Friday, July 11, 2008

AA to USB, part V

This is the continuation of my little project to build a device to power a USB port off a pair of AAs. Click for part I, part II, part III, and part iv.

And you had thought that, just because I hadn't breathed a word of this project in three months, that it had gone away. Ha! Just because I'm putting the bathroom back together, have had some busy times at work, done some traveling, and am awaiting my firstborn, doesn't mean I've lost interest in getting my geek on. Today's update: circuit boards.

These little suckers are the implementation of the design I had been doing on the computer. My earlier, prototype board, was made by one of my colleagues using a sort of Dremel on an x-y table, cutting away copper-clad board. These little boards were made by photo-etching. Copper clad board gets coated with a photoresist. The circuit pattern gets shone onto that resist, and the unexposed resist is washed away. Then the whole thing gets dunked in acid, which eats away any copper not covered by photoresist. There are some other steps, like drilling many little holes and plating them with metal, then covering most of the top and bottom surfaces with that green plastic.

This is, in the main, a bit outside the realm of amateurs - it is best done by a "board house." It's not all that different from, say, getting a stack of wedding invitations made up. In my case, I used a small company called BatchPCB. For a handful of really small circuit boards, this was the least expensive option I could find. This company works by gathering lots of smallish orders, from hobbyists and makers, and lays them out on computer to fill large panels, which then get sent off to a board house in China. The full-sized panels get sent back to BatchPCB, who then cuts them up into individual boards, assembles the orders, then sends them on their way. So, the turnaround time is pretty long: about 4 weeks in this case. But, I was charged only $2.50/board, since my board is about one square inch in size. Other places I know of, like Advanced Circuits, can turn out a board in 2-3 days. Usually, for their smaller orders, they charge a flat rate for a board in a given size range. My 1-sq in board would have cost as much as a 10-sq in board, some $20-60, typically with a minimum order order of several boards. That's actually a good deal for a medium or large size board, but not cost effective for tiny ones like mine. If it were on the company's dime, and a project's momentum counted on a fast turnaround, spending a few hundred for 2nd day prototype boards is no big deal. When producing thousands or millions of a circuit board, the cost is pretty small, only a dollar or two for a cellphone or ipod's. For a hobbyist, the priorities are different.

Yup, it's a pretty small piece of work - a lot is crammed in there. There's an equal amount of stuff going on underneath as well. I'm pretty pleased with the results so far. Now I just need to find some time to populate the board and see if it works!

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