With the electrical design completed and parts/suppliers chosen the next step is to layout a board.
This means actually drawing lines that will end up being traces (wires) in the finished printed circuit board (PCB). In other words the fun part! Actually it's quite a bit of work, and not too easy to fit all the traces on the board.
A Note to Past Me
Make a note of this when I finish the time machine and go back in time to do everything right: EagleCAD is the de facto circuit design software out there. All the pro's I've ever met live in it. And, thus, plenty of help is available. This is what I've been working in. It has a free version (that runs on any OS!) but it's limited. It turns out one of the limitations is in the size of the board. -- only a max of 8 cm x 10 cm. Which isn't too too small, but I would have preferred a little more breathing room.
There is an open-source design tool called gEDA that was widely panned by some of my EE friends, but it turns out that there has been a lot of progress made recently and is actually usable now. Too late to switch for this project, but in the future I'll try to use gEDA instead, if for no other reason to support the project. Being open-source it has no limitations -- except features that have not been programmed yet. It's also an open standard with which to share projects.
So here is how layout works: First you create a new board and Eagle dumps in all the parts you designed (hopefully correctly -- read the datasheets carefully) on the screen off to the side. All the pins are connected according to the schematic with straight yellow lines called a rats nest -- see the screenshots below.
In the first image I moved a couple of parts onto the board, but nothing is actually connected yet. I rotated and placed everything in an attempt to minimize the overlaps of yellow lines.
Then I stated routing traces. I started with the LED drivers and LEDs. By the 4th image I have most of the LEDs routed to the driver chips in the middle.
Red and Blue
The colors in the layout tool represent the front and back of the board. Green parts are holes that will be drilled that let electricity go from one side of the board to the other -- called via's.
The idea is to make sure no traces cross each other on any one side of the board. So you go along until you get stuck and you duck down to the bottom of the board through a via and get past and obstacle like another trace or chip on the board then you pop back up where you need to.
By the 6th image I routed most of the traces except for power and the power supply chips.
At this point I had enough done to start adding in some polish, like a ground plane. Instead of etching away 90% of the copper on the board and just leaving the traces I'm leaving most of the copper there connected to ground. This makes it convenient to hook up the ground pins on parts and helps shield sensitive parts from noise.
Because it's a bit confusing what all is going on with the bottom and the top visible at once the last image is of just the top layer with the power pins highlighted. So you can almost make out what the board will look like. Anywhere there is red will be copper on the PCB, black will be bare fiberglass (not conductive) and the green dots will be holes either filled with solder or the leads of a part, depending on where it is.
Next Week: Finishing up the layout and fixing any errors