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New design (includes special WS2813 footprint)

A project log for RGB seven-segment display [RSSD]

My journey to create a big but thin, evenly illuminated seven-segment display.

JanJan 11/12/2018 at 11:564 Comments

Font overhaul

The layout of the segments has been revised. Dimensions are as in the following drawing.

You can see that everything is now nicely symmetrical, aligned and has even dimensions in mm for all important features (segment width, supports between segments, character height). I did this to have it scale down nicely to 50mm (segment-width is 5mm in that case) and bigger is easily scaled too.

Optical correction of the corner-segments

This is something that always annoyed me with this font. The marked corners do no look aligned, though they are perfectly in line. They seem to stick out a bit. This is an optical thing, which needs to be corrected by an offset.


Here's the version with corrected corners:


Here's how little correction is needed (0.3mm):

Foundation of my work is the digital-7 font which is free for personal use. By the way. There even is a Segment Displays Wiki on the net, exclusively dedicated to segmented displays and fonts. Wow :)

LED's per segment

For the 100mm font-height 3 LED's per segment are used to achieve the "top-to-bottom rainbow effect" I want to have with the finished 4-character-display (plus separating colon)

I want to show the time as 24 hour format plus maybe temperature and/or date.

Modularity

Many different PCBs for each segment would result in no bulk discount at most PCB manufacturers, so I created one PCB, which can be used for every 7-segment-letter including the colon.

Later there will be four "dumb" segments with just LED's and one smart segment, which in turn controls the four others.

Each segment has all LED's in series plus an input and output connector. A solder bridge tells the "smart segment" which one is first and which is last.

Control of the Segments

One master-segment has all the parts placed to control further segments. The programming is done via USB (CH330N USB/UART bridge). It uses the same driver as the well known CH340 from all those cheap Arduino clones.

It has three pots (yes pots, not rotary encoders) to set the time and color of each/all segments. As the clock works in HSV-color mode, there's one pot for each value.

Reducing the overall height

This step includes two aspects:

  1. setting the LED's in into the PCB
  2. adding more diffusion because there's less overhead space for the LED's light to mix now

Setting-in of the LED's

This is done by creating a slightly undersized cutout of 4.9x5.3mm in the PCB for each LED. This makes soldering much easier. LED's are pressed in by hand with little force. There's no need for further aligning, just solder them from the bottom-side and you're done.

PCBs are from Aisler this time as they only take 8 days from uploading Gerbers to your mailbox if you live in Germany. I already asked them about their cutout-quality. Edges are not as straight as I am used to by OSHPark...

PCB - bottom
WS2813 - they align differently because I used desoldered ones from an RGB LED strip
bottom side

You can download the KiCAD footprint here: https://cdn.hackaday.io/files/20142861755328/WS2813_sunken_final_Aisler.kicad_mod

It is not possible (at least not in KiCAD 5.0.1) to draw lines on the Edge Cuts layer. If you wonder how it's done: draw the cutout on the DWG User layer, save the file, open it in a text editor and substitute your "layer dwgs.User" with "layer Edge.Cuts":

Save your file and close/reload your KiCAD. Be aware that modifications to these lines in the KiCAD footprint editor will give you errors, telling you it has detected segments on unsupported layers. Just click "Cancel" and you're fine.

Discussions

Mike Szczys wrote 11/29/2018 at 15:37 point

That's a neat KiCAD trick for getting the shapes you want on the edge cuts layer. How is the soldering process? Is it easy to bridge the gap with solder only or are you using some scrap of wire for a more robust connection?

Font looks really great. It's an interesting point about optical illusion on the edge. Thanks for taking the time to write about thatone!

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Jan wrote 11/29/2018 at 17:19 point

Hi Mike,

soldering is easy. LEDs are pressed in and soldered in place. Absolutely no need to use wire whatsoever. The gap looks bigger than it is. In reality it's more like 0.1mm or so. One LED is soldered in a few seconds.

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Robert Mateja wrote 11/28/2018 at 20:28 point

That's second time I see horrible/hobby grade pcb cut from Aisler. All boards are like that? What is your experience? Thx.

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Jan wrote 11/29/2018 at 08:12 point

I'm in lively contact with them at the moment. Felix from Aisler is very cooperative and honest here. Hats off for that!

The very crude board edges from the very early days were because they tried to cut the boards after everything else was done (manually). They very quickly switched to the standard-method other board houses use.

The edges above are that way because of quite high feed rates. We'll stay in contact regarding that issue.

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