I have constantly been looking at ways to make my home workspace more convenient and safer for PCB fabrication/prototyping. I switched to all lead-free products several years ago but have been using a pretty janky setup for fume extraction: a 120mm fan with the charcoal filter attached to one side. It worked well enough, but I wanted to make something a little better. I wanted to have the fan out of the way, with some hose, so it would not take up too much room on my actual table. I decided to design something in Autodesk Inventor to be fabricated out of acrylic. The idea is that a standard hose would be attached at the top and that the whole assembly can be mounted on my rack. The results are pretty good, the suction works well, with the key downside being that it is louder than commercial units which are more expensive. The files are attached, both in Inventor format and as DXF outlines. The material thickness was 0.250″ for the backplate and 0.175″ for the rest. The DXF units were in inches, so the tube OD can be measured there.
We finally have everything squared away for Maker Faire 2013 and we should be able to show off an open-source LED controller. That design is not complete, however, I still wanted to be able to give something away. I settled on creating a PCB business card which doubled as an SMT prototyping board on the back. I did a few iterations and settled on something that was all surface mount. There are pads for an SMT mini-USB port, a 32qfp,80tqfp and 64 pin IDC side-connector. All of the quad footprints will accept smaller packages with the same pitch and the IDC pads are designed in such a way that the 0.8mm PCB can be soldered between the pins of a standard 2.54mm/0.1″ connector. There is also a grid of pads that have 1.27mm pitch in one direction and 0.65mm pitch in the other direction so they can accommodate a SOIC one way and TSSOP the other way. The finish is ENIG with a high performance white solder mask so it should stand up to soldering without discoloring.
Stop by the mobius.io table and just ask if you would like to get one of these. Gerbers for this card are here: mobius_io_card2013 .
As designers, we often give PCB function the highest priority while making the mounting/enclosure options (see above) an afterthought. This is especially problematic if the design will be constructed more than once or twice, where custom drilling/milling/manual processing will become very expensive and hard to reproduce. Having worked custom mold-injected enclosures, custom metal enclosures, and off the shelf enclosures, here are some things I have learned along the way:
- Check with your PCB assembly house about component placing near PCB edges, a lot of places will typically want >1.27mm. This is a process issue for removing individual boards out of panels and being able to clear the cutting tool. More is better as it makes it easier to handle the board after de-panelizing without touching mounted components.
- Label component keepout areas on the PCB print if you need to clear anything, especially when using ridges to support PCBs in plastic cases. Locations of these keepouts can be obvious when initially designing the PCB, but you will soon forget and may accidentally place components there in a future revision. Changing a PCB fab is usually a lot cheaper than changing the injection mold, so you will be redesigning your PCB in situation.
- Make sure that your board only fits into the chassis one way. This can be done by using a non-symmetric mounting hole pattern or by having specific cutouts for mechanical pins or ridges.
- Comply with standard drill sizes (and keepout areas) for standard mounting hardware. Tim Hausherr has a good overview here. It is a lot easier to be able to use standard fasteners instead of drilling arbitrary holes in PCBs and then trying to find something that fits.
- Specify a torque spec and use an electric driver with torque setting when driving any kind of threaded fastening hardware. If you don’t do this, you can expect to get anything from cracked PCBs from overtightening to loose hardware/PCBs in your chassis.
- If you expect a lot of vibration, avoid using threaded fasteners and use clips instead. This is especially true for mounting speakers to chassis. It is tempting to use threadlock, but this is a bad idea from a serviceability standpoint (see below).
- Expect that you will get failures in the field and that you may have todisassemble units coming back from the field. This is the primary reason I don’t like using threadlock unless it is going into standoffs which are stamped into the chassis and will not turn when you try to back your fasteners out. This is also the reason to give a second thought to conformal coating/potting your circuits unless you have a specific performance reason to do this (IPXX rating, isolation). Coating your PCB/filling your case with epoxy will not stop people from being able to reverse engineer the physical aspects of your design.
- Use 3D models if your connectors, or at least mark their outlines on an assembly layer, so you can align them with your chassis. The large gap between the USB and Ethernet connectors on the rpi makes it challenging to mount PCB in an off the shelf rectangular case.
The time has come to bring microblog back to life.
I have spent the past three years working for a large medical device company in Silicon Valley designing surgical electronics. This has been a lot of fun, but I want to get back to working on open source electronics and posting about them. I have been working on a couple of designs in my spare time and have been accepted to Maker Faire 2013 to demo them, fairly excited about that. I can’t post any links yet, but watch this space for updates and hope to see you San Mateo, CA May 18-19th. I am going to try to write
at least one post a week (who was I kidding?) periodically focusing on more quality than quantity. I will also keep up with moderating comments.
I have made few attempts to keep this space updated and relevant over the past year, but ultimately failed. I have nearly completed my PhD so I have gotten extremely busy with finishing experiments and writing/editing my thesis. I still do analog and digital design in my day-to-day life, except, most of that work cannot be published here directly. I no longer plan on updating microblog, however, I will leave it up for at least the forthcoming months until the end of the internet in case anyone is interested in saving some information. Thank you readers, it has been a great ride!
Update: I had no idea that there were so many readers, so I will not disappoint and will do my best to keep this content up indefinitely. I may add in the future, but it will not be nearly as frequent as it was in the past.
P.S. I will be looking for an industry job in medical device design in the coming spring, so don’t hesitate to contact me if you are looking for an engineer.
I have been very busy at the lab lately, too busy to update ublog regularly, and this set of formulas from Gamma Instruments has saved me a bit of trouble. Several air core inductor designs are outlined which allow you to make your own custom inductor, within reason, if you need to test something and you don’t have the part on hand. Typically, the inductance values will be modest, however, it is much easier to make an inductor yourself than a reasonable capacitor. Just think of all of the folding!
( pancakewheel )
I have been reading James Gleick’s Chaos and I must confess that I am very impressed with the book so far. I am beginning to realize some of the practical applications of non-linear dynamics to analog circuit design, however, more on that later. What has been very interesting is the slow change in the mentality of the scientific world, from the notion that a small change in a systems initial conditions only warrants a small change in the output, to the reality that small changes in initial conditions can generate wildly different results. One of the pioneers in this field was the late Edward Lorenz. He discovered that a slight change (less than 1%) in the initial conditions of his deterministic weather model, which was numerically integrated, would cause the outcome to diverge from the unperturbed simulation to the point that the two weather systems were completely different after several days. The error in his integrator could not account for this disparity, therefore, he went through some analytic computations and found that simple differential equations can have very complex behaviors that were very dependent on initial conditions. He published his results in the Journal of Atmospheric Sciences, a paper that is well worth looking over. For those who are not mathematically inclined, looking over the introduction and conclusion should provide some insight into the paper. Additionally, this is the paper where the often duplicated Lorenz attractor, or butterfly attractor (figure 2) makes its first appearance.
It took me a few months to finally read this book, but it was well worth it. I have been reading it prior to sleep as it was so full of information that it was difficult to read more than ten pages without taking a break to think about all of the new ideas. Furthermore, the information was presented in such an accessible manner that even those who are not specialists in relativity, topology or physics can appreciate the message.
I selected this book because I figured the topic was far away from electrical engineering that it could give a new perspective on understanding what is implied by measuring time and distance. Sure enough, this book provided many insights into the nature of our universe through the relation of time and space measurement. I will avoid summarizing the book, however, I will mention that it would be a pleasant read for those interested in non-Eucledian coordinates and the effects of gravitational fields. The book is extremely well written and reads much like a lecture series where the audience does not need to be able to carry out all of the steps of each operation, but acquires a taste for the process and a deeper appreceation. From the point of view of technical written English, this was one of the most understandable books on a physical subject that I have read in some time.
I have many AS and LS series logic gates lying around, so I figured that I would image some as they would be more instructive than looking at the latest CPUs coming out of Intel. I have a few more lined up so please comment if you want me to follow this reasoning. Today’s subject is a hex-inverter, the 74AS04. Below are images of the whole chip and a blowup of the top-right inverter gate.