Gas station without pumps

2018 January 17

An evening out at MAH, Limelight Cafe, and Penny Ice Creamery

Filed under: Uncategorized — gasstationwithoutpumps @ 21:03
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My wife and I try to eat out at least once a week, and tonight was our night out.

Before dinner we went to the Museum of Art and History to see the exhibit of Tom Killion’s prints.

A low-resolution image of one of Tom Killion’s works, from the MAH website. This low resolution does not do justice to the fine detail of the woodblock print.

His website has better images of his work than this little one, but they are still nothing like seeing the details on the real prints. We’ve always admired his work (we have one of his books), but none of the prints listed as available on his website currently screams “buy me!”. MAH did a good job of hanging the exhibit, though they may need to work on the accuracy of their labels (claiming over 100 years for the date of one print, for example)

While we were in MAH, we noticed that Santa Cruz Core Fitness had massage tables and other stuff set out in the lobby—MAH seems to be making a concerted effort to connect with a wider swath of the community, and some of the partnerships they come up with seem a bit strained.

We had been thinking of eating at Abbott Square (the courtyard behind MAH) after seeing the exhibit, but the space seemed noisy and uncomfortable, despite having rather few customers. We’ve eaten there a couple of times and never found it very comfortable—the concrete walls make the space too “live” and conversation is often difficult. It might have been ok outside, but tonight felt a little too chilly to us, and their outdoor heaters are not as effective as others around town.

Instead we went to Cafe Limelight, a very quiet panini shop on Cedar Street which has very eclectic decor and which we find very comfortable. (The panini are good too.) When we eat there we often see one of my colleagues, so we made a little bet—if he was there we would go for ice cream at Penny Ice Creamery on the other side of Cedar after dinner. As you have probably guessed from the title, he was in his usual place at the counter—I think he may eat there every Wednesday.

Over dinner, we discussed MAH’s willingness to pair with almost anyone and started thinking of strange partnerships for MAH’s atrium. The best idea came from my wife, who suggested that MAH partner with a funeral home, which could tie in with MAH’s stewardship of the Evergreen Cemetery. The funeral home could display coffins, headstones, urns, and other funerary art. It could even be an interactive exhibit, inviting people to lie down in a coffin (maybe giving them a little bell to ring?).

At Penny Ice Creamery,we each had a mini-double. My wife had candy-cap mushroom ice cream and a buttermilk salted-toffee ganache, while I had pumpkin ice cream and Meyer-lemon with poppy seed. The candy-cap ice cream is about as good as other candy-cap ice cream we’ve had in town (from Mission Hill Creamery), which is to say it had a rather intense maple-like flavor. The Santa Cruz Fungus Fair was last weekend, so this is the time of year for candy-cap ice cream. (We didn’t go to the Fungus Fair this year—we figured that with the very dry December weather, the fungus would not be in peak condition this year.)  The pumpkin was good, though my wife claims it is not as good as their gingerbread (which they rarely make).  The Meyer lemon with poppyseed was also good with the pumpkin, though I probably would not order it by itself, as the flavor in not intense enough for it to stand on its own (at least not for me—some people may prefer the more delicate flavor).  I did not try the buttermilk salted-toffee ganache—my wife reported it as having a good base, but marred by the lack of crunch in the toffee.

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Long weekend, little done

Filed under: Circuits course — gasstationwithoutpumps @ 10:01
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Last weekend was a long weekend (Martin Luther King Day on Monday), and I briefly had fantasies of getting a lot of stuff done.  The trouble is I never settled on exactly what it was I would get done, so I ended up doing very little.  I read science fiction, I slept a lot, I tinkered with my book a little bit, I got the live Christmas tree out of the house, and I adjusted the brakes on my bike. but that’s about it.

The main addition to the book wasn’t even a very important one—this picture:

Some of the most common packages for FETs.

We used to use TO-220 and TO-251 packages for the class, since that is how power FETS are most commonly packaged, but the power FETs are getting expensive (the cheapest ones keep getting discontinued—maybe they were cheap because they were end-of-life, or maybe they were discontinued because there wasn’t enough profit at the low price point).  We had problems last year with TO-251 packages not staying in the breadboards—the springs seemed to pop the leads out rather than grabbing them.

This year we’ll be using the SOT-23 transistors, which are much cheaper, and soldering them to a breakout board.  I’m a little worried about how many of the students will have trouble with hand-soldering the small parts.  They’ll have had a little more practice soldering by then, so I’m hopeful that it will go ok.

The other changes to the book were mostly typo fixes for problems found by my students.  The students have been very good this year at reporting problems to me, and there were a lot more typos than I expected (averaging about 1 every 3 pages).  So far they’ve not pointed out any substantive errors, though there was one omission that I’ve fixed—it seems that some students have not heard of raster image formats, and thought I was trying to say “faster image formats”, so I’ve added a couple of paragraphs about image formats.  The changes that I’m making this quarter will be in the next release, which will probably be in March, during spring break.

2018 January 13

Marcus in umbrella

Filed under: Uncategorized — gasstationwithoutpumps @ 16:05
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Last week when we had rain, Marcus found drying umbrellas to be a great new toy. (Everything is a toy to Marcus.)

2018 January 3

SOT-23 FETs for half H-bridge

Filed under: Circuits course — gasstationwithoutpumps @ 20:53
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In Breakout board for SOT-23 FETs, I gave the schematic and layout pictures for a half H-bridge breakout board using SOT-23 surface-mount FETs.  The boards arrived today, 12 days after I ordered them. The boards cost $4.86 plus $23.79 shipping, but I had them panelize the design, and they sent me 13 copies instead of 10, so I ended up with 546 boards (instead of 420), making a cost of 5.24¢ each.

One of the panelized board. The panels are just separated with V cuts, so the corner rounding is not very good, but there is some, and I did not end up with sharp corners after cutting off a row of boards with tin snips.

With the transistors, capacitor, and headers, each half H-bridge will cost under 40¢ in 100s—much less than the approximately $1.37/half H-bridge that separate TO-220 FETs cost.

Today I tried soldering on a  PMV20XNER nFET (14.9¢ in 100s) and SSM3J332R pFET (12.4¢ in 100s), a 5-long right-angle header, and a 10µF ceramic capacitor. I wanted to do this with pretty much the same tools the students would have, so I did not use a board holder nor cross-lock tweezers (both of which would have made the job slightly easier).  My technique was simple:

  • Put the board face up on the bench.
  • Place one FET using sharp-pointed tweezers.
  • Tape the FET and the drain side down with a tiny piece of blue painters’ tape.
  • Solder the source and gate.
  • Remove the tape.
  • Solder the drain.
  • Repeat for the other FET.
  • Put the headers through the holes (from the component side).
  • Flip the board over and solder the header.
  • Put the header pins into a breadboard at the edge of the board.
  • Insert the capacitor from the component side.
  • Prop the breadboard up so the solder side of the board is exposed.
  • Solder the capacitor in place and trim its leads.

Soldering the first board went well.  The second one was a little harder (I had a bit of hand tremor), but still only took a few minutes.  Having made the lands huge (big enough for wave soldering) made alignment fairly simple—I did not have to be exact.

I tried one FET without the trick of taping the FET in place—that did not work at all, as the FET moved completely off the pad when I tried to solder.  I had to remove solder from the board with a solder sucker and redo the FET using tape.

Here are the front and back of the boards before and after populating, along with the pointed tweezers I used for placing the FETs.

Here is a close-up of one of the two boards I soldered (the one with the worst alignment—see the pFET at the top left).

I spent the rest of the afternoon checking that the boards were OK.  I used essentially the same setup as I used for Ron vs Vgs for pFETs and nFETS, with a 24Ω load and a 10V ramp that gradually turned the transistor off.  Because the test was the same, I plotted the results together with the old results:

The PMV20XNER transistor has a much lower threshold than the other nFETS I’ve looked at, but a comparable Ron to the other power nFETs.

The SSM3J332R pFET also has a low threshold voltage and the on resistance is in the same range as others we have used in the past.

It looks to me like the half-H-bridge will be a perfectly reasonable way for the students to get FETs for the class-D amplifier.  The current will be somewhat limited by the power dissipation of the pFET, but with an 8Ω speaker and 0.1Ω pFET, the power to the loudspeaker should be 80 times the power lost in the pFET, so the 10W limit on the loudspeaker should be reached well before the half H-bridge overheats.

Update: the EAGLE and Gerber files for this board are available at https://users.soe.ucsc.edu/~karplus/bme51/pc-boards/

2018 January 1

Blog stats for 2017

Filed under: Uncategorized — gasstationwithoutpumps @ 11:47
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According to WordPress.com, my blog had 60,609 views and 35,810 visitors in 2017 (down from 68,443 and 42,499 in 2016).  The reduced viewership is not too surprising, as I posted only 114 blog posts in 2017, rather than the 200 of 2016.  (My biggest year was 2014 with 116,359 views and 69,179 visitors.)  I don’t know how much the decline in my readership is due to a general decline in how much people read blogs and how much is specific to my blog—I’ve not been able to find good statistics on the readership of “average” blogs.

Here are my year’s most-viewed pages (almost all of which are from previous years):

2017-01-01 to 2018-01-01

Title Views
Home page / Archives 19,956
Where you get your BS in CS matters 1,820
Making WAV files from C programs 1,601
Tools and parts list for Applied Electronics W2017 and S2017 1,387
Algorithmic vs. Computational thinking 967
Sum of probabilities in log-prob space 800
How many AP courses are too many? 632
Journals for high school researchers 606
Problems rewriting the Class-D amplifier lab 497
Pressure sensor with air pump 442
Getting text from Amazon’s “Look Inside” 438
Conductivity of saline solution 407
Where PhDs get their Bachelors’ degrees 401
Installing gnuplot—a nightmare 371
More on automatic measurement of conductivity of saline solution 365
Circuits course: Table of Contents 361
Why Discrete Math Is Important and The Calculus Trap 354
Lying to my students 341
Pressure and volume lab 334
labhacks — The $25 scrunchable scientific poster 315
Pullup vs. transimpedance amