Tuesday, April 29, 2008

AA to USB, part IV

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.

Well, the first prototype worked out pretty well. But, hey, one can always do better. So, I sought to make the board smaller by squeezing the components closer together and, more importantly, making it a two-layer board by putting traces and components on the top and bottom. One other major change is that I put a little slider switch on the board for turning the thing on and off. Other devices people have cobbled together often don't include an on/off switch, and I don't really know why. Even if there isn't anything plugged into the USB port, the lack of a switch will cause the batteries to slowly drain away. I had always had a switch in the design, but figured it would be somwhere off the board, a part of the case, and run wires between the two. Then I realized that there wouldn't be much free space in the case, so the switch and the board would be sitting right next to each other anyway. Soldering something to a board is a lot easier than soldering tiny jumper wires between things.

Anyway, this is how the new design looks in Eagle. Looking at board layouts takes a little getting used to. The red is the copper on the top layer, blue is the bottom layer. The backwards writing is on the bottom layer, too - it looks right if one were to flip the board over. The green circles are called plated thru-holes which, as the name implies, means that they are holes that go through the board and are plated with metal to be conductive and solder-able. Some thru-holes are places to attach components, others connect signals between the top and bottom, called vias. This is just a two layer board - imagine the complexity of a six or eight layer board! When things get that complicated, PCB layout folks rely heavily on auto-router programs that use various optimized algorithms to connect signals together. Eagle does have an autorouter, but this I did by hand.

Next I need to have a colleague route this out so that I can populate it. I am also working on a mechanical CAD model of the board, batteries, etc., so that I can design the case just right. Stay tuned.

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