Long time readers of my blog may remember that I’m interested in rainbows (not unicorns, just rainbows). A while ago, I wrote a simple simulation that showed the formation of the primary and secondary rainbows by simulating the refraction of water inside a single raindrop. These two bows appear opposite the sun in the sky. But I never thought to try to simulate the higher order rainbows (caused by greater numbers of refractions inside water drops) because, well, I didn’t think that they existed.
But they do. And this week the first known photos of them appeared.
Unlike the primary and secondary bows, these are actually in the direction of the sun. Very cool. I bet that I could modify my old rainbow tracer to make pictures of these things. Perhaps if I get a few moments this weekend.
I got a tweet from twisst, the ISS pass prediction robot yesterday indicating that I’d have a good pass around 8:25PM. While I am fighting off a cold, the weather was beautiful and nice, and so I ran some path predictions to see what the path looked like, and also checked on ARISSat-1’s path to see how it was doing. I hadn’t recorded ARISSat-1 since it’s first launch weeks ago, and hoped that in spite of it’s rapidly increasing battery problems, that it would still be in sunlight, and would therefore still have a strong signal. ARRISSat-1 would lead the ISS by about 23 minutes, rising around 8:02 or so, but since I have a tall horizon to the north where it would rise, I wouldn’t expect to pick up a good signal until it cleared the hills, about 8:06 or so.
It turned out to be a really good pass: I got three different SSTV images, and some really clear audio telemetry. The first SSTV image and the last were pretty marginal, but the middle one was really clear (sad, since it was the least interesting). When I first recorded ARISSat-1 shortly after launch, I had periodic fades which I hypothesized as tumbling of the satellite: those appear to be entirely gone. I’ll have to try again to see if I can get a better and more interesting SSTV image.
After ARISSAT-1 set, I waited until 8:25 to see if the ISS would come up. I tuned into 145.825 (the ISS packet radio frequency) and waited with my iPhone camera ready. By then, it was surprisingly dark, so my camera recorded mostly just blackness, but toward the end of the clip, you can see a faint dot in the recording (and very little else). Not too exciting, but I left the audio from the radio playing in the background, so you can hear the digital packet signals being echoed through the ISS. The ISS was predicted to peak at magnitude -3.1, which made it brighter than any star in the sky, and it was very easy to see.
Here’s the resulting video. WARNING, spoiler: I forgot to edit out the “secret word” in this recording. Blame it on the cold medicine I’m on.
I’ll probably try to record another pass soon. Stay tuned.
This is awesome! MIT has created an interesting course as part of the their Open Course Ware project: it describes how radar can work, and as a final project, students were expected to build an test a simple radar system. Their description:
Are you interested in building and testing your own imaging radar system? MIT Lincoln Laboratory offers this 3-week course in the design, fabrication, and test of a laptop-based radar sensor capable of measuring Doppler, range, and forming synthetic aperture radar (SAR) images. You do not have to be a radar engineer but it helps if you are interested in any of the following; electronics, amateur radio, physics, or electromagnetics. It is recommended that you have some familiarity with MATLAB®. Teams of three students will receive a radar kit and will attend a total of 5 sessions spanning topics from the fundamentals of radar to SAR imaging.
I haven’t read a lot of this, but I’m bookmarking it for future perusal.
A couple of months ago, Collin’s Lab featured a story about making your own piezoelectric crystals from Rochelle salt. Collin stopped short of making an actual microphone though: he just demonstrated that the salt crystal would generate a series of voltage spikes when whacked with the handle of a screwdriver. Leafcutter John followed pretty much the same recipe to make crystals of his own, and then clamped the crystal between the jaws of a little panvise, and hooked it to an audio amplifier. When a small music box was held near the crystal, a surprisingly high fidelity recording resulted. Check it out!
While looking up some references on amateur nuclear fusion (don’t ask!) I found that Raymond Jimenez had written a cute 40 page book on his own experiments with a Farnsworth Fusor. You can apparently order a dead tree version from Lulu for $12.50, but it’s also available as a free download.
I subscribe to the Sixty Symbols YouTube channel which is produced by the University of Nottingham, and today, I noticed they had a new video on a subject near and dear to many a physicists heart: Guinness.
