ZephyrEye: Board Layout
Have you ever wondered, if you were a cat, if you would have been one of those cats that would have chased the laser pointer mark across the wall, again and again and again? Eight hours a day? If yes, then there’s a promising career for you in circuit board layout!
Actually, it’s not quite that bad. I have to admit, though, if I were a cat … well, anyways, it’s kinda fun in a weird way to lay out a circuit board (which is probably what the cat would say too). You get to put all the components in correct positions, and it gets you outside of the flat earth of schematic-land and thinking a little in 3D. These parts have to fit in an enclosure, and that’s usually the trickiest part of board layout: How is this PCB going to fit in its final resting place? Will the charging plug fit? Where will the screen go? How will buttons make it outside the enclosure? Then you hope there’s enough space left on the surface of the PCB to connect them all together with flat copper wires.
I have to lay out a disclaimer here (hmmm … probably should have a few blog posts ago …) that I was working towards a proof of concept more than something that could actually be used. The approach I took led me to design a project that would absolutely shatter into a gooey, paintbally mess of plastic and electronic parts if it took a direct hit. That … actually makes it sound kind of fun … (No Brad! Don’t do it! Don’t go into the cave with your lightsaber!) I’ll explain later how I intend to make this thing field-hardened and able to withstand the worst abuse that even a cartoon Nelson (“Haw haw!”) could throw at it.
So here’s a list and short description of the approach I took for the enclosure and mechanical interfaces:
- ABS Plastic enclosure from Polycase.
- Edit: The case is actually a PacTec enclosure, from the PP series. Memory lapse caused this unfortunate mistake.
- This one from SparkFun is a little bulkier, but might work as a decent replacement (no guarantees!).
- 1/16″ Acrylic (Plexiglass) from a local hobby store.
- I carved into the case with a drill, flush cutters, and an Xacto knife to make a viewing hole for the LCD display.
- Cut the acrylic with scissors, and stuck over the viewport on with double sided tape.
- 3/8″ tall tact switches, with stiff contact force to prevent accidental presses.
- Drilled using a 1/8″ drill.
- 2.1mm Barrel jack (planned) for charging from a wall wart.
- Since I only charge it from my bench power supply, I decided to just leave it with male 0.1″ pin headers.
- I left the top plate off until I was sure things were working. Then I left it off because it was working – why rock the boat? The charging connector therefore required no drilling.
- An electrical component for sure, but a very large one with special placement considerations.
- Fits flat behind the PCB inside the enclosure.
- Again, an electrical component but it’s large and needs to be placed carefully.
- Originally, I mounted it flush to the back with foam tape. It seemed like this caused it to take way too long to get a good lock on the satellites…
- So I placed it perpendicular to the face of the enclosure heading out the back direction. Again, double-sided foam tape. It’s just about as important as a soldering iron for most projects I do.
- A simple slide switch was used.
- Just cut the positive terminal from the battery and soldered this in series. Again, no top plate makes installation easy.
So, on top of all the electrical connections, we’ve also got to worry about all the mechanical placement when it comes to board layout. So let’s get down to it. I chose a two-layer PCB, meaning you can put copper on top and on bottom with copper plated holes (vias) that connect signals from one side to the other. The red denotes where copper will be placed on the top side of the circuit board, and blue denotes where copper will be placed on the bottom side of the board. Without further ado, here it is:
The inverted looking silk screen helps show which parts are to be mounted on the back side instead of the front.
Eagle actually has an autorouter, which works OK even though the results are usually ugly as sin on Sundays and it’s generally hard to fix things if you add new connections post-autorouter. Nevertheless, on large boards it’s still a useful and time saving tool that’s worthwhile. I used it for this project, and then went through and thickened higher current wires and corrected a few other items of importance. I also laid out some large ground planes using polygons (see the big read lines that go around the perimeter?), which snap a large copper plane around existing traces without overlapping or shorting other traces.
That’s about it for the board layout process. I used Sunstone Circuits to fabricate the circuit boards, they are in-state to me and ship it the following day (free ground shipping, which is overnight to me). There are lots of other places that are probably cheaper, but I’ve never had a quality problem and I get them REALLY quick. Patience has never been a virtue of mine. In fact, it’s never been a virtue – that was a lie spread by the magical powers of ligers in the late 18th century.
Uh, I ordered a few extra boards (price breaks levels work solely to generate revenue), so if anyone wants one, feel free to request in the comments! They’re not perfect, but can be made to work.
Next time: I’ll go over what I found (and didn’t find!) during testing.