<br><br><div class="gmail_quote">On Wed, Dec 26, 2012 at 2:28 AM, Arnim Littek <span dir="ltr"><<a href="mailto:arnim@actrix.gen.nz" target="_blank">arnim@actrix.gen.nz</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div>> > On Tuesday 25 December 2012 11:29:25 you wrote:<br>
> > > I am getting some hard questions on my KickStarter<br>
> > > project<<br>
> ><br>
> > <a href="http://www.kickstarter.com/projects/1378252804/681865516?token=bfb4" target="_blank">http://www.kickstarter.com/projects/1378252804/681865516?token=bfb4</a><br>
<br>
</div>Maybe I'd better get a higher level view before I say anything about the<br>
schematic. Simply the fact that the design stems from 2004, means that most<br>
of a decade later, you might be wise to consider your options in light of your<br>
highest level objectives.<br>
<br>
a) understandability for your students - OK, got that.<br>
<br>
b) serial control of lab instruments - is RS232 still the best way for you to<br>
do this?<br></blockquote><div><br></div><div>This depends on whether it is my instrument, or logging data off of an existing instrument. That probably means that those details should all be on a daughter board.</div><div>
<br></div><div>For a new instrument, I would not use RS-232.</div><div><br></div><div>Some of the applications that are unique to me are leaking into this "general" device. Actually, if I call this "a case study" it is a good thing.</div>
<div>
<br></div><div>My specialty in chemistry is analytical chemistry. It is important to weigh items to a reliable 0.000 1 g. I have students each weigh the same set of clean nickels (US $0.05 coin that is nominally a metric 5.00 g standard, readily available in the US, resistant to wear, and has "serial numbers" embossed on them) on each analytical balance in the lab, and then take apart the data to see how each balance, and each student, are performing.</div>
<div><br></div><div>To take this lab to the next level, I want to record the time, barometric pressure, temperature, humidity, and the balance output. The analytical balances cost about $5,000, so I am pretty much stuck with legacy interfaces or a converter board. In the case of the Mettlers in our lab, this is RS-232 out of a DB-9 connector. The environmental stuff should be recorded inside the weighing chamber of the balance without heating it up, the RS-232 port is on the outside.</div>
<div><br></div><div>So, for this project, the daughter board should receive RS-232 and a host of other transducer inputs. It should transfer, with very high reliability, the data to a database server. I probably want to use WiFi or IR for this. I don't know the practicality of IR. </div>
<div><br></div><div>All of this is backstory stuff. I think I should hide it from the KickStarter project, but I am just guessing there.</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
c) you've got a socket for nearly everything. Is that because you weren't<br>
confident in your design, or for other reasons? Sockets raise the price, and<br>
reduce the reliability of the finished assembly.<br></blockquote><div><br></div><div>Interesting that you notice that. I am astonished at how much the sockets and headers add to the cost. I am pretty dissatisfied with the every I have thought of.</div>
<div><br></div><div>I socket the LEDs so that I can swap them out, and generate current-voltage curves for different parts. I could accomplish this in a lab by having different models of board with directly soldered parts.</div>
<div>I socket the hobby part so that I can swap out whatever I am working on that moment. That probably could be a customer added socket.</div><div>I have mixed feelings about the transparent latch. My firmware defaults to transparent and I don't think I have ever actually pulled the chip to do an experiment. I might want to have 1 or 2 socket-ed in a lab, and have the rest of them directly soldered. Another situation for 2 models, I suppose.</div>
<div><br></div><div>I pulled parts from one prototype to build out the next prototype. LOL. I even de-soldered sockets to go to the next board. I think this was a bad time-money tradeoff decision, but I grew up poor and have some poor people habits that keep me poor.</div>
<div><br></div><div>Since the board is also a programmer, I have put a ZIF socket on some revisions. That really adds to the cost. The current board is a little too tight for a ZIF socket. I worry about students destroying chips prying them out of a socket. I have good luck with a chip puller, or even a slim bladed pocket knife inserted under the chip and twisted, but some students have bad attitude and a streak of vandalism.</div>
<div><br></div><div>It seems feasible to make this a customer decision in the KickStarter environment. I would then have hard numbers at purchasing time, and I could buy 5% excess of everything for those who make an after purchase change of mind.</div>
<div><br></div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
d) ref. b), if you use USB, then you can simplify the power supply. However<br>
there are provisos on power via USB. For my own part, I find FTDI USB<br>
interface chips good for anything I've wanted, and because the drivers are<br>
widely, and well, supported, which matters to me as a Linux proponent.<br></blockquote><div><br></div><div>The original revision, back in 2008, was to go to the PIC 18F4550 part. I changed the board to take power from USB, and to have fairly short traces for the USB data. <a href="https://docs.google.com/open?id=0Bx35YZ-0A4ffajNRVzd0R1BxZVE">I uploaded those EAGLE files</a>.</div>
<div><br></div><div>I forgot about the FTDI part in the past 8 years. I dropped an FTDI daughterboard into a prototype board, and ran jumper wires over to my board. It worked really well. I have laid out artwork where that chip is on my board. I can't remember if I ordered that artwork or not. I seem to have a pipeline of built out prototypes, bare board prototypes, files with ECOs on them, files with unrelated projects that might fill an empty spot in a prototype run. It's getting too complex for me to manage.</div>
<div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
e) how much help did all of those LEDs provide you on the existing samples of<br>
the board? Are there better ways to communicate the same information?<br>
<br></blockquote><div><br></div><div>The LEDs let me and a group of students see what is going on very quickly and without any new tools intruding in the experience. If I directly solder them, the total support is only about USD $1.10 for the board.</div>
<div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
I'll leave that as a starting point, and we'll see where it takes us...<br>
<br>
Arim<br>
</blockquote></div><br><br clear="all"><div><br></div>-- <br><font face="'times new roman', serif">Aubrey McIntosh, Ph.D.<br>211 E. 5th St.<br>Morris MN 56267</font><div><div><span style="line-height:20px"><font face="'times new roman', serif"><a href="tel:%28512%29-348-7401" value="+15123487401" target="_blank">(512)-348-7401</a></font></span></div>
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