If you think that beer is beneath the interest of physics, you should surf on over to Amazon.com and check out Craig Bohren’s Clouds in a Glass of Beer. It is an excellent accessible book on atmospheric physics that examines (among other topics) the way that bubbles form in beer. It’s a creative, informative book that presumes no great knowledge of physics, but can help remove some of the many misconceptions we have surrounding atmospheric physics and weather.
As an astronomy/telescope buff, I have built simple telescopes for looking at the sun, but I haven’t done much of that lately, and I have only recently become interested in observing the sun’s effect on the Earth’s radio environment. I’ve also had a fairly longstanding interest in VLF communications, and so the prospect of building a radio system for monitoring radio for SIDs (Sudden Ionospheric Disturbances) seems like a good project. I recall that Mark Spencer, WA8SME whom I’ve had the pleasure of meeting at Pacificon and even chatted with via the AO-51 satellite had published some articles on designing such radios for amateur and educational construction.
The most common way to monitor for SIDs is to try to detect the signal strength of the VLF station in Cutler, ME on 24Khz. Changes in signal strength can indicate the presence of solar flare activity as the ionosphere is bombarded by high energy particles. One common circuit that lots of people use is called The Gyrator VLF circuit, which you can find here:
I had never really looked at this circuit very carefully before, and looking at it tonight, I realized that it was actually fairly interesting and employs a technique which I hadn’t seen before. Instead of making a tuned front end using a (fairly large, because of the low frequency) inductor, they create an equivalent circuit using two op amps and a collection of resistors. This creates an inductor with very high Q, with the side effect that is easier to make, using operational amplifiers that you can still get at Radio Shack. I’ll probably try to simulate some of the basics with LTSpice to gain some intuition as to what’s going on, but it’s a simple enough circuit that just building it would be pretty easy.
The basic circuit has undergone a couple of iterations, and now the Gyrator III schematic is recommended by the AAVSO. You can check out the details here (and surf around, the AAVSO has lots of information on this stuff).
This morning’s massive coronal mass ejection from the Sun got me scrambling around trying to remember details of how amateurs can monitor solar flare activity during the current solar cycle. Mark Spencer, WA8SME, had some articles on building a small monitoring station that detected SIDs, or “sudden ionospheric disturbances”. The basic idea is to create a VLF receiver to pick up signals such as the strong one megawatt transmitter on 24Khz in Cutler, Maine, and then graph the overall signal strength and record it on the computer equivalent of a strip recorder. I know I have the QEX article from 2008 where he described this system, but I can’t find it. You might find this article to be of interest, as well as the great efforts of the AAVSO (American Association of Variable Star Observers). Here is another receiver circuit that I ran across.. All good stuff.
When I was still in grade school, I (and this will be a shock to my readers) spent a lot of time in libraries. Our library used to have a free bin, where they would toss things that they no longer wanted in their collection. One day, I came by and found a pile of more than two decades of Scientific American magazines. Being a bit of a science nut, I carted them home, and spent many a happy evening reading Martin Gardner’s Mathematical Games and C.L. Stong’s Amateur Scientist column. Stong’s column was really cool, and had great articles about building plasma jets, X-ray machines, diffraction ruling machines, and all sorts of other good stuff.
It was then with some suprise that I read Nyle Steiner’s article on making FET transistors from cadmium sulfide photocells, because he made reference to a 1970 article by Roger Baker that was about making home made FET transistors. I didn’t remember any such article. Luckily, I bought a CDROM which contains all the Amateur Scientist columns, and found it. And, indeed: it’s the second half of a two part column, and talks about the deposition of thin films. I guess I had never read it closely enough to see that it included the manufacture of a thin film transistor.
The article also makes reference to THIN FILM MICROELECTRONICS: THE PREPARATION AND PROPERTIES OF COMPONENTS AND CIRCUIT ARRAYS which was available in preview mode on Google Books, and it seemed enticing enough that I decided to track down a copy via online search (I managed to find a copy for only $6 + $3.50 shipping). It seems like the level of this book might be within the reach of the dedicated amateur.
Doing a bit more reading, I found out that the equations that make up the Lorenz attractor (which are derived from a simplified model of 2D fluid flow with a superimposed temperature gradient) can also be thought of as governing another physical system. Imagine a water wheel, with a number of buckets spaced evenly around the perimeter. These buckets filled at the top of the wheel. As that bucket fills, any offset will generate an imbalance, and the wheel rotates. That will rotate another bucket into position. The amount of water in that bucket is less because it spends less time under the faucet. But eventually, the buckets all fill up, the wheel is balanced, and the friction of rotation causes the motion to cease.
But now, imagine that each bucket is leaky: that some of its water drains out. What happens then? Well, it turns out that depending on the speed at which the water is pumped in and leaks out, the wheel can exhibit chaotic motion: spinning at radically different speeds and often reversing itself. Very neat. Here’s a video of one with a particularly simple design (you can google for more examples):
Over sushi this evening, Tom mentioned “Chua’s circuit”, or “Chua’s oscillator”. I knew that I had seen this somewhere before, but failed to remember that Chua was also the guy who first theorized about the memrister: a circuit element whose resistance is proportional to the sum of the charges that has been passed through it. Chua first imagined this circuit back in 1983, and it is probably one of the most well studied and well understood chaotic circuits ever proposed. It’s also quite simple. The page linked here should a simple circuit, with just a single op amp and a handful of other discrete components. I’ll ponder it some more:
This is a graph of the so-called “Lorenz attractor”, first described by mathematician Edward Lorenz in his paper Deterministic Nonperiodic Flow back in 1962. I learned about this kind of stuff probably back in highschool by reading Scientific American. Anyway, the equations themselves are pretty simple, but describe paths which are nonperiodic, and which are unstable: for two points very near each other, their evolution rapidly diverges, and they no longer follow identical paths.
Making these graphs was really just a diversion: I’ve got it in my head that creating an analog circuit that simulates these equations might be a fun thing to do. This is boldly going where others have gone before, so here are some links:
Build a Lorenz Attractor has a nifty little circuit that has two analog multipliers and three op amps. Checking with digikey, the MPY634 multipliers seem pretty spendy for something that is just a lark ($28 a piece, ouch), but Analog Devices makes some devices which seem like they will perform adequately, and cost $8 a piece.
And, of course, we need a YouTube video. Jeri and Chris show off a hardware implementation of the first circuit that Chris assembled:
I didn’t realize until later that the first article was written by Paul Horowitz, one of the authors of the incredible book The Art of Electronics by Horowitz and Hill. Digging a bit more, I found an actual lecture by Paul Horowitz on the subject, posted by user harvardphysics on YouTube:
Addendum: Here are some more links.
From the analogmuseum.org website, a nifty page that points out that if you change the value of the integrating caps, you can effectively change the speed of calculation, allowing the analog computer to directly drive a pen plotter.
I got pointed to the Analog Museum from this page, which in addition to demonstrating the Lorenz attractor running on an analog computer also has schematics and parts lists for actually building an analog computer, again using the AN633 analog multipliers from Analog Devices. Neat.
I’ve always been interested in crystals: their outer beauty hints at a certain kind of inner beauty, caused by the orderly arrangement of molecules at the atomic level. When I was a kid, I made crystals from sugar, salt, alum, and copper sulfate, but never tried Rochelle salts. Rochelle salts are interesting because they are piezoelectric: in response to mechanical deformation, they generate an electrical current. Similarly, when stimulated with an electric current, they deform. This makes them useful for all sorts of cool applications such as microphone pickups.
In an attempt to give you some value added, beyond just the link, I did some searching on Google Books. Somewhat interestingly, it turned up the Nov 11, 1946 issue of Life magazine, which ran an article on Rochelle salt crystals, which had the tag line:
ROCHELLE SALT CRYSTALS
Pretty girls in sunsuits grow a unique mineral that can turn pressure into electric current
Perhaps of greater interest to the experimenter is this article from Popular Science, November 1945 which talks about the history of microphones in general, and includes a bunch of experiments (somewhat tersely described) which can be done.
Interesting. A group of students are launching a high altitude balloon “some miles” from the launch site of the Shuttle Discovery at Cape Kennedy, and will be streaming the video of the event as recorded by a pair of Android phones on board. I suspect that any video so streamed will be less than stellar, but I like the basic idea. I’ll have to check back and see how they did.
