Mike Ziwisky's Place to Write Stuff
Electronics. Software. Helicopters. etc.

PCB Design

Bare and stuffed boards

Now that I had an idea of how the hardware needed to be connected, it was time to design a PCB. For this task I used the free version of Cadsoft Eagle. I hadn’t ever used Eagle before, so it was a bit of a learning curve. But after several hours and a few online tutorials, I found the program to be quite capable without being overly complicated. A word of advice for anyone new to PCB design — it’s a step that should be done in parallel with searching for parts from your favorite retailer.

Eagle makes PCB design easy by allowing the user to first build a schematic, then use the netlist of that schematic to make sure you don’t forget to connect anything that needs connecting when you’re routing your PCB traces. Eagle makes you choose a package for each component before you add it into the schematic, before you even begin to think about the board layout. You don’t want to spend hours designing a board for a component in a particular package only to find out that your favorite retailer doesn’t have it in that package, or they do but only in quantities of 5,000 or more. On the other hand, you don’t want to jump right in and order a bunch of parts before doing any designing, and then realize when your schematic is together that you forgot to order two $0.10 capacitors for which you now have to pay another $7 to have shipped. Instead, whenever you want to add a component into your schematic, find it on your favorite retailer’s website first. Knowing what they have available, decide which package you’d like to get the component in, and then add that one to your schematic. Keep adding parts to your shopping cart as you add them to your schematic. The Eagle libraries are extensive, but they don’t have everything. If you can’t find a part you need in the package you want in the libraries, then you’ll have to either find it somewhere online or create it yourself. I had to create a few components for this project, and I found a nice tutorial on Instructables to help me out with that.

I encountered few speed bumps when building the schematic for the heli lights. One of the most time-consuming decisions was actually finding a pair of connectors to use to plug a battery into the board. Problems without a solution are difficult to deal with. Almost as difficult are problems with thousands of solutions. I eventually (after many hours …ugh) found a Molex brand connector that was about the size and shape that I imagined the board should have, so I ordered some males and females. This was one of the parts I could not find in the Eagle libraries, so I had to create my own. As soon as the schematic and part selection was done, I placed an order for the parts I decided on and then began laying out the board.

Heli lights schematic

One of the decisions I had to make before designing the boards was how to mount LEDs along the length of the blade and get them all wired to the board. I found a thread at the HeliFreak forums that described how to wire up some night blades with adhesive-backed copper tape. I liked the idea of using copper tape because it would allow me lay out traces right on the blade and I could stack up layers of traces with some insulating tape between the layers. It sounded like a good way to wire up the LEDs without screwing up the airfoil too badly. So I placed a 4×8 array of pads near the edge of the PCB for contact points for the copper tape. Minimizing the size of the board was a top priority, and it took me many hours to complete this layout.

PCB design

The next big step was finding the best deal I could for getting these boards fabricated. I surveyed many fab houses. For this particular project, I needed a handfull of rather small, identical boards. The minimum trace size was modest, and a soldermask and silkscreen were not necessary, but would be nice. Considering my requirements along with the cost and turnaround times of the different fab houses, I settled upon a Bulgarian company named Olimex. Their website is a bit of a mess to navigate, but I eventually collected enough information from there to figure out how to properly prepare my Gerber files in the way they required for fabrication. I believe they will also accept an Eagle .BRD file, but I wanted to learn about creating the Gerber files. In doing the Gerber processing, I found that I was using a non-standard drill size. This would have resulted in an extra fee for fabricating the boards, but I was able to modify my Gerber drill file in a text editor to make those holes smaller and avoid the fee.

Turnaround was not quite as quick as Olimex would lead you to believe on their website. With FedEx shipping, I received the boards exactly two weeks after sending in a completed PO. However, I am very pleased with the quality of the boards I received. Olimex includes a soldermask and single-side silkscreen in their base price. This not only gives the boards a more professional look, it also makes soldering the components easier and helps protect against electrical shorting. The photo below shows a board fresh from Olimex alongside one that is stuffed with components.

Bare and stuffed boards

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