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2013 July 6

Disseminating the applied circuits labs

Filed under: Circuits course — gasstationwithoutpumps @ 11:41
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The post I just published about academic conferences lead me to thinking (again) about how I should disseminate my course design (or individual lab designs) for the Applied Circuits course.  I suppose I should write up a paper and submit it to a conference or some journal like the Journal of Engineering Education.

One problem is that my course designs are not education research—they are attempts to solve particular curricular problems by taking advantage of my strengths as a teacher.  Some of the course design generalizes to other teachers and other institutions with somewhat different curricular needs, but there is no controlled experiment to say that my course design is “better” than another is some predefined, measurable way.

I’m not sure where this sort of here-are-some-good-ideas-you-may-be-able-to-use course design work fits in academic publishing.  If I were teaching physics, I would probably submit to The Physics Teacher, but I don’t know what the closest equivalent is in engineering—particularly interdisciplinary stuff like teaching circuits to bioengineers.  There don’t seem to be good journals intended for disseminating instructional labs and curricular design. Maybe Advances in Engineering Education would be a better fit than Journal of Engineering Education, even if I don’t feel that my course design contains “significant, proven innovations in engineering education practice, especially those that are best presented through the creative use of multimedia.”

Of course, distilling down the 200–400 pages of notes on my circuits course that I’ve collected on this blog to a conference presentation or a journal article is a daunting task—one that I’ll probably keep putting off until it is so stale that even I’m not interested in it any more.  It might even be easier to turn the notes into a book than into an article, since I would not have to do 100-to-1 compression.

I’m not sure who the right audience for such a book would be—instructors trying to create a new course, students taking my course, hobbyists wanting to learn the material at home, … ? That is, should I be writing about a case study in course design, should I be trying to create a textbook for the course, or should I be trying to put together a self-study book that could accompany a kit of parts for people to learn electronics at home?

Again, the book project is big enough that I’d probably keep putting off indefinitely.  If I was sure there was an audience, I’d be more inclined to put in the effort to disseminate the material beyond this blog, though.

Another approach for disseminating the course materials would be to put together stand-alone kits for each lab (with detailed tutorials) that could be sold independently.  Releasing one lab at a time would be a more incremental effort than doing all 10 labs in one package, but would require some redesign, both for reduced expectations of lab equipment and to make the kits more modular.

I suspect that one could do most of the experiments of the circuits course for about $360 in lab equipment and tools, using something like the Velleman Lab2U unit (which PartsExpress sells for $210), a $40 multimeter,  a $17 soldering station, an Arduino, and the $66 bag of tools and parts I put together for the course.  The Lab2U is only a single power supply, so some of the projects that used a multiple supply would need to be redesigned—most notably the class-D power amp.  Unfortunately, $360 is too big a chunk of money for anyone but a dedicated hobbyist, who quite likely already has most of the needed equipment.

Making the kits more modular might be difficult. For example, many of the labs require students to choose resistors and capacitors from a large set of possibilities, since their lab kit contained 10 each of 112 different resistors and 10 each of 25 different ceramic capacitors.  It is easy to justify the cost of those parts spread over 10 labs, but harder to provide that much selection for a single lab.  Perhaps one would have to sell a core kit (with breadboard, resistors, capacitors, …) to use with the Arduino and add-on kits for each lab.  The core kit would need to have some fundamental experiments (like RC time constants), so that it would be instructional even without add-ons.

I wonder if there is a market for such lab kits, and how I would find out if there were a market (without sinking months of my time and thousands of dollars). I wouldn’t want to assemble or market the resulting lab kits either, but would want to distribute them through a company like Sparkfun electronics, Adafruit Industries, or Seeedstudio, who have already set up the necessary business infrastructure.

I also wonder whether I’m capable of writing the tutorials at an introductory enough level to work for hobbyists, while still covering enough of the theory.  I’ve never cared for kits that have great assembly instructions, but treat the way the things works as too difficult for the purchaser of the kit.

Writing instructions that included the use of an oscilloscope or multimeter, when there are many different ones that the person may be trying to use, would be a very challenging task.  Oscilloscopes in particular have evolved to have many radically different user interfaces, some of which are very complicated.

Also, all my writing has been for well-educated people: college professors, university students chosen from the top 10% of high school students, my loyal blog readers, … .  Can I make my writing intelligible for an average, or slightly above average, high school student, without sounding condescending or patronizing?  From the rather unsuccessful attempts I’ve seen in kit instructions in the past, that is not an easy task.

2012 July 7

Building a function generator kit

Filed under: Circuits course — gasstationwithoutpumps @ 22:35
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My Elenco FG-500 function generator kit that I mentioned in Speakers and function generator arrived yesterday, so I spent some time this afternoon putting it together.

Because I wanted to run it off of wall warts, rather than a 9v battery, I made one mod to the kit:  I replaced the mini-phone-plug power input with a standard 2.1/5.5mm power barrel jack, so that I could use the 9v or 10v wall warts that I have.  This mod required two changes:

  1. I drilled out the power jack hole to 5/16″ to accommodate the larger barrel jack.  I was a bit surprised that the front panel was metal, rather than plastic.  but drilling it was easy.  I did end up using my wife’s Dremel motor to deburr the drilled hole—that’s the first time I’ve used the tool.
  2. Both the phone jack and the power barrel jack have switches so that the 9v battery can be used when the power is not plugged in, but the phone jack switched the +5v and the power barrel switched the ground lead, so I had to do a little rewiring to make the negative battery connection switched instead of the positive battery connection.

I was a little disappointed with the poor connections of the power barrels in the jack—wobbling them caused loss of power.  On reading the specs for the jack, I see that the center pin is a 2.0mm pin, not a 2.1mm pin, as Digikey had it listed, so I may need to buy another jack, perhaps the Switchcraft 722A, which has a tapered center pin.  (I’d not gotten that originally, because it costs $3.52 rather than $2.43 for the CUI PJ005A, and I didn’t see a significant difference.  Now I think I know what the difference is, and I would have saved money by buying the more expensive part the first time.

The instruction manual that comes with the kit is quite good, reminiscent of the old Heathkit manuals.  It was from reading this manual that I realized that I would need to get a power barrel jack to use the kit the way I wanted. It starts with a checklist for the parts list, but some of the parts are had to check.  For example, there are three nuts for the binding posts and two for the potentiometers. They look the same, but have different threads. As it turns out, they sent me 5 that match the binding posts (5/16″-18) and only 1 that matched the potentiometer (which they say is 7mm, but is probably 5/16″-24), and I did not have a spare nut of the right size. If it really is an M7 nut that is needed, I’m going to have a hard time finding one—just about everyone skips that size, and uses M6 or M8, so I hope it is a 5/16″-24, as those are readily available.  (I’ll even check the hardware store tomorrow, though they are unlikely to have the skinnier jam nuts—I’ll probably have to order from Amazon.)

[Update 2012 July 10: the hardware store had both M7 and 5/16″-24 jam nuts.  The M7 nut did not fit, but the 5/16″-24 did.]

I got everything assembled without mishap, and it worked right away with either the 9v or the 10v wall wart.  I suspect it would work best at 12v, since that is what the  XR2206 chip is spec’ed at.

I tried using my old Fluke 8060A multimeter to measure the frequency (using the 10v wall wart):

  • Range 10: ?–15.35Hz (meter cuts out at 11.3Hz)
  • Range 100: ?–161.96Hz (meter cuts out at 11.3Hz)
  • Range 1K:  97.0Hz–1640.9Hz
  • Range 10K: 970.4Hz–15533Hz
  • Range 100K: 9.738kHz–142kHz
  • Range 1M: 107.8kHz–? (meter cuts out at 200kHz)

I would guess that the Range 100 setting goes down to about 9.7Hz and the Range 10 setting from about 0.97Hz to 16.2Hz.  Using the oscilloscope to estimate the time for the square wave pulse, I get

  • Range 10: ?—61msec (?–16.4Hz)
  • Range 100:  95msec–5.8msc (10.5Hz–172Hz)
  • Range 1K:  9.6msec—570µsec (104Hz–1750Hz)
  • Range 10K: 960µsec–62µsec (1.04kHz–16.1kHz)
  • Range 100K: 97µsec–6.65µsec (10.3kHz–150kHz)
  • Range 1M: 8.8µsec–0.905µsec (114kHz–1.1Mz)

I’m getting rather different readings from the Fluke multimeter and the oscilloscope, and I suspect that the problem is an uncalibrated scope. The top range starts about 11 or 12 times higher than range below it, because the capacitor used for the top range is 820pF rather than 1000pF.  I’ve still not figured out why they did that in the design, and I’m considering unsoldering it and replacing it with the expected 1000pF.

I tried using the Arduino (with a slight modification of the Superpulley code my son wrote) to time the lower frequencies:

  • Range 10: 1.0878s—65.05msec (0.919Hz–15.373Hz)
  • Range 100:  102.77msec–6168µsec(9.731Hz–162.127Hz)
  • Range 1K:  10.28msec—? (97.2Hz–?) starts missing ticks around 600Hz

The Arduino timing confirms that the Fluke multimeter is roughly correct, and that the timebase in the oscilloscope is a bit off. I don’t know what the Fluke timebase uses, but the Arduino crystal should be better than 0.3%, which is the discrepancy between the Arduino and the Fluke.  I checked around 20Hz and 200Hz, and the two agreed to 0.1%, which is the limit of reporting on the Fluke.

The Arduino program also gets good measurements for the very low frequency end, which was not feasible for the multimeter or the scope (a digital storage scope would have worked well, but the analog scope I’m using faded too fast).

No one would mistake the Elenco function generator for a professional function generator, as the output “sine wave” is awful. Perhaps the worst feature is a strong –5mV pulse at every rising edge of the square wave (the minimum of the sine wave) and a 10mV pulse at every falling edge (the maximum of the sine wave).  These pulses last about 60nsec, much less than the rise time of 220nsec, but comparable to the fall time of 60nsec for the square wave.  The amplitude of the noise is roughly constant, independent of the amplitude of the sine wave, but is slightly less if using the triangular wave form, rather than the sine-shaped one.  I don’t know whether this noise is introduced in the XR2206 chip, in the wiring to the outputs, or coupled through the power supply. Any of these are possible, as no bypass capacitors were used (other than electrolytics).  I wonder if adding a 47µF ceramic capacitor across the power leads (between pins 4 and 12) would help any.

2012 June 23

Speakers and function generator

Filed under: Circuits course — gasstationwithoutpumps @ 12:54
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In a previous comment, I had remarked on the difficulty I was having finding cheap loudspeakers.  I did a little more searching today, and found a good source: Parts Express.  I’ve bought stuff from them before, and they seem to be a reliable supplier.  They currently have a couple of small speakers for $2 each:

Even better, we could get a pair of 3″ 4Ω speakers in cabinets for 99¢, which could be used as is or cannibalized for parts.  All these deals are special offers, and so may not be available when we need to run the class, and probably can’t be purchased with University purchase orders.  I certainly wouldn’t want to be the one having to deal with the University purchasing system to get them.

But if we wanted audio output for any of the labs, we could afford to use loudspeakers.

When I complained about my function generator having died, Mylène suggested a kit generator.  I looked at the Elenco FG-500K Function Generator Kit she suggested, and read the PDF manual for the kit. The manual reminds me a lot of the HeathKit manuals that I grew up with, though with a bit less explanation of how things work.  I might get one just for the nostalgia value of building a kit like that again.

The function generator itself seems almost as functional as the much larger bench generator that died on me. The only feature that seems to be lacking is a frequency-modulation FM input, which I don’t expect to use much anyway. It uses the Exar XR-2206 chip, which does have FM capability (with a current input), but the circuit doesn’t use that capability.  It looks possible to add FM input, if needed, since the FSK input and the second timing resistor input are currently unused and hooking them up for FM would not be difficult.

The XR-2206 is supposed to run off 12V (10V to 26V, according to the data sheet), but is being run off a 9V battery. I wonder how much this under-voltage affects the behavior of the function generator. The function generator can be run off an external power supply, and the electrolytic capacitors are 16V capacitors so running off a 12V supply should work, if there are problems with a 9v supply.  I would want to change the jack they use for power, though, from a earphone jack to a barrel that matches one of my wall warts (probably the same size as is used for the Arduino, a 5.5mm/2.1mm center-positive barrel). I wish that power barrel connectors were more standardized—every wall wart I have seems to use a different connector, and there is no relationship between the voltages and the connectors. (Actually, 5.5mm/2.1mm seems to be the most common—I have 3 wall warts with that connector, each with a different voltage.)

They also push the high-frequency spec a bit to get 1MHz (which is the typical spec for the upper limit, but the min spec is only 0.5MHz)—their smallest timing capacitor is 820pF, while the spec suggests a minimum of 1000pF.  They don’t push the lower end, going down to 1Hz, while the spec claims the chip can go down to 0.01Hz.  The Elenco design switches the timing capacitor (100µF, 10µF, 1µF, 0.1µF, 0.01µF, 820pF) and uses a potentiometer to vary the timing resistor from 620Ω to 10620Ω.  Since the XR-2206 chip produces an output frequency at 1/(RC), the lowest frequency range should be 0.94Hz to 16.1Hz, and each switch up should provide a factor of 10 increase, except for the last one, which is 115kHz to 1.97MHz.  I wonder why they didn’t use a 1000pF capacitor to stay in spec—I might substitute a part there, if I get the kit.


2012 April 20

Make: Kit Reviews | The Ultimate Kit Guide

About a month ago, Make magazine released their reviews of various kits, Make: Kit Reviews | The Ultimate Kit Guide.  I have been a big fan of kits as a way to get kids into the habit of building things and knowing how they are put together.  They provide an intermediate point between ready-made consumer goods and hand-made artisanal goods.

I’ve talked before about my fondness for Heathkit electronics kits when I was growing up (Thanks, Dad!) and about how I was glad to see that they were finally back in the kit business. The kit issue of Make has a number of cool things in it ranging from the $3 Learn-to-Solder badge to $800 model submarines, $1000 mini CNC milling machines, $1300 3D printers, and $863 wood-fired hot tubs.  Although there are few kits in the issue that I really want, it is cool to see just how much is available in kit form these days.  Some are old-school kits (tube amplifiers! crystal radios! Nixie tubes!) and some are very modern (RFID breakout boards, quadracopters, drone planes).

My son has made a number of kits over the years (like the Velleman MK150 shaking dice kit or the K5300 Stroboscope with a xenon tube), and he is now moderately competent with soldering iron, solder sucker, diagonal cutters, and long nose pliers.  I suppose I should get him doing some surface-mount soldering, as my fine-motor control is a little shaky for 1mm × 2mm capacitors and 0.05″ pitch leads on ICs.  (Yes, I’ve seen instructions for making solder reflow ovens out of toaster ovens, and doing soldering with a skillet, but I’m not yet convinced that those are functional enough to be worth the investment in time and fried parts.)

Leads torn on pressure sensor.

The point about SMD soldering comes up this week because the pressure sensor superglued to the inside of the dry box for the underwater vehicle had its leads torn apart. This is probably my fault, since I had suggested the idea of supergluing the pressure sensor to the inside of the dry box without giving any consideration to the forces on the tiny little leads of the pressure sensor.

I had some spare sensor boards, but I had to order more pressure sensors from Digikey and assemble a new board for them.  This weekend, they’ll drill yet another hole in the drybox and glue the replacement pressure sensor in place, but this time there will be a couple of pieces of plastic glued to the PC board (about 4.8mm thick, to match the thickness of the pressure sensor body) also glued to the inside of the dry box, so that unplugging the cables will not put strain on the tiny wires of the pressure gauge.

The new hole will make the 6th penetration of the dry box.  Somewhat amazingly, none of these penetrations have leaked, although we have had problems with the underwater connector that they designed for the motor wires.  We’re hoping that problem will be fixed this weekend.

2012 February 20

Heathkit is back in the kit business

Filed under: Uncategorized — gasstationwithoutpumps @ 19:52
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Last year, in my Thanks, Dad post, I mentioned how much Heathkits had meant to me growing up, and how I missed the combination of detailed instructions and how-things-work of their manuals in more recent kits.

Heathkit is now back in the kit business. They have started with only one kit, a Garage Parking Assistant,which is of little use to me as I have no car (nor any room in my garage if I had one).

They plan to add more kits in four categories:

The kits will almost certainly be more expensive than buying already built versions of the products, since it is much cheaper to integrate and assemble things in a factory than to count and package parts that can be hand assembled.  The value of electronic kit building nowadays is more in learning and sense of accomplishment than it is in saving money.

I understand that the audio and amateur radio kits will be based around vacuum-tube technology, which is not so easy to get commercially, and for which kit building is competitive with commercial assembly.

I welcome Heathkit back to the kit-making business, and I hope that they do well.

(I found out about Heathkit’s return to kits from the special kit issue of Make Magazine—an issue with many fine kits in it, though some of the most attractive ones are beyond what I’m willing to spend on a toy at the moment.)

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