Design Challenge: AM radio
Once again I had a group challenge as part of the electronics final. This was something I first tried the last time electronics was offered (two years ago, see laser tripwire). I felt that it was just as much fun as last time, and the class certainly engaged with it and came through victorious.
The format for the final was a 1.5 hour written exam (individual) and a 1 hour group challenge. I provided a stack of parts, some basic test equipment, and an outline of the challenge:
You are stranded on a desert island after a plane crash. Fortunately the plane was full of physics majors and you all survived. The following parts and equipment were not incinerated after the fiery crash:
- two 555 timers
- an NPN transistor (2N3904)
- various capacitors and resistors
- numerous scrap wires
- one breadboard
- a battery-operated oscilloscope (don’t leave home without it!)
The oscilloscope is on loan from your local hackerspace so you cannot tear it open for parts! To survive you must build a transmitter capable of sending a morse code message over the AM band. You estimate that the search party will be nearby within an hour, your transmitter must be working by then. Good luck!
Since we had not discussed AM radio in any substantial way, I told students who finished the written exam early that they were allowed to search for information about AM radio and radio transmitters. They were allowed to take notes on what they find, but the internet would not be available during the challenge.
This turned out to be crucial… otherwise I would have had to get them started with some “outside information” and that wouldn’t be as fun. Several of them had enough time to jot down schematics but since they didn’t know what parts would be available, these weren’t as useful. The most useful notes had general information on the frequency range for AM radio, and the basic idea that a carrier is modulated by an audio signal.
During the semester, we had briefly covered the use of transistors as mixers and the class all remembered their most basic use as “one current controlling another”. These pieces were enough for them to recognize that one strategy was to design two oscillators with astable 555′s (one for the carrier and one for the signal) and then mix them on the transistor.
It turns out to have worked very well and I was easily able to tune in their “signal” on an old clock radio. The range wasn’t great with just a single wire antenna (no impedance matching or amplification) but they satisfied the spirit of the problem.
I learned that it is tricky to do these group challenge problems with 11 students. Last time around I had 7 in the class and that was just fine… two natural groups of 3 and 4 students. This time, there were always one or two people standing around. They broke out into four groups, with two groups working on the carrier and two on the signal. This let them speed up the prototyping & testing process. The adjustment I had to make this time was to provide extra parts to tinker with (duplicates from the parts list above). That way there were enough parts so everyone could be prototyping something. When someone got the oscillator to work they could say what component values they used and that circuit could be built into the final answer. If the course is this large again, I’ll have to be creative about finding a challenge that can be easily distributed.
The class learned about AM radio and I’m willing to bet a longitudinal study that they will remember how to use a transistor as a mixer. I’m using the testing effect here to teach some concepts that weren’t central to the course (i.e., they didn’t cram for them the night before) but they are relevant, useful, and fun to know.
Overall, it was another fun challenge, and the result is a circuit that’s fun to play with. Below is a schematic for the answer the class turned in. One nice feature of this problem is that there are actually many answers that work as long as the basic criteria are met. Due to the huge number of harmonics generated by the astable oscillators, you can pick up this signal all over the AM dial. In fact, the fundamental frequency of the carrier doesn’t even have to be in the AM band, even if a higher harmonic fall in the AM you can still hear the transmission. Low specificity is bad for radio stations, but good for rescue beacons! It is important to avoid challenge problems that have tight tolerances or that require very specific solutions.
An AM radio built from two 555 timers.
Data manipulation
This isn’t a political post, it’s a post about data, and ethics. When you read a graph, you start with the assumption that whoever made the graph followed a set of basic guidelines. Breaking these guidelines can happen by accident, but with computerized tools, this is rare. With the availability of graph software, making an X-Y graph misrepresent data requires some effort. Sometimes you can even prove it was intentional.
The case study here is a television graphic presented last week. Here is the original on-air version:
Original air date Dec 12, 2011 (Fox News)
There are two big problems with this graph. I’m not going to address graph design, chartjunk, or any other aspects beyond the fundamentals. Once this graph is technically correct, we could argue those points. Here we’ll focus on graphs 101… the basics.
The scale is clearly wrong on the left of the chart. 9% roughly lines up with the data but the peak at 9.2 almost lines up with 9.5 on the scale. The valley (8.8) is close to 8.5 on the scale. Despite the error, this doesn’t really matter because most line graphs are used to indicate a trend. As long as the trend is correct, we can forgive an error on the scale.
The trend is not only wrong, but it has been manipulated. This can be shown by comparing the data presented above to the actual data. First, in order to even compare these charts, we have to fix the scale issue. It turns out that the scale was wrong by exactly 1/2 so it seems reasonable to assume that an error was made in the preparation where either the data or the scale were reduced in size in order to make things fit. Perhaps it was more pleasing to see 8 and 10 as round numbers on the screen. The data doesn’t vary by much so that scale would have made it seem too flat (a legitimate trick for skewing data perception). This kind of mistake happens… it shouldn’t, but it does. With the scale fixed, the data sets overlap very well except for two key areas:
Unemployment data (blue) and TV news graphic (red).
You can spot the difference without my help. Just to be open, honest, and clear I have added error bars that represent the confidence interval for gathering data from the screenshot presented above. Thanks to GraphClick software, this is an easy thing to do. The blue line is the original data from the source, and the red is the (scale corrected) TV version.
Consider the process of making a graph:
- Start with a data set
- Import data into software
- Choose fonts, colors, points, backgrounds, lines, labels, etc.
- Show graph on TV
Somewhere between steps 3 and 4 two things have happened. First, the scale shrunk (possibly to make 8 and 10 fit nicely on the TV screen). Second, at least two data points got tweaked. Perhaps you want to give the benefit of the doubt and assume that the last point was accidentally duplicated to show two points at 9 instead of one at 9 and one way down at 8.6. Ok, I’ll let you think that. How can we explain why the next lowest point (March, 8.8) got shifted so far up? And why did the February point move down? Moving one point down doesn’t cancel out moving another one up! This is data manipulation, this is what the same network continues to call Climategate!
Sure, they could say “we gave you the raw data in yellow.” Yes, but the graph is supposed to reinforce the numbers and represent them accurately. There are many ways to screw this up by accident… this was not an accident; someone grabbed those points in the graph and moved them around.
LaTeX in Prezi
I’ve become a regular Prezi user in the past year, but one thing was holding me back: LaTeX math had to come in via file upload… until now.
The codecogs equation editor has an HTML integration scheme that will let you import a SWF image by entering LaTeX code directly in the URL. If you like this, please donate to CodeCogs… in a few minutes you’ll see why you owe them for this hack and not me.
The URL scheme is described at the HTML integration page. In particular, something of the form:
http://latex.codecogs.com/gif.latex?1+sin(x)
will generate a gif image of 1+sin(x) in nice LaTeX. Replace gif with swf and you have an swf (flash) file that is ideal for importing to Prezi. For the import process, start editing a prezi and go ahead to the dialog for importing an image. In the image dialog, past the codecogs URL into the field as shown below, and hit enter (be sure to use the swf version of the URL). Prezi recognizes the file as an SWF and goes about importing it as expected.
For the example, I use the URL:
http://latex.codecogs.com/swf.latex?1+sin(x)
Paste the codecogs URL into the image URL field in Prezi
Prezi works its magic...
There will be a short bubble notice that Prezi is thinking… but don’t worry, it will do what you want it to.
Next you will see the glorious LaTeX result added to your Prezi in SWF format with all the speed and scalability of flash (the native Prezi format).
Please keep in mind that it is the awesome work of CodeCogs that makes this hack work… and they ask for nothing in return. Do the right thing and support them with a small donation.
Viola, LaTeX in Prezi... with full SWF scalability!
I can also suggest that spacing can be added either with LaTeX space commands (\, \quad, etc.) or a single space can be URL-ized with %20. The url with a literal space will not be processed correctly (at least it wasn’t when I tried it).
As always, your mileage may vary, but let me know if it helps.
TI Launchpad vs. Arduino
I was excited to see the TI Launchpad platform, especially given the low price ($4.30). Unfortunately, there are two troubling statements in their wiki… these are dealbreakers for a growing number of hobbyists:
“There is currently no official support for the Linux operating system.”
“There is currently no official support for the Mac OS X operating system”
While windows is still dominant in the electronics profession, the hobby crowd is much more varied. These issues will make it hard for the launchpad to compete with Arduino. I’ll probably order one to play with, but when I already have a dozen Arduinos (Arduini?) lying around, and the latest IDE on each of my computers (linux & mac alike) it is hard to imagine going through much trouble just for a cheaper board. For me, the value of Arduino is in the community resources that are already out there. The codebase is huge, you can find thousands of great examples and tweak them to your particular use: search google or youtube for what you want to make, upload the code to arduino and you are done.
Programming Arduino is like plugging in to the matrix… a few clicks and keystrokes and suddenly “I know kung fu”
Cold-Atom Lab in a Box
The ColdQuanta miniMOT atom trap
I was quoted in a recent Photonics Spectra article “Cold-Atom Lab in a Box“. The article describes the new Rubidium miniMOT available from ColdQuanta. My Photonics and Quantum Optics Lab at Pacific University was among their first customers. We’ve really enjoyed being involved with the early stages of this product and look forward to contributing to MOT research at the undergraduate (and perhaps high school) level.
For pictures of our trap in operation, see my earlier post “MOT!“. We have done a variety of experiments and have trapped atoms in several different geometries since then. More details are described in the talk given by Simone Carpenter at the Northwest APS section meeting: Off-the-shelf atom trapping [PDF] and the corresponding poster Off-the-shelf atom trapping.
Please feel free to contact me with any questions.
Student errors in computational physics models
An interesting post about recent work on the implementation of python-based modeling in physics courses. Read more at Mark Guzdial’s blog.
The post is part 2 of 3 that summarize some wonderful points in Marcos Daniel “Danny” Caballero’s PhD thesis at Georgia Tech. Danny is the first Physics Education Research (PER) student from GT.
I am interested in the possibility of catching some common exceptions and using python features to replace boring error messages with something that speaks to the underlying physics. I doubt this could be done on a general level, but perhaps it could be done for certain types of problems, and included invisibly by the instructor. I need to brush up on how to catch exceptions in python, but I’d like to hear from anyone who is familiar with python exceptions.
Our article on optical precursors is out now, visit the abstract at:
Near a narrow resonance, an optical pulse can pass through a strongly-absorbing atomic vapor for the first few nanoseconds. The part of the pulse that makes it through the material is called a precursor. Our paper explores the form of the precursors as the carrier frequency is tuned off resonance.
Experimentally obtained transient transmission intensity (black solid lines) compared with two theoretical analysis: the asymptotic analysis (red dotted lines), and the weakly dispersive narrow resonance (blue dashed lines). Transient transmission taken near the 4S1/2 (F = 1) ↔ 4P1/2 (F = 2) transition for (a) ∆ = ∆(1) ∼ 5δ, (b) ∆ = ∆(2) ∼ δ, and (c) ∆ = ∆(3) ∼ 0.
NOVA | Absolute Zero | PBS
Absolute Zero: No it’s not a vodka mixer, it is the lowest possible temperature; the temperature at which atomic motion ceases. In the lab, we try hard to get there… and we can cool atoms to a few millionths of a degree above absolute zero. Read more about the history of this field and where it may go in the future:
On Being Wrong – Kathryn Schultz
A great talk on the benefits of being wrong. The most important part is always remembering that you may be wrong, and to not fear it.
http://www.ted.com/talks/kathryn_schulz_on_being_wrong.html
