How to Build a footprint

Well, not really how to build one in a technical sense, but some thoughts on how to better ensure that you get it right. In theory, it shouldn't be that difficult. You download the datasheet and build the land pattern based on the information in the datasheet. That usually works, but not always.

I had a thru-hole battery holder that didn't match up with any of the land patterns in my library, so I modified one that was close. That worked mostly okay, but there was one measurement in the data sheet that was a little ambiguous. I ended up with the mounting holes being off by a millimeter or so. Not too much, but enough to make the fit difficult.

I went in and shifted the leads over by the same amount, used it again, got another PCB fabbed and discovered that I had shifted the pins the wrong way! Then it hit me. In the first application, I had the battery holder on the bottom side of the PCB but I had looked at it through the mounting holes from the top side of the PCB. D'oh! One reason why I'm not a professional designer.

The other part was a little tiny SMT trim pot. Since there are pretty close to a million different little trim pots, the likelihood of me finding an exact match in my CAD library was precisely zero. I didn't want to Gieger VR mistake close re-invent the little zig zag resistor symbol, so I just found a part that looked the same. Well, it was almost the same. The footprint I found is for a 4mm x 4mm part and the part I ordered is 3mm x 3mm. That's a tiny trim pot. Somehow, when looking at the datasheet, I got the measurements wrong. Once the part came in the mail, it was quite obviously too small.

The pad pretty much ends right at the edge of the trim pot. We won't be able to reflow that part. No solder paste would be touching the pads on the trim pot. I'll see if our guys on the floor can figure out how to get the thing soldered on there. If they can't, I'll need to look for a larger part to put in it's place.

Fortunately, I physically looked at the part and the PCB before assembly. Unfortunately, I got the measurements wrong. If at all possible, get some sample parts before you order your PCBs. Then you can print out a 1:1 image of your PCB and lay the parts out on it. That would have saved me in both of the above cases.

Duane Benson
Is it "datasheets" or "data sheets"?

IS IT VALUE? IS IT PROTO?

SMT Geiger PCB

Looks like it to me. I got a couple of nice PCBs from Sunstone here. The parts, except for the Geiger tubes are here from Digi-Key too. The tubes are someplace between the Ukraine and Canby. All I have left to do is kit it up and place the order here at Screaming Circuits. It may seem silly that I have to go through the effort of placing an order on our web site when I work here. But it's not.

Doing so does two things. It reminds me of what it's like to be a customer. Always a good thing. And, it doesn't disrupt the shop floor with something that is outside of our process.

In case you haven't been following, this is my SMT re-layout of the open source Geiger counter designed by Jeff Keyzer of MightyOhm.com.

Duane Benson
Tick. Tick. Tick. Tick. Tick. (actually, I hope not)

More Geigering

SMT Geiger 2nd I didn't get back to this at the same bat channel and same bat time. Sorry if anyone tuned in and found the Penguin instead. I think I'm ready now though. I did a bit more layout tweaking, moved the MCU bypass cap closer to the supply pins and added in a MAX3232 so that I won't need an external driver / transceiver board if I want data on my PC.

I'm calling it done. I have my gerbers in a .ZIP file, my centroid and the completed BOM . Next step is to get some PCBs fabbed up. I'm trying out the ValueProto service from our partner, Sunstone Circuits. As I said before, if, after assembly, this design actually works and counts Geigers, I'll post all of the design files as needs to be done with Open Source hardware.

Here we are at ValueProto. The PCB is 4" x 2.7". I'm not going to do a quick build and my zip is 97013. $57.40 for one. $34.90 each in quantity ten. That's all the questions I need to answer. I upload my .ZIP file, check a few boxes and the order is now placed. Next, I'll get some parts, kit it up and send it through Screaming Circuits. The NOS Soviet Geiger Mueller tube is one the way from the Ukraine.

I'm not sure what I'll use as a radiation source to test it out with once it's built. I hadn't really thought that far ahead. I'll have to come up with something.

If you're going to be at the Embedded Systems Conference in Boston next month (September 27, 28), we'll be there in booth 615, across the isle from Element14. Stop in and take a look at it. While you're there, ask for fabulous prizes and gifts. We have a smattering of Screaming Circuits shirts and flashlights for the asking.

Duane Benson
No baked beans, please

ValueProto from Sunstone

I think I'm done with the geiger counter layout. Now I just need to get the thing built up to see if it works. I'm pretty sure, but you never know. I have an idea... I'll build a prototype. And... I'll build it in as self-serving a way as I can. How might I build a prototype in a self-serving manner?

First, I'll use my company (and our partner Sunstone Circuits) to build it. Second, I'll write about it here. Technically, you're not really supposed to review your own stuff, but I really don't get to order things very often. I know all about Sunstones's PCB fab services, but I haven't used their ValueProto service so I'm using this as an opportunity to do so. This PCB looks like it should work for their "ValueProto" service as well as with Screaming Circuits' "SimpleProto" service. Small quantity, no leadless parts. Perfect for the simple and value services.

DPAK in gieger I actually made a small change since I last wrote about this design. The particular high-speed, high-voltage transistor in the original design isn't available in an SMT package. I could have still left that one part in thru-hole, but I didn't want to so I didn't. This is one of the reasons I understand the difficulties of parts substitution. I found two simialr parts. One in a SOT-23 and the other in a larger DPAK. I really wanted the smaller package, but the specs of the DPAK part were closer. The DPAK is quite a bit bigger than the SOT-23, but it fits.

When I pulled up the layout to take this screen capture, I notices that the "Q1" label was slightly on the big pad for the DPAK. That's not good. When I find a last minute error like that, I usually take that as a sign to go back and give everything another once-over. I'm going to do that tomorrow, so stay tuned.

Duane Benson
Same bat-channel. Same bat-time. (Different real-time though)

Open Source The Eskimo

Not long ago, I found myself in the vicinity of the former Trojan Nuclear Power Plant. I say former, because it's not there anymore. The reactor's long gone and the cooling tower was exploded a few years back. I was just under a mile away, and downwind, from the tower when it went down. The cooling tower didn't have exposure to radioactive material when it was in operation, but the dust cloud was still pretty annoying. Interestingly, prior to the nuclear power plant being there, the land was owned by the Trojan Powder Company, maker of explosives. I wonder if the blasting material was made by the Trojan Powder Company. Today, there's just a park and a few abandoned buildings left on the spot.

While I was there recently I pondered the possibility of small amounts of residual radiation in the area. I quickly forgot the thought until I ran across a post by Jeff Keyzer (mightyohm.com) about an open source geiger counter he designed.

SMT Geiger Well, who could resist the chance to build a geiger counter and run it around looking for bad stuff at the former sight of a nuclear power plant. Not me. Here at Screaming Circuits, we specialize in surface mount, so I thought it would make sense to re-layout the PCB for SMT components. Which I did. That's the cool thing about open source. (one of the cool things) The design files are accessible.

The only real challenge I had was in finding a few parts substitutions. The exact match wasn't available in SMT for some of the components. Hopefully, I picked good subs. Doing that is frequently not as easy as it might seem. Especially when the original design is not mine so I don't necessarily know which parameters are in the criticle path.

Now I have to get some parts from DigiKey or Element14, PCBs from Sunstone Circuits and I'll have to run over to Eastern Europe to pick up an old Soviet era geiger muller tube. If my layout actually works, I'll pack up the SMT files and BOM and make them available as all open source is supposed to be. If it doesn't work, I'll quietly try to disavow any knowledge of every having tried to mess with it.

Duane Benson
Everybody's building ships and boats
Except us. We're building prototypes.

 

Check Again.

How do you know? How do you know what? It could be how do you know if that new restaurant has good food, or how do you know that the car you're about to buy isn't a lemon. It could be a question of how do you know that the cigarettes you're smoking will mess your lungs up. Wait. You do know that answer to that one.

But I'm talking more about substitutions. When choosing a part, there are a wide variety of parameters to check. Some mater for your design and some don't. If you run into a part that exactly matches all of your parameters, you'll probably be okay. If that specific part is in short supply, how do you go about finding a suitable substitution?

A good example is the CDBW0520-G, Schottky diode. I had used that part in the past because it was physically small enough (SOD123) and had the stats I need. I pulled that same part number out of an old BOM to use in a new design. When I went online to check the price, I found that they were almost out of stock. I remembered when I originally searched for that part, I had a lot of trouble finding anything in that particular package. I could go to a physically bigger part, but I really didn't want to. Space isn't super tight, but tight enough.

I needed as low a forward voltage as possible, and this part drops just over a third of a volt and can pass half an amp through. My first instinct was to look at higher current versions, but they all had bigger packages. Next, I looked within the same manufacturer for a higher voltage part. I found one with a 40V max in the same SOD123. That was fine. The original was 20V.

The only bummer was that the CDBW0540-G drops half a volt. Not a great difference, but when your supply is 3V, you need to keep as much as possible. For some reason, a few days later, I searched for the part again and must have taken a different route down the parametric search because I found one from a different manufacturer with 340mV drop and a package just a hair smaller. And, it has a higher current rating to boot. That makes me happy and content.

Duane Benson
...because I live in a split level head.

Flutter

If there was a lot of twittering going on, I think I'd call it "flutter." I could call it "a bunch of Twitter tweets", but that's too long and awkward, so I'm good with flutter. Probably because it's short and rhymes with clutter. If there's really a lot, then we could call it flutter clutter.

Regardless, I'm still in my quest to determine if Twitter really does have a use that matches up with something I might need or find useful. I'll just take a few examples. What I'm finding is, in addition to the "I ate a cheeto" noise, there seems to be useful information. I regularly pass through a fair number of websites, but there are more that I would like to keep up on.

If the website owner does a good job, I can keep posted on their doings and I can know when I need to pop over for more detail. Adafruit is a good example of that. I'm not currently in the market for anything they sell, but they are one of the most influential members of the open source hardware community. By following them on Twitter, I can just glance at their announcements quickly and quickly jump over if I want more detail. That works pretty good for keeping up with the OSHW folks. I have a number others that I follow for similar purposes..

I also like to keep up with the mood and mindset of the engineering community. I read the trade magazines (or their websites) but there is more to it than that. I don't follow many periodicals because the volume of tweets tends to be too high. I have few (SilconFarmer, Chris Gammel, Mighty Ohm and freaklabs) that I follow specifically for that purpose. That's useful.

MaxMaxfield AKA Max the Magnificent always has interesting things to say. Some just his own thoughts and some teasers for interesting articles he's written over on the eeTimes website. And he posts just about the right amount. Enough to be worth following but not so much as to become noise. Mike Buetow over at Circuits Assembly magazine does a very good job of keeping me informed about what's going on in the EMS industry. Very valuable.

Okay, so that's not everyone I follow, but it's three different types of Twitter streams that I follow and find useful. I think that means that whether I like it or not, I do seem to be finding use in all of the flutter clutter. I won't call myself completely sold yet, or even a Twitter fan, but I may be getting there. I still do my best to avoid the "Cheerios are good" crowd.

That's three uses. Any other good uses for it in the technical community that I've missed?

Duane Benson
Burmashave

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