Eagle class at [NSL]
There is always a lot of interest in learning Eagle, it cheap and popular. It is not the best PCB layout tool around but for most people it does what they need.
Thusday, We decided to make a Evil Mad Science Lab’s Larson Scanner, but make it out of all surface mount and try to make it single sided PCB.
Using Camtasia I recorded the screen for each part of the class, at just under 1900×1200, whoops, lessons learnt there, most projectors can’t handle that and it makes massive files. The create a part video was 78GBytes.
So lesson learned, check the native resolution of the projector and go from there! Most people probably don’t run 1900×1200 screens either.
I grabbed the PDF schematic from EVMSL, then drew it into the Eagle layout, Windell uses gEDA for his schematic.
The original schematic. Click for a bigger version.
So its always been difficult for me to source Atmel chips, I’m cursed. Digikey had the MLF package ATtiny2313, which is tiny and basically leadless. Since this is [NSL] that is how we do things.
Next step was to order up enough to make 50 boards, 75 CPUs, 50 switches, 5000 resistors, 5000 capacitors and 500 LEDs. Total cost came into to around $200, not bad for that many boards and we’ve now got some spare components for the next project.
A nice switch was found, and as a bonus it had no Eagle footprint I could find, so this was the perfect part to use for the ‘Make a Part’ video. Another lesson was about to be learnt, don’t concentrate too much on the video or what to say, and pay attention to the part you’re making, more on that later.
Friday, I laid out the board, managed to get it all single sided, and cut the board. Found a new feature of my PCB CNC , Black lines means that the distance is too small to cut, and they don’t cut even in force mode, so the first three boards had missing pads… Oh well it was enough to verify the design and the parts hadn’t arrived yet.
Without the GND polygon processed. you can see the trickiness employed to route the ICSP. But the VCC is missing, which I’d never have gotten in this way without a jumper
Saturday, the parts arrive from Digikey. I re-laid out the board, had a chat with Chris over at AccurateCNC who’s knowledge about his product and PCB making never fails to amaze me, we chat a bit about the errors of the pads, its interesting that in Eagle if you make a part and it looks the same, the slight math errors that creep can make the difference between an 8mil and a 6mil gap. You’d never see it in eagle without looking at the actual numbers. I’ve seen this a few times on different parts. One pad of an SMD 0603 resistor CNC’d the other didn’t. This is probably due in some part to floating point numbers.
The bad boards, notice the missing pad on the LED on the right, the 0603 resistor to the right of that and one of the ICSP is missing the isolation, which turns out to be the VCC(3V) too.
I make the first board, and the MLF package turns out to be really easy to reflow, i basically tinned each of the traces and pad in the middle, placed the part and hot plated it, no problem, except when i test the CPU no go.. Ok add the 3V to the ICSP that i forgot, still no good. And yep the chips in 90o off…. Pull the chip and place another one, this one also drops in easily its almost as if the parts want to sit in the right place, it wasn’t hard to align it at all.
After fiddling about with the AVR dragon a bit, setting the speed in ICSP mode I flash the chip with Wendell’s HEX file, flashes and verified !
Next is the addition of the resistors and LEDs, I dab a drop of leaded solder paste on each pad and add the components with tweezers, throw it on the hotplate and everything’s tickety boo. Add 3V and all the LED’s start their dance.
MMCA says hey try the switch, no go… hmm what up i think, bad joint. Investigate and that’s odd, the schematic said the switch part was lengthways right, nope, its the short side… and the PCB is cut in such a way that all three pads need to be modified. I use an exacto knife to cut the trace , then cut away at the ground plane to isolate all but one of the existing grounded switch pads. This is bringing me back to before I’d rotated the part design by 90o and so the schematic didn’t match the pads. Easily fixed but a pain. Of course I recorded the video and I’m watching myself create a schematic with an obvious short , but wasn’t paying attention to the diagram..
After much late night not enough coffee stuff, its all together and patched. Here is it with a 3V CR2032 cell as comparison.
The red wire is the patch to add 3V to the CPU.. which wasn’t routed in the original design… whoops. you can see the cutting of the copper around the switch to make it work too..
By Saturday morning i have the first pass done, the PCB cut and a working prototype, the changes are back into the board.
Sunday, I’m watching the video and thinking, have to redo the video in a lower resolution and write up a blog post about it, before i forgot half of what i did. I’ve re-laid out the schematic, made it smaller, its now square, removed the second header, moved Opt1 and opt2 to opposite sides to allow chaining, added the 3V. Changed the ICSP 6 pin from 3×2 to 6×1 and also changed the order, since it was difficult to reroute the board single without messy traces and 6mil widths. BatchPCB couldn’t cut the PCB as it was.
Here’s a video of it in ‘action’
And the switch works.
Hopefully on the 29th, a lot of people will have made theirs and add them to pumpkins all over.
Now to finish up work stuff and then cut the hopefully final PCB, test a couple and then cut 50 PCB’s, i hope my new V90 bits last..
No thermals, and i rushed the rubout cut, so didn’t set the depth , but its fine. Only one warning this time and sure enough it had an issue at that corner but right side of the cpu, so i just have to move that trace ever so slightly, I’ll put the polygon thermals back on , as i think it’ll make it hard for people to solder. about 28mm square now. Shame i rushed the rubout as the cut itself was perfect.
After it’s made.
Programming with the non standard connector and pogo pins
Size compare with the previous revision
Hackerspacing up the battery, superglued onto the FR4, loop a wire around the outside of the battery, then another wire under duct tape. Soldering to coin cells is bad, i didn’t do that other one.