Author Topic: The Living PCB Design Thread  (Read 203316 times)

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Offline bpiphany

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Re: The Living PCB Design Thread
« Reply #800 on: Thu, 03 January 2019, 16:41:16 »
Hi,
I'm currently trying to build my own full size keyboard with a custom layout. After reading through the whole thread (I actually just made an account so subscribe to this thread  ^-^) I still have some questions:

  • I've read that you should not place traces below the oscillator and surround it by a ground plane. Is that only specific to that layer or should I keep the traces on all layers away from the crystal?
  • I want to use a ceramic resonator. Do these even have the same issues with interference as crystals?
  • Originally I wanted to add LED backlighting but now I'm unsure: At >100 LEDs in parallel, drawing 20mA each, thats more than 2A (and therefore more than the 500/900mA USB provides and way more than the 20/100mA the Atmega32u4 can drain). What are the tricks to get low power LED backlighting?

Thanks a lot.

1. The surrounding ground would be to shield off the crystal. Preferably a plane underneath it, and a ring off GND around it.
2. Ceramic oscillators may not be stable enough to work properly with USB.
3. You don't want 100 LEDs driven at 20mA each continuously at the same time. You would be blind. A few mA probably is enough, but you would still need to be careful in the design to meet the current limits.

Offline kuerb

  • Posts: 3
Re: The Living PCB Design Thread
« Reply #801 on: Fri, 04 January 2019, 08:53:10 »
 Thank you for the response.

@1&2) I designed the PCB around the Adafruit Feather 32u4 Bluefruit LE because that already works with QMK. I guess I'll think about switching to a quartz. But then I'd have to retrace a lot of wires for the extra caps and bigger component :eek:

@3) Now I feel stupid about the LEDs ;D I only thought about reducing the duty cycle to keep the overall power consumption low and didn't even think about what would happen if I connected the LEDs to a low current power supply (resembling the pin).

I just tried a setup of 25 parallel green* LEDs. With 5 mA on the power supply (that should be equal to 100LEDs@20mA) each LED still gets to around 180 uA but the power supply is also lowering the voltage to 2.2V. But at least the LEDs stay visible with artificial lighting. With keycaps on the LEDs, they're only visible in the dark but that might be just enough for a very subtle backlight. Powering the LEDs through a Teensy (LC) was also possible but with a bit of flickering.
*or rather the green parts of RGB LEDs

I might hold back on adding per-key backlighting and just add a few dedicated LEDs instead. I can always upscale for later projects :))

Offline bpiphany

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Re: The Living PCB Design Thread
« Reply #802 on: Sat, 05 January 2019, 14:35:51 »
1&2. I haven't looked too hard but I couldn't find out what the part used actually is. According to the datasheet the 32u4 should have an internal oscillator that's supposedly possible to trim to make it good enough for USB But there's very much not a promise in there that a resonator will be good enough in general.

3. It's not a fantastic idea to just parallel up LEDs, google for thermal runaway. And you'd probably want to drive them through something else than the micro, a dedicated LED driver or simply with transistors.

3++. Using a switching regulator to lower the voltage for the LEDs can save a lot of the power consumption. You should probably still really have a current limiting resistor in series with the LED. but you can shift more of the power to be dropped actually producing light.

Offline kuerb

  • Posts: 3
Re: The Living PCB Design Thread
« Reply #803 on: Wed, 09 January 2019, 08:14:32 »
@3) I know that parallel LEDs weren't the best idea, I just wanted to test the output before scrapping the idea of LED backlighting. But in the end I removed all of the backlight LEDs from my PCB and opted for four dedicated LEDs instead.


I want to add support for multiple layouts. But then I'll get holes overlapping pads (as well as overlapping holes) like in the picture. Can someone recommend a manufacturer who will produce it that way?
I asked support at JLCPCB (the people behind EasyEDA) and they told me that they can't produce it (although I'm not 100% sure that my question was understood correctly).

Offline bpiphany

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Re: The Living PCB Design Thread
« Reply #804 on: Thu, 10 January 2019, 14:44:49 »
It's probaby going to depend on their process and tooling, if they are willing to do it at all or at the price point. I think you'd generally not want to overlap holes by too much when drilling. A bottom cutting end mill may be able to do more awkward cuts, but be more expansive and not doing it as fast as a drill bit. You want to have enough wall for the bit not to snap into the previous hole and break.

I've had this made https://geekhack.org/index.php?topic=67713 with overlapping holes just like your design.

Another option is to actually make the overlapping holes into a single slot, which will be milled instead. Like on the phantom https://deskthority.net/w/images/3/3e/Phantom_PCB-Small.jpg

The manufacturer may also be reluctant milling into copper pads, especially plated through holes. It can tear the copper and cause shorts, and they may still do it at your risk.

3+++) There are different (probably more than you can count) LED drivers that will drive any number of LEDs you'd ask for through any serial interface you'd ask for (probably). They can often do constant current delivery to simplify your BOM, and life in general.
« Last Edit: Thu, 10 January 2019, 14:50:45 by bpiphany »