PCB Layout for the Amplifier and Cell Connector
The second semester of Senior Design is all about making our project deployable. We want to put all of our circuitry on a printed circuit board (PCB), which is a nice professional option for a durable product. It is also a nice alternative to the board we have now with wires all over the place.
We’ve heard that it’s pretty easy to go wrong with PCB design, especially on your first design. Nobody seems to be able to tell us exactly where it is so easy to go wrong, so it must just be the combination of so many different things that all have to go right. One of the biggest challenges for this design is time, so we decided to get a jump on things when we can.
I spent some time over our winter break designing a PCB to test the process out. Getting boards fabricated can be fairly expensive, so we kept this one to 7.5 cm², which is about $20 to get fabbed, including a setup fee and shipping. This is not our entire board, or even a significant portion of it. We took one portion of our project, the amplifier circuit for the local alarm, and laid that out all on its own. It’s a simple design that will let us see if we’re missing anything important in the PCB design process. This part of the circuit can stand on its own, so we can hook up an iPod and a speaker to this circuit and see if it works once we get it all soldered together. We already have some extra components for the amp, so this seemed like an ideal circuit for our testing.
I also decided to include a connector for our cell module on the board. It’ a completely separate circuit from the amp, even though it will be built on the same board. We wanted to do this component, because it has very small circuit-mount leads that we want to make sure we can solder by hand. The leads are about 0.25 mm wide, and there are 60 on the device. For this test, I ran 6 of the pins out to headers that we can connect to for some basic functionality. That will let us power the board, and connect to a serial terminal to verify our connections. If we can get that far successfully, we’ll feel pretty confident about putting our full circuit on a board and having it made.
Doing the design was fun. It makes you think about circuits in a new way. Layout is a puzzle, so I just tried to think about it like a game. I hope the layout of the entire circuit is so enjoyable; I imagine it will be if there is enough time set aside for that task.
The image at the top of this page shows the final PCB layout for this test, minus some changes to the silkscreen layer.
Robert Murrish 