Saturday, December 8, 2018

Wiring up display PCB for DSKY substitute, working on keyboard and chatting with DSKY wizard


We got together with Craig, a silicon valley engineer who is working on building a replica of the DSKY that is as faithful to the real object as possible. He brought along some of his circuit boards, assemblies and parts to discuss.

His attention to detail was very impressive. He designed the parts to look identical inside as well as on the face, with as few variations as is humanly possible. He has built a special high resolution 3D printer to build parts and owns a very nice laser cutter to create components.

He calculated the angle of dispersion of the LEDs to match the angles that were emitted by the real electroluminescent panel, for example, and used the exact fonts and paint colors. He is using Neopixels to allow him to dial the exact color for the alarm light panels.

Craig gave me a set of keycaps in different sizes as well as some sizing templates, to help me create the right look for the DSKY substitute. Since my objectives are much looser than his, I will be satisfied earlier than he would be.

My display panels will produce digits that are only .390 inch while the real DSKY digits stand .500 tall. Mine are in the shape and at the tilt that comes from the standard 7 segment display modules, while Craig's are the exact shape, tilt and size of the DSKY itself.

He gave me some good ideas, and we talked together about means to produce a new electroluminescent panel to the NASA/MIT specs. I have been noodling around with methods that use a PCB to establish all segment shapes, but focused on ways to hide the vias that may be necessary to connect to each segment. Craig had other methods to block the connector traces such that he wouldn't need vias into the segment copper itself.


Board arrives and is unboxed

The shipment from arrived while we were working at Marc's house, giving us the opportunity to open it, see the boards and bring some to our homes to begin soldering. The boards looked great. I will do tests to verify connectivity of all points, duplicating what was already done at the factory with their flying probe tester, just out of paranoia.
Backside of raw PCB, testing power leads tacked on 
Board construction begins

I worked out methods to install and test subsets of the components, building up the board in stages. This lets me spot errors rapidly and makes rework easier due to a partially populated board. It also helps me to make fine adjustments, since the resistors that set the LED illumination levels will be added once the digits are displaying so I can match the brightness of all light sources.

First up are the LEDs which I can install and check out by holding a test resistor temporarily for each position. I don't want to add the actual resistors because their precise values will be determined later in the build process.
Installing the larger LEDs
I discovered that the white surface mount LEDs I had ordered were, unfortunately, super-microscopic and won't fit on the pads on my board. Some online research yielded a cool white LED of the same size (PLCC-2) which I quickly ordered. These will need to be added to the board later, once all the other parts are in place.

The small LEDs used to form the lines and the signs are 0603 size. It took a while to lay these on correctly and I had an annoying failure rate where the LED was cooked by the solder or rework process. A couple sprang away from the tweezers to parts unknown. I installed as many as I had on hand, with the rest to be soldered on later. I did finish two register lines and two of the plus/minus signs. The last register line is partially done and the last sign is empty.

Next will come the AGC input interface circuits, which I can test with a 28V source and scope to verify proper operation of each of the 23 circuits. These are all the parts that depend on the 28V power plane inside and are unconnected to the 5V parts (other than an input pin on the Arduino, once that is installed). Each circuit is two 10K resistors, a .1uf electrolytic capacitor, a PNP transistor, a 220K resistor and a 47K resistor.

AGC input interface circuit
After that, I installed the AGC output interface circuits, which are relay drivers that will pull down the AGC lines to ground when activated. I put on all parts except for the diode and relays, which are relatively large. That way, I might press down or tack solder one diode and one relay to each position to test. At a minimum I need to test one circuit to be sure the parts work properly together.
AGC output circuit


Design issue discovered

I had realized that I did not think through the method of assembly of the keyboard, specifically how I would get the plunger part inserted into the honeycomb part. The issue stems from the two tabs that jut out of the side of the plunger about .12 inch on each side. You can tell from this that I am not a mechanical engineer nor experienced in designing for someone else to manufacture.

The plunger is .81 x .81 and fits in a channel just .875 x .875. That gives only .065 margin. The plunger can't be tilted very far and the .24" of tab can't be forced down the .065" margin. I can't rely on the honeycomb walls to move .18" on average, which would be necessary to force a plunger into place. This is particularly unlikely since the tabs sit near the corners of the honeycomb channels, where there is essentially no give at all.

In about a month I will have the actual honeycomb in hand, which would permit some experimentation, but I can't burn the money to make the plungers until I am certain I could assemble them. As the design sits, I couldn't

I am thinking about alternative methods of building the plungers and other parts, since I have already ordered the honeycomb which normally would be the easiest part to modify. I am depending on accuracy of the honeycomb and the plunger to establish the right height and depression depth for all 19 button positions. Any alternate construction method has to provide at least as much control.

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