Gas station without pumps

2014 July 18

How to sell a widget

SparkFun has a tutorial on how to sell “widgets” through them at How to Sell Your Widget on SparkFun – Learn.SFE:

Have an awesome electronic widget that you want to get to market? Great! We are always listening for new ideas from our customers and the community. We get many inquires on this topic, so read this tutorial carefully to keep your product pitch from getting lost in the shuffle.

We’re lucky, here at SparkFun, to have an amazingly creative and talented group of customers. Not only can they identify gaps in the catalogs of electronics suppliers, they can create a gizmo that fills that hole. But, going beyond a prototype or even a limited-quantity production run, often the hardest task in getting your world-altering product out there is producing, marketing, and/or selling it to the masses. That’s where we come into the picture.

The advice there is seems fairly reasonable.  They offer a choice of two models: make it yourself and have them sell for you, or have them make it and pay you royalties.  They tell you how to pitch products to them and how to design for them to be able to manufacture stuff.  Most of the stuff they sell is PCB boards, so they give quite a bit of advice about PCB design to fit their pipeline—they use Eagle, but ‘All parts are placed on a 0.005″ grid. If possible, use a .05″ grid.’ That must get irritating with modern parts that are convenient round metric numbers, not mils.   They also want version numbers in the bottom copper, which is reasonable for some designs, but not all.  They also encourage people to use their Eagle libraries, but my son and I have found their pad layouts to be very sloppy (putting silkscreen over SMD layers, getting the keep-out areas off by a little bit, not fixing the fonts on the “>NAME” and “>VALUE” labels to meet their own requirements, and so forth).

Still, it is good that they put out their design rules and provide clear guidelines to new designers.

I’ve thought a few times about putting out some of my designs through SparkFun or Adafruit Industries—perhaps an improved version of the blinky EKG as a kit.  SparkFun now sells EKG electrodes and snap leads for them, and even have a heart-rate monitor board (based on the AD8232 chip) and the “BITalino” biomedical board, so I suspect that they are interested in the market.

The BITalino is outrageously expensive and their EKG electrodes are about 3 times the price of buying them at Amazon, but the AD8232 chip actually looks like a nice one for building an EKG front-end and reasonably priced, so I’m not sure they’d have much interest in a through-hole part kit for do-it-yourself EKG that isn’t quite as good, unless it could be sold very cheaply or as an educational product (which is what the blinky EKG is aimed at, anyway).

I have some other ideas for products that I might be marketable, but I don’t know whether I have the time to refine them to the point of pitching them to SparkFun.  I can justify some time spent on doing electronics as a hobby, some as necessary learning for teaching my applied electronics course for bioengineers, and some as engineering-for-manufacture experience (something I never had any instruction in, despite my years as an engineering professor). But when the electronics work starts cutting into the time I need to spend on writing my book, teaching my classes, or doing collaborative research with other faculty, then I have to draw the line.  I’ve also got a lot of administrative responsibilities now (undergrad director and faculty adviser for two BS degrees, Program Chair for bioengineering, and Vice Chair for the Biomolecular Engineering Department), so writing time and research time have gotten doubly precious.

I do have one project this summer that I’m going to try to get fabricated for me—it is all SMD parts, including some that are hard to solder by hand (pads under the chips), so I don’t want to do it myself.  The project also calls for a lot of identical boards (20 to 50 of them), so a prototyping house seems like the way to go.

I’m looking currently at Smart Prototyping to do the PC board fabrication and assembly—they may not be the cheapest, but they have a comprehensible pricing scheme on their website, and they replied within 12 hours of my request for a quote. They also have a nearly turnkey system—I send them the Gerber files and the Bill of Materials (BOM), and they’ll make the boards, buy the parts, and assemble the boards.  They’ll even test them for an additional charge, though these boards are simple enough that I can test them myself at about 5 seconds a board, so their testing would not be worthwhile unless they guaranteed their assembly (which none of the prototype houses can afford to do with untested designs).

I also considered Elecrow, which has a similar service, but their pricing information on the web page is rather vague: “For BGA or IC with pads under IC, The quotation will be a little higher.” and “We will give a discount for the PCB assembly service according the some factors (assembly time,Hard or Easy to assemble or requirements etc.).” I prefer sites that have clear pricing even if it is slightly higher, so that there are no surprises. I suppose I could ask Elecrow for a quote and see if they respond as promptly as Smart Prototyping did.

Incidentally, my design does not follow all SparkFun’s guidelines—for one thing, I placed parts on a 0.5mm grid, not a 0.005″ gird, and the board is not rectangular.  Still, if the design I’m working on turns out well, I might pitch it to them, as I see some potential for it appealing to the open-source hardware market, and the violations of their design guidelines made good sense for this application.  Note: I’m deliberately not saying what the design is—I’ll reveal it once I’ve gotten a working prototype, when I’ve decided whether I want to commercialize it or not.

2013 March 15

Action potential lecture and EKG lab

Filed under: Circuits course — gasstationwithoutpumps @ 00:43
Tags: , , ,

I did not lecture on Wednesday, but had a guest lecturer from the biology department, who gave a lecture on action potentials, based on a similar lecture she gives in a neuropsychology course.  The lecture went a bit slow for the bioengineers, I think (with everything repeated 3 times), but there was some good information about the use of the Nernst equation to get resting and action potentials, the sodium and potassium channels, and the use of a voltage clamp to measure permeability of a squid axon to ions at different potentials.  If I were to give the lecture, I would probably pick up the pace a bit, but use a longer wait time when asking the class a question—the biology professor tended to answer her own questions as soon as she got any sound from the class.

The EKG lab this afternoon had mixed results—everyone finished with a working EKG board, but the last group to leave took 7 hours, so I was in the lab from 2 p.m. to 9 p.m.  I started the lab by demoing the board that I had build Tuesday morning, so that they could see that the project was possible.  It was a good thing that I did so, since several of the EKG boards the students designed and built worked fine with the electrodes I was wearing, but not with their electrodes on their bodies.  I’ve not figured out why this happened.  Had their electrodes dried out?  Was their skin prep or electrode placement poorer? Is my pulse larger than theirs, or my skin more conductive?  Some of the groups did manage to get their boards working with their own electrodes, and everyone’s worked with some set of electrodes.

One of the slowest debugging problems was a component value error: assembling the board with a 4.7µF capacitor, rather than the 4.7nF capacitor they had designed in their schematic, resulting in a low-pass corner frequency of 0.05Hz instead of 50Hz, eliminating their signal.  Another component value error by a different group was using an 8Ω gain resistor (giving a gain of 10,005) when a gain of 8 was desired—this resulted in saturating the amplifier.  There were other component errors that were less catastrophic (a 47nF capacitor where a 4.7nF capacitor was intended, eliminating the 60Hz ripple, but also eliminating a big chunk of the signal), but all were eventually found and fixed.

Almost everyone had a lot of 60Hz ripple in the output (as did I, though they generally had more with their electrodes).  I was confused on Tuesday about what I saw as a 40Hz ripple, but of course that was just aliasing of the 60Hz ripple with a 100Hz sampling frequency (with a Nyquist frequency of 50Hz, a component at 60Hz and a component at 40Hz are indistinguishable).  I think that most of the 60Hz ripple is coming from electromagnetic pickup in the loop from where the wires stop being twisted together and spread out to the three electrodes, but some may be capacitive also.  I may play around with better wiring to see if I can reduce the ripple.  I could also try sampling at 60Hz, so that the ripple is aliased to DC.  The only risk there is that it may not be exactly 60Hz, and aliasing to near DC could interfere with the meaningful part of the signal.  I believe that professional EKGs sample at a higher rate, then use a digital notch filter to remove the 60Hz ripple.

2013 March 11

Twenty-sixth day of circuits class

Filed under: Circuits course — gasstationwithoutpumps @ 20:53
Tags: , , ,

I started today with feedback on the writing.  The students have been getting much better about the content and organization of their reports, and the writing is pretty good from at least half the class, but everyone is having trouble with not checking the details, particularly in the schematics.  I’d really like to give some A’s this quarter, but I can’t give an A to a lab report that has a power-ground short in a schematic or a missing connection.

After the writing feedback, I talked a bit about safety in biomedical equipment (battery operation, opto-isolators, isolation transformers, wireless, …).  I also talked about how skin was an excellent insulator at low frequencies and low voltages, but how at high frequencies the skin capacitance is no barrier, and that at high voltages, like with Automatic Electric Defibrillators (AEDs) the skin dielectric is broken down.  Even with EKGs, running at low voltages, they need to add series current-limiting resistors to all their electrode leads, to ensure that currents remain below 50µA, even if the leads are accidentally shorted to the highest voltage in the system.

After the safety discussion, I gave them some information about the AC signal they were trying to amplify (±0.5–1mV, with a DC offset of ±300mV, and a frequency range of 0.1Hz to 50Hz). Rather than covering the block diagram as a whole class, I had them split into 3 groups of 4 and try to design the block diagram for the EKG.  I circulated around the room and answered questions. I did tell the class that every design I’ve seen for an EKG puts the high-pass filter after the first-stage instrumentation amp.  I’m not sure why that is standard, but it certainly seems to be.

The main point (which some groups came to quickly and others slowly), is that the ±300mV offset constrains how much gain you can ask the first stage for, so that a second stage is essential.  One group asked if they could do a particular gain in one op amp, so I got a chance to remind them of gain-bandwidth product (the frequencies here are so low that the gain was easily feasible).

Over the weekend the whole class had gathered to work together (mainly on rewriting lab reports—I offered the class in the syllabus that they could rewrite any lab report whose grade they were not happy with).  They sent me a photo of the class working together—I think this is the first time I’ve seen a spontaneously formed study group consisting of an entire class.  Today I got 15 redone lab reports (about 2 weeks worth of grading, with more expected on Wednesday, not to mention the class-D amplifier reports that are due on Wednesday.

Since we are not having a final exam for the course (the quizzes and the 50 pages of design reports from the students is enough to evaluate them on), the students asked if we could meet during the exam period for beer (with the constraint that it must be a place that serves non-alcoholic beverages to those under 21, since not all the students are drinking age).  I agreed that we could, but I’ve not thought of an ideal location.  Perhaps Cafe Pergolesi would do—they serve beer and wine, but are primarily a café serving coffee, tea, and hot chocolate.  I don’t know how much of a hipster hangout it is on Tuesday afternoons, but I suspect that a dozen of us could occupy one of the “geek” rooms.  I had planned to make a detailed survey for the students to fill out about things that were good and bad about the course—they’ve been pretty good about giving me feedback as we go along, but I’m planning on redesigning the course somewhat and revising the course approval forms, so I’d like to know which aspects of the course worked best (and worst) from their perspective at the end of the course.  I’ll also be asking them to send me PDFs of all their lab reports, so that I can show them to the EE faculty to try to convince the faculty that this applied circuits course is acceptable preparation for the signals and systems course and the bioelectronics course.

I think I’ll be spending tomorrow on 3 things:

  • trying out a different EKG circuit that uses active feedback of the common-mode signal.
  • grading redone lab reports—if I can get half the stack done tomorrow I’ll probably be able to handle the load for the rest of the week.
  • trying to catch up in physics with my son—it kind of slid for the past 2 weeks as both he and I were crazy-busy.

Wednesday I have a guest lecturer coming in to talk about action potentials and excitable cells, which should be good preparation for understanding the EKGs they will be making on Thursday—I’ve not really talked about where the voltages they’ll be measuring come from.

2013 March 8

Twenty-fifth day of circuits class

Filed under: Circuits course — gasstationwithoutpumps @ 21:33
Tags: , , ,

I started class today with a do-now problem:

What is the gain at 0Hz? ∞Hz?At what frequencies does the behavior change?

What is the gain at 0Hz? ∞Hz?
At what frequencies does the behavior change?

Only a couple of students had any clue how to do this problem, though they’ve had all the pieces needed to do it for quite a while. I got them to do it as a class (making a big thing out of the block diagram of op-amp, impedance, impedance for a negative-feedback amplifier. The did eventually figure out what the circuit did, with a whole lot of prompting, but I worry about how little ability they seem to have to apply stuff they know well in even slightly unfamiliar contexts. Even if they never do electronics again, the same skills will be called on to make up new lab protocols in molecular biology, looking at what each protocol procedure does and how the needs of one protocol step constrain what steps can be done before or afterwards.

I think that there is way too much rote learning in the bioengineering program (particularly in the chemistry and biology courses, but even in the math and physics classes), and that students are not learning how to solve problems, but just applying solutions learned by rote—a fairly useless skill.

We then did quiz review for the quiz they took last Monday, looking at the stuff that lots of people got wrong. For example, one question asked what the gain equation (Vout as a function of Vin) was for a negative-feedback op-amp circuit, then asked what the constraints on Vin were for the equation to be valid. I had provided power-supply voltages for the op amp, so Vout was constrained to be between the power rails, and it was a simple matter of rearranging the inequalities using the formula for Vout from the first part of the problem, but no one thought to do that. I showed them the technique, and talked (again) about propagating constraints through a block diagram.

After that I explained why a number of students had been seeing a spike on their gate voltages in the class-D amplifier, on the edges of pulses. The problem is the reverse transfer capacitance: coupling the large voltage swing on the drains of the FETs back to the gates. We did not address how to reduce the spike, other than to suggest playing with the size of the pull-up resistor on the gate.

I showed the Old Spice ad that demonstrates skin-electrode EMG (which I mentioned in an earlier blog post), then started talking about EKGs. We started with the need to have current-limiting resistors to keep currents below 50µA, even if the highest voltages in the system get connected to the leads.  We then talked about the differential signal between left arm and right arm, and I talked about electrode placement below the collar bones to avoid interference from other muscles (see Better electrode placement for EKG blinky). After a lot of questioning about how to keep the LA and RA signals between the power rails of the instrumentation amp, I finally got a student to suggest the standard solution: adding a reference electrode connected to Vref. We did not have time to talk about active cancellation of the common-mode signal.

We also did not have time to talk about the DC offset in the electrodes, and how the ±1mV signal we are interested in can be buried in a ±300mV DC offset due to different half-cell voltages at different electrodes.  Monday will have to cover the full block diagram for an EKG, since Wednesday will have a guest lecturer on action potentials and how the voltages for EKGs arise.

2012 August 31

EMG video

Filed under: Circuits course — gasstationwithoutpumps @ 10:38
Tags: , , , , ,

I just got pointed to a video that seems relevant to the EMG/EKG lab:

For those without the patience to watch a 76-second video, it shows the use of 20 EMG channels from skin electrodes being used to control musical instruments (mostly percussion).  It probably takes some practice to be able to control each of the muscles independently with enough precision to play music, as the muscles involved are mostly large ones.

This video might be a good one to show as an introduction to the EMG/EKG lab.

Next Page »

%d bloggers like this: