Thursday, December 27, 2018

Plastic honeycomb arrived and test fit done with keyboard stack of DSKY substitute


Keyboard portion

My honeycomb part was finally on the truck ready for delivery today. I need to test fit the sample plunger into the honeycomb to ensure that the design is sound before I order the other 18 plungers. I can't insert the plunger without cutting out slots from the bottom of the honeycomb, except for the four end positions which might allow me to wrestle the plunger in.

At any case, after trying to get a plunger into the side as the part was produced, I discovered that the plunger can be snapped into any position. No need to cut the honeycomb, thus I get the target depth of movement I want to provide.

I tested the entire stack of PCB, coil spring, plunger, keycap and cover plate. It is important that the key press cleanly and return reliably. It is also important that I achieve the 1/4" travel and approximate resistance force to mimic the behavior of a real DSKY.

What I discovered were two issues. First, the pins that were soldered for the cable connector and for the Arduino Nano raise the honeycomb slightly; I need to relieve those portions of the honeycomb slightly to get a good solid fit. Second, the plunger doesn't come high enough in the faceplate opening; I want the surface of the keycap just below the level of the faceplate, but it is down too much.

To solve the problem with the plunger height, I have to add a sliver of material to the top of the plunger. I submitted the order for the 19 plungers at the new height, at the same time as the bottom and top covers for the display (see below).

I just need to fine-tune the coil spring - too light a resistance and it doesn't depress the pushbutton switch, too firm and it activates without the full 1/4" downward travel. I have some springs that seem to work reasonably, although I will need to test the switching action to be sure I don't need to tweak the spring type.

Three open issues with my keyboard design:

  1. I suspect a need to build additional supports under board to avoid cracking PCB from key presses.
  2. I believe light will leak around the outside of the keycap, since the plunger is translucent and the honeycomb walls are white. This should be minimized.
  3. It is possible that the LEDs and plunger won't deliver enough light through the text on the keycaps - the keycap is relatively thick and the plunger wasn't designed to focus light. 

Display portion

I swapped in 10K resistors for some of the white LEDs on the left side panel, which had been too bright with the 3K ohm resistors I had originally placed onto the board.The new values worked great, lowering the output of the white LEDs to perceptibly the same brightness as the yellow LEDs and the right hand side of the display.

The top of the display portion consists of two frosted plastic panels, containing text and other graphics, held in place by plastic covers. I did 3D modeling of the covers. These are built in two halves, which will be glued together with the frosted plastic panel inside. These were ordered from and will arrive within two weeks.

I need to get some clear acrylic 1/4" thick panels cut to size in order to begin adding the text, graphics and frosted color backing. I expect I will buy them from TAP Plastic which is nearby.

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