Gas station without pumps

2016 May 12

Soda bottle rockets used again

Filed under: Uncategorized — gasstationwithoutpumps @ 20:48
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I had another chance on Tuesday this week to play with soda-bottle water rockets, which I have not done since my son was taking home-school physics and we wrote the timing program for measuring the ascent of the rockets that later turned into PteroDAQ to go along with the homemade Lego “superpulley”.

My wife volunteered me to help out at the Spring Hill School’s Family Science Night—a tradition I started 9 years ago (here are my notes from the 2 years I ran it).  She had thought I could set up the “Dr. K’s Applied Electronics” display I’d used at the Mini Maker Faire, but I didn’t have the time it would take to set up the display (over an hour, to do it right).  Instead, we agreed to revive the soda-bottle water rockets, which are always a hit with elementary-school kids.

I raced home from running the pulse-monitor lab for the Applied Electronics course, loaded up my bike trailer with empty mineral water and soda bottles, a couple of floor pumps, and some rocket launchers.  My wife also brought a few copies of the instructions for making the PVC launchers, though not the Spanish-language version, which we first created in 2001, when my son was in kindergarten (a post I wrote in 2011 talked a bit about the activities I did with the Kindergarten kids to explain how rockets worked).

The rockets were a big hit at Family Science Night last night, though the kids preferred the commercial launcher that I believe I bought from Arbor Scientific (which can be seen in the superpulley post and the water-rocket simulation posts 1 and 2).  Nonetheless, several families did take instructions for making the simple PVC launchers, with the intent of doing it as a fun project this summer.

It was fun to do the science outreach, though I only managed to talk with one family about how rockets worked, and it gave me a good excuse for not doing any grading that night (after 3 days of doing not much besides grading, teaching, and supervising the students in lab, a night supervising kids sending up soda-bottle water rockets was a welcome relief).

2016 May 11

Don’t put pulse monitor first

Filed under: Circuits course — gasstationwithoutpumps @ 21:25
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In Revised microphone pre-amp lab too long I said,

I think that the soldering lab should not be the first op-amp lab, but I still like the idea of the students having to solder up their microphone preamps. So I’ll have to do a major reorganization of the book this summer, to move a different lab into the first position.

Currently, I’m thinking that the transimpedance amplifier and pulse monitor lab would be a good choice as the first op-amp lab.

After a rather rough start in the first half of the transimpedance-amplifier lab yesterday, I no longer think that is a good idea. The instrumentation-amp lab went much smoother, so may be a better first choice.  Among other advantages, the instrumentation-amp lab with the pressure sensors has no analog filtering and simpler sensor sensitivity calculations.  One disadvantage of moving away from the mic preamp is that the microphone and loudspeaker characterization in the first half of the class would be separated from the audio amplifier design in the second half—not a problem in a single quarter class, but potentially a bigger problem in a two-quarter sequence.

I’m not sure why the transimpedance amplifier lab went poorly yesterday, and I’m hoping the second half will go smoother tomorrow.  It may be that prelab was not a good match for the lab this time.  Or it may be that over a third of the class didn’t do the prelab this time.  (I’ve threatened the class with a quiz worth as much as a design report if they don’t shape up by next week—I’m carrying around enough redone reports to grade that I really don’t want to do more grading, but I’ll follow through on the threat if so many students continue slacking.)

It turns out that I had several errors in the draft of the book that the students were using for the prelab exercises for the optical pulse monitor.  I’d decided in the summer or fall to switch to a new 700nm LED, but I’d only updated about half the scaffolding for the sensor sensitivity, so there were still a number of things that were only accurate for a 623nm LED.  Also, I’d been using a datasheet for the WP3DP3BT that I’d gotten when I first started using the part, but Kingbright has improved their datasheet a lot between V3 (which I had) and V5 (the current one), so there is now a spectral sensitivity curve on the datasheet, and the spectral sensitivity is quite different from what I was expecting.

I’ve started editing the book to correct the errors, but even after I fixed everything, the estimates for the current from the phototransistor were about 5% of what students were measuring in the lab. The model I had created, which worked fairly well for the previous LEDs, does not seem to work for the longer wavelength of the new LED.

I’m considering simplifying the model by eliminating the modeling of scattering, to see how well that works.  I should check the model with at least 3 different LEDs: the current 700nm one, the shorter wavelength ones I used to use, and an IR emitter.  If I can get the estimates to be within a factor of 10 of measured values for all the LEDs I have, then the model is good enough to include in the book.

I might also want to consider switching phototransistors to one with a wider spectral sensitivity, so that the estimation is not thrown off as much by the filter that blocks so much of visible light.  That would allow me to try a green LED as well.

I’m still thinking about doing a log-transimpedance amplifier as the first stage (not for the class, just for a demo unit) so that the pulse monitor can work in varying light levels up to full sunlight.  The fluctuation in light from the pulse seems to be about 1%, which should be a variation of about 850µV out of the logarithmic amplifier (based on the 9.8mV/dB I measured in Logarithmic amplifier again).  That’s a somewhat smaller signal than I’ve been getting with well-chosen gain resistors, but it may be worth it to get greater independence from the overall light level.

2016 May 5

PCB CAD tools

Filed under: Uncategorized — gasstationwithoutpumps @ 20:58
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I’ve been using Eagle for designing printed circuit boards for a few years now, and I am reasonably happy with it as a free tool. However, I’m a little annoyed by the low quality of the schematics and by the awkward creation of new footprints for components, and so I am willing to consider other tools, and am looking for recommendations for free PCB tools that are better than Eagle.

Two I’ve heard of (but not tried yet) are

  • EasyEDA , which is web-based, and
  • DipTrace, which (like Eagle) is a commercial package with a free, but limited, hobbyist version.

I’ve not used either of these yet, and I don’t have any PCB designs to do right now (nor time to do them until the quarter ends), but I’m curious whether any of my readers have tried EasyEDA or DipTrace, particularly if they can compare them with Eagle.  I’m also curious whether there are other PCB tools out there that run under both Mac OS X and Linux and that are free, easy to use, and robust.

My son and I are planning a couple of boards this summer as part of the LED theater lights project, so there will be an opportunity then to try out different PCB tools, if anyone has ones to recommend.

Democratic primary very important this year

Filed under: Uncategorized — gasstationwithoutpumps @ 20:48
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The Democratic primary is very important this year, because it gives US voters the opportunity to choose between a New Deal Democrat (Sanders) and an Eisenhower Republican (Clinton).  This is the first time since George McGovern ran in 1972 that such a choice has been offered to the US people.

The Republican party itself no longer offers serious candidates for president (and increasingly often doesn’t for other races either), so the Democratic party has taken over most of the Republican policies and positions and offers Republican candidates a place to run for office without having to associate with the clowns who have made the GOP into a circus sideshow.

Personally, I’m a progressive Democrat, so I strongly favor Sanders’s policies, but I can see a lot of Republicans voting for Clinton, whose stand on almost every issue is a late 1950s Republican position (except that Eisenhower warned us about the military-industrial complex, while Clinton supports it whole-heartedly).

Because the Republican Party is incapable this year of running a serious Presidential candidate, the Democratic primary is doubly important—it is the only chance voters will get to choose between a classic Democrat and a classic Republican.


2016 May 3

Revised pressure sensor lab went very well

Filed under: Circuits course — gasstationwithoutpumps @ 21:20
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Today I ran a revised version of the pressure sensor lab (see
Pressure-sensor lab went well, Class-D lab revision didn’t work, Blood pressure monitor, Blood pressure lab, and Blood pressure lab part 2 for descriptions of the old labs).

The revised lab included both blood pressure cuffs and breath pressure using the simplified breath pressure apparatus of Simplified breath pressure apparatus:

The ½" elbow is small enough that I can put my lips around the opening, which would have been a bit difficult with the 1" tee.

The ½” elbow is small enough that I can put my lips around the opening, which would have been a bit difficult with the 1″ tee.

To make the apparatus, the students had to drill 2mm holes in PVC elbows, so I packed up my drill press last night and hauled up the hill in my bike trailer this morning.  For those unfamiliar with Santa Cruz, that is a 3-mile ride with a fairly steady 4% slope, resulting in a climb of about 715′ (218m).  Needless to say, I went slower than usual uphill!  There is a drill press only about 150′ from the lab the students were working in, but the bureaucracy for getting the students access to the drill press is incredible (I tried, and failed, to get a dozen students access last quarter). So it was easier for me to haul my own drill press up the hill on my bike than to deal with the dysfunctional bureaucracy at UCSC to use the drill press supposedly there for student use.

I explained to each pair of students how to use a drill press, including basic safety precautions, and had them drill a 2mm diameter hole in their PVC elbows.  There were no problems with this, and I plan to do the same for the lab in future.

Each pair of students designed an instrumentation amplifier with an INA126P chip as a first stage and an op-amp as a second stage, wired it up on breadboards, checked the calibration, and recorded both breath pressure and blood cuff pressure.  A few students used extra time to play around with some toys I brought in: a hand vacuum pump, a Lego pneumatics pump, and an aquarium air pump.  One group even tried using the pressure sensor as a microphone, using a loudspeaker with a 300Hz sine wave for input (the pressure sensor could detect the 300Hz input without problems, though I suspect that it was not registering the full pressure fluctuation, as I think that the sensor has about a 200Hz bandwidth).

Most groups were done with this week’s lab in the 3 hours of today’s lab, so Thursday’s lab will consist mostly of students doing make-up work to redo old labs, with a few finishing up this week’s lab.  I expect to spend most of Thursday’s lab time grading design reports (I’m about 2 days behind—I got the design reports that were turned in a week ago done and returned yesterday, and I got the prelabs turned in yesterday done and returned today, but I haven’t started yet on the reports turned in last Friday, nor the stack of redone work turned in last week).

The instrumentation-amp lab went surprisingly well this year, despite adding the drill press.  I think that the big advantages over previous years are that they did not have to solder the inst amps this year and that they had already done a multi-stage amplifier for the microphone amp.

I think that I should rewrite the book to introduce multi-stage amplifiers as the default design (since every amplifier they do in the course is now multi-stage), and talk about how to choose the gain for each stage in general, before getting into individual labs.

One minor problem in lab today—students didn’t have the short pieces of tubing to connect up their breath-pressure apparatus.  This turned out to be my fault—I hadn’t included them on the parts list for this year!  Luckily the BELS staff had some pieces leftover from last year, and I had about 20 feet of my own tubing in the box of stuff I’d packed for the lab, so we had enough for everyone to get 6″.


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