will be fitted. To the right of that area is a pair of solder jumpers labelled JP1 and JP2. These are present to allow setting up the same PCB for either left or right handed usage and control which pins of the TRRS connector are connected to the power and ground lines.

Each jumper consists of three rectangles. The middle of the three needs to be bridged to either the left or the right pad using either excess solder or a short section snipped off of one of the diode legs. Controlling excess solder can be difficult, so I would suggest using a diode leg trimmed to size if you're not confident, or you don't have a solder sucker on hand to undo.

• JP1 must be bridged to 3V3
• JP2 must be bridged to GND

It's not the end of the world if you mix this up, so long as each jumper either goes to GND or 3V3. They must not both be set to the same thing or you risk damaging your electronics.

This image shows the jumpers for the LHS:

Assembly - Right Hand

The bottom of the PCB faces up for the Right Hand. You'll see a graphic showing a right hand in the bottom right corner of the PCB.

You will place the key switches on this side of the board for the right hand but solder them in on the other side.

If you are right handed and also want to connect a touch pad, you will connect the controller and the touch pad on this half.

If you are left handed and also want to connect a touch pad, you will connect the SX1509 on this half.

I2C Jumper configuration

Similar to the note on the Left Hand, but the Right Hand absolutely must have the opposite settings to the Left Hand!

• JP1 must be bridged to GND
• JP2 must be bridged to 3V3

While the PCB is mostly reversible, the jumper blocks are only present on one side of the board, so you need to flip it over to configure them.

This image shows the jumpers for the HHS:

Errata

The first batch of these boards have a minor issue with the SCL line; due to a typo it isn't actually connected to the TRRS jack. You will need to bridge the misnamed SDL pads to the SCL pads on each side.

Patching the RHS

Patching the LHS

Recommended Order of Assembly

1. Diodes
2. The TRRS connectors
3. Jumpers (I2C and the Errata section above)
4. Header strip for the controller/expander
5. Key switches
6. Touchpad

Diodes

The diodes are directional; they allow current to flow in a single direction. Their purpose in the circuit is to prevent ghosting. If you attach one in the wrong direction it can be hard to debug and isolate if you don't have a multimeter, so take care to make sure you've placed them correctly.

The diodes have a black line at one end. The black line must be aligned with the thick white line in the diode outline on the PCB. The solder pad on that side is square, which is another visual cue; think black, thick, square.

Bend the legs of the diode so that you form a slender n shape, with the diode body at the top of the n. The goal is to have the diode body lay flush against the PCB within the outline on the pcb, so you want the legs to bend and fit the holes at either end of the outline.

I'd suggest taking your time with the first one and perhaps solder that one before placing the rest to make sure you understand how it will fit. Make sure that you have it oriented correctly before soldering!

To solder the diodes, the technique is important. You're placing the diodes on the side of the board facing up but you need to solder them on the back side of the board. I bend the legs to form a V shape to keep the diodes in place when you flip the board over:

Once you've placed the diodes, check again to make sure that you have them facing the right direction (black, thick, square) and solder them in. You want enough solder that you fill the hole with a nice shiny blob, and you want it to be very slightly convex.

Be careful with the diode legs: you can easily prick yourself on the legs and the human reflex to suddenly move away from the prick can be dangerous if one of your hands is holding a hot soldering iron!

After soldering, double check the orientation of the diodes again: it's not super easy to correct at this point, but it will get much harder if you discover a problem later because the next step is to snip off the diode legs.

Use your angle/flush cutters for this; lay the edge of the cutters flush with the board and snip off the leg where it comes through the hole. Ideally you want to trim it such that you're just left with a small convex mound of solder around the hole. I recommend holding the leg with one hand while you snip; the picture below doesn't show this because I was holding the camera to take the picture. The reason for holding the leg while snipping is that it can jump off your work surface and land on the floor. If you're not diligent at tracking and picking up the legs from the floor, it is easy for someone to stub/stab a toe with a diode leg. While they're not super sharp, they're small enough that they can jab in a few millimeters and draw blood.

TRRS Connectors

Attaching these is pretty straightforward; look for the little notch in the PCB near the I2C jumpers. The notch makes room for the circular opening in the connector. Place the legs of the component through the holes and solder on the other side.

If you are using a plate with your keyboard, check to see whether you have enough clearance to mount the connector on the switch side of the plate. If you have an acrylic plate than I suspect that there won't be enough clearance. If you're using a 1.6mm plate then it looks like it might just fit.

In the pictures in this guide, I built without a plate and placed the connectors on the top side of the PCB.

Header Strip for Controller and Expander

I recommend using some header strip to mount these components. The controller comes with appropriately sized strip but you'll need to obtain some more header to mount the IO expander.

Similar consideration as the TRRS connector: make sure that you have enough clearance to place the components and your plate if you're using a plate. You may need to mount the components on the opposite side of the board to accomodate your plate. Note that you can also buy short header strip for lower profile builds.

This image shows the header strip placement for the IO Expander:

When soldering the header be prepared: GND is attached to a larger ground plane fill in that area of the board; there's large region of copper under the surface coating and that means that it will soak up the heat from your iron and be more difficult/take longer to solder than the surrounding pins. You may notice that the PCB in that region is noticeably much hotter. T