[GreenKeys] John Nagle's USB <-> 60 mA loop board

[Pete Lancashire]

I love Seeed, used them in the past, and on a bench is a Seeed / Beaglebone
CANBUS setup. Highly recommend.

May not be the cheapest although many times they are but the quality is
there.

-pete

PS IF there is room for a Big Teletype(r) Logo in the silkscreen that would
be sweet.

On Thu, Feb 22, 2018 at 6:57 PM, John Nagle <nagle at animats.com> wrote:

> On Wed, Feb 21, 2018 at 3:24 AM, Bruce Phillips
>>
>>>
>>> I was planning to build a few in the very near future.......I'm in
>>> Australia, but postage should not be too much for a small item like that
>>> if
>>> there is any interest. I was very conscious not to tread on anyone's
>>> toes.
>>> I planned to change the layout of the pcb to as much smd as possible, but
>>> that was just my preference for my own assembly. In any case I will have
>>> probably 20 pcb's made, so don't mind sharing them around.
>>>
>>> Regards,
>>> Bruce Phillips
>>> Australia
>>>
>>
>    Feel free to use the design. That's why I put all the files on Github.
>
>    Blank boards are the easy part, and cheap. I have mine made by
> Seeed, in Shenzhen.  It's the assembly that's hard.
>
>    Notes:
>
>    - An all-SMD design would be nice, but there are some problems.
>      The CP2102 USB to serial converter is packaged in a QFN28 package.
>      Those mount via pads under the part, 0.4mm apart.
>      While it's possible to hand solder such parts, the technique
>      is complicated. See it done:
>
>      https://www.youtube.com/watch?v=lhOKSqipwqg
>      https://www.youtube.com/watch?v=ljyGMOi5JPo
>
>      Data sheet:
> https://www.silabs.com/documents/public/data-sheets/cp2102n-datasheet.pdf
>
>      That's why the daughter board for the USB to serial converter.
>      I'd like to get rid of that, and if you're going for assembly
>      using a solder paste stencil and pick and place machine,
>      go for it.
>
>    - C1 and C2, the 1uf ceramic capacitors, are through-hole
>      because the new multilayer surface mount large-value
>      ceramic capacitors (MLCC) on't work. Their capacitance
>      decreases with voltage, down to about 20% of normal
>      value. They're OK for filters but not for energy storage,
>      which is how they're used here. Watch this tutorial on
>      MLCC capacitors.
>
>
> https://www.digikey.com/en/ptm/m/murata-electronics-north-
> america/high-capacitance-hi-cap-mlcc/tutorial (see slide 7).
>
>   - The layout around the switch controller U1 is critical. Trace
>     lengths must be very short in that area, or it won't work at all.
>     See the data sheet.
>
> http://cds.linear.com/docs/en/datasheet/3750fa.pdf
>
>     This is a switching power supply operating around 300KHz with
>     2ns rise times. Keeping those spikes out of the computer that's
>     powering the thing, or generating RF noise, requires
>     tight layout and various bypass caps and ferrite beads on the
>     board. Steve Garrison says the thing is RF-quiet; he's not
>     hearing it on his receivers or seeing noise on a spectrum
>     analyzer.
>
>   - The board could be made shorter. There's a price break if you
>     get all dimensions below 100mm.  I just used that size because
>     there's a nice case available for 125mm x 75mm boards.
>
> It took seven revisions of the board to get this working. It's
> non-trivial.  If you redesign the board, be prepared to debug it.
>
>                                 John Nagle
>
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