There’s a reason we often use the phrase “It ain’t Rocket Science”. Because real rocket science IS difficult. It is dangerous and complicated, and a lot of things can and do go wrong, often with disastrous consequences. It is imperative that the lessons learned from past failures must be documented and disseminated to prevent future mishaps. This is much easier said than done. There’s a large number of agencies and laboratories working on multiple projects over long periods of time. Which is why NASA has set up NASA Lessons Learned — a central, online database of issues documented by contributors from within NASA as well as other organizations.
Unfortunately, all of this body of past knowledge is sometimes still not enough to prevent problems. Case in point is a recently discovered issue on the ISS — a completely avoidable power supply mistake. Science payloads attach to the ISS via holders called the ExPRESS logistics carriers. These provide mechanical anchoring, electrical power and data links. Inside the carriers, the power supply meant to supply 28V to the payloads was found to have a few capacitors mounted the other way around. This has forced the payloads to use the 120V supply instead, requiring them to have an additional 120V to 28V converter retrofit. This means modifying the existing hardware and factoring in additional weight, volume, heat, cost and other issues when adding the extra converter. If you’d like to dig into the details, check out this article about NASA’s power supply fail.
Have you ever wired up a piece of test equipment to a circuit or piece of equipment on your bench, only to have the dreaded magic smoke emerge when you apply power? [Steaky] did, and unfortunately for him the smoke was coming not from his circuit being tested but from a £2300 Clare H101 HiPot tester. His write-up details his search for the culprit, then looks at how the manufacturer might have protected the instrument.
[Steaky]’s employer uses the HiPot tester to check that adjacent circuits are adequately isolated from each other. A high voltage is put between the two circuits, and the leakage current between them is measured. A variety of tests are conducted on the same piece of equipment, and the task in hand was to produce a new version of a switch box with software control to swap between the different tests.
This particular instrument has a guard circuit, a pair of contacts that have to be closed before it will proceed. So the switch box incorporated a relay to close them, and wiring was made to connect to the guard socket. At first it was thought that the circuit might run at mains voltage, but when it was discovered to be only 5V the decision was made to use a 3.5mm jack on the switch box. Inadvertently this was left with its sleeve earthed, which had the effect of shorting out a DC to DC converter in the HiPot tester and releasing the smoke. Fortunately then the converter could be replaced and the machine brought back to life, but it left questions about the design of the internal circuit. What was in effect a logic level sense line was in fact connected to a low current power supply, and even the most rudimentary of protection circuitry could have saved the day.
We all stand warned to be vigilant for this kind of problem, and kudos to [Steaky] for both owning up to this particular fail and writing such a good analysis of it.
Many of the Fail Of The Week stories we feature here are pretty minor in the grand scheme of things. At worse, gears are ground, bits are broken, or the Magic Blue Smoke is released. This attempt to smooth a 3D print released far more than a puff of blue smoke, and was nearly a disaster of insurance adjuster or medical examiner proportions.
Luckily, [Maxloader] and his wife escaped serious injury, and their house came out mostly unscathed. The misadventure started with a 3D printed Mario statue. [Maxloader] had read acetone vapor can smooth a 3D print, and that warming the acetone speeds the process. Fortunately, his wife saw the looming danger and wisely suggested keeping a fire blanket handy, because [Max] decided to speed the process even more by putting a lid on the pot. It’s not clear exactly what happened in the pot – did the trapped acetone vapors burp the lid off and find a path to the cooktop burner? Whatever it was, the results were pretty spectacular and were captured on a security camera. The action starts at 1:13 in the video below. The fire blanket came in handy, buying [Max] a few seconds to open the window and send the whole flaming mess outside. Crisis averted, except for nearly setting the yard on fire.
What are we to learn from [Maxloader]’s nearly epic fail? First, acetone and open flame do not mix. If you want to heat acetone, do it outside and use an electric heat source. Second, a fire extinguisher is standard household equipment. Every house needs at least one, and doubly so when there’s a 3D printer present. And third, it’s best to know your filaments – the dearly departed Mario print was in PLA, which is best smoothed with tetrahydrofuran, not acetone.
Anything else? Feel free to flame away in the comments.
If you are someone whose interests lie in the field of RF, you won’t need telling about the endless field of new possibilities opened up by the advent of affordable software defined radio technology. If you are a designer or constructor it might be tempting to believe that these radios could reduce some of the problems facing an RF design engineer. After all, that tricky signal processing work has been moved into code, so the RF engineer’s only remaining job should be to fill the not-so-huge gap between antenna and ADC or DAC.
In some cases this is true. If you are designing an SDR front end for a relatively narrow band of frequencies, perhaps a single frequency allocation such as an amateur band, the challenges are largely the same as those you’d find in the front end of a traditional radio. The simplest SDRs are thus well within the abilities of a home constructor, for example converting a below-100kHz-wide segment of radio spectrum to the below-100kHz baseband audio bandwidth of a decent quality computer sound card which serves as both ADC and DAC. You will only need to design one set of not-very-wide filters, and the integrated circuits you’ll use will not be particularly exotic.
But what happens if the SDR you are designing is not a simple narrow-band device? [Chris Testa, KD2BMH] delivered a talk at this year’s Dayton Hamvention looking at some of the mistakes he made and pitfalls he encountered over the last few years of work on his 50MHz to 1GHz-bandwidth Whitebox handheld SDR project. It’s not a FoTW in the traditional sense in that it is not a single ignominious fail, instead it is a candid and fascinating examination of so many of the wrong turnings a would-be RF engineer can make.
The video of his talk can be found below the break, courtesy of Ham Radio Now. [Chris]’s talk is part of a longer presentation after [Bruce Perens, K6BP] who some of you may recognise from his activities when he’s not talking about digital voice and SDRs. We’re jumping in at about the 34 minute mark to catch [Chris], but [Bruce]’s talk is almost worth an article in itself..
Physics gives us the basic tools needed to understand the universe, but turning theory into something useful is how engineers make their living. Pushing on that boundary is the subject of this week’s Fail of the Week, wherein we follow the travails of making a working magnetic flowmeter (YouTube, embedded below).
Theory suggests that measuring fluid flow should be simple. After all, sticking a magnetic paddle wheel into a fluid stream and counting pulses with a reed switch or Hall sensor is pretty straightforward, right? In this case, though, [Grady] of Practical Engineering starts out with a much more complicated flow measurement modality – electromagnetic detection. He does a great job of explaining Faraday’s Law of Induction and how a fluid can be the conductor that moves through a magnetic field and has a measurable current induced in it. The current should be proportional to the velocity of the fluid, so it should be a snap to whip up a homebrew magnetic flowmeter, right? Nope – despite valiant effort, [Grady] was never able to get a usable signal out of the noise in his system.
The theory is sound, his test rig looks workable, and he’s got some pretty decent instrumentation. So where did [Grady] go wrong? Could he clean up the signal with a better instrumentation amp? What would happen if he changed the process fluid to something more conductive, like salt water? By his own admission, electrical engineering is not his strong suit – he’s a civil engineer by trade. Think you can clean up that signal? Let us know in the comments section.
It’s more of a half-fail than a full fail, but [Basti] is accustomed to getting things right (eventually) so it sticks in his craw that he wasn’t able to fully realize his ferrofluid dreams (German, translated here). Anyway, fail or demi-fail, the project is certainly a lesson in the reality of ferrofluid.
We’ve all seen amazing things done with ferrofluid and magnets. How hard can it be to make an interactive ferrofluid wedding present for his sister? Where ferrofluid spikes climb up a beautifully cut steel heart in a jar? (Answer: very hard.)
There’s a saying among writers that goes something like “Everyone has a novel in them, but in most cases that’s where it should stay”. Its source is the subject of some dispute, but it remains sage advice that wannabe authors should remember on dark and stormy nights.
It is possible that a similar saying could be constructed among hackers and makers: that every one of us has at least one motor vehicle within, held back only by the lack of available time, budget, and workshop space. And like the writers, within is probably where most of them should stay.
[TheFrostyman] might have had cause to heed such advice. For blessed with a workshop, a hundred dollars, and the free time of a 15-year-old, he’s built his first motorcycle. It’s a machine of which he seems inordinately proud, a hardtail with a stance somewhere closer to a café racer and powered by what looks like a clone of the ubiquitous Honda 50 engine.
Unfortunately for him, though the machine looks about as cool a ride as any 15-year-old could hope to own it could also serve as a textbook example of how not to build a safe motorcycle. In fact, we’d go further than that, it’s a deathtrap that we hope he takes a second look at and never ever rides. It’s worth running through some of its deficiencies not for a laugh at his expense but to gain some understanding of motorcycle design.