Monday, April 30, 2018

RC2018/04 wrap-up

At the beginning of April., I entered the Sinclair Scientific Calculator emulator in the Retrochallenge 2018/04.

The following was the description of the goals associated with this challenge:

"For this retrochallenge. I want to completely redesign the PCB so it has the same dimensions as the original: 111 mm tall / 50 mm wide. While the display will remain the not period-accurate LED, all the components will be placed so that their centerline is in the same position as the originals. The board might end up being used with an enclosure, so components will be placed away from the edges such that a smaller version can be be quickly manufactured. Lastly, for aesthetic reasons, the board will have the same color scheme as the calculator and the traces will be discreetly run in the back, as much as possible."

The calculator at that time looked like this:

While it was an improvement over the Green V1 PCB, it was not quite the right dimension, the buttons were not placed with the correct spacing and there were some visible traces in the front. The circuit was also based on the KIM-1 and while it worked, I now realize that it was not the most efficient use of I/O pins for this particular combination of keys and digits.

The initial goals were somewhat easy, redesign the above circuit so it is size accurate and no traces in the front.

At the time, I already knew how to conceal vias by tenting them (placing soldermask and silkscreen over them. The following project log shows the technique:

I had already learned how to do a keyboard label using negative silkscreen as shown in the project log:

Another PCB design technique learned during this month was how to create custom PCB shapes. This can be as discreet as a rectangle with rounded corners or as exotic as an insect shape board:

By playing with an actual Sinclair Scientific calculator and pressing multiple keys, the connections between the keyboard the the display were deduced without disassembling the unit:

The following schematic was drawn using the information learned:

Using all these techniques, the following board was designed. The placement of the components matches the position of those of the real calculator. Most of the copper is in the bottom. Some unavoidable pieces are placed as long horizontal lines in the front. A couple of tracks that needed to go in the front were hidden under the 8 key.

View of the back of the board copper layer:

Copper in the front of the board:

An assembly guide was put together showing how to solder all the components;

And here are pictures of the evolution of this project:


The latest version is placed next to the original

Overall, I am satisfied with the progress of this project. The latest version (V6) implements a calculator that is dimensionally and electrically correct.

All of the lessons learned are documented in the following page:

Friday, April 20, 2018

A review of @Jon_Raymond_ Banana Ruler

It all started when the following tweet showed up on my timeline.

What a brilliant idea, we can finally have a PCB ruler to accurately measure objects in fractions of a banana.

The order was placed with @Oshpark and 2 weeks and $9.50 later, three of these showed up in the mail.

It's absolutely beautiful. The copper layer is exposed to form the shape of the banana, the numerals and the manufacturer logo in the back. The color of the ENIG (electroless nickel immersion gold) coated copper perfectly represents that of a ripe banana.

Technically, this is a beautiful board. The PCB is a custom shape, a rectangle with adjacent corners having a different radius, but opposing corners radiused the same. The copper layer is the same shape as the PCB, but kept away from the edge 1/22 banana. The PCB version number has been written as a negative space in the top(?) copper layer. The use of negative space continues in the soldermask layer. Both the shape of the banana and the numerals are in negative, exposing the copper underneath. No such tricks were employed in the silkscreen layer. The words "Banana Ruler" and the ruler line are unremarkable. A generous hole has been provided to attach this to a keyring. The inside of the hole has been plated.

I received v1.1 of the board. I am unsure what are the differences to v1.0

And now, for the question I am sure everyone is asking themselves. What is the conversion factor between the banana scale, standard and metric units.

The banana ruler end tick mark was carefully aligned with a Stanley tape measure.

The other end of the ruler seemed to be a cat hair over the 1 1/8" mark.

A set of digital calipers were used to measure the inner distance between the end marks.

28.34mm seemed too little. 28.35 was too much.

And there you have it. I highly recommend everybody gets their own Banana Ruler to insure the smooth flow of commerce and exchange of technical information. This is a quality product that with proper care should last you for a long time.

Here is a link to the Dirty Engineering's Banana Ruler shared project at Oshpark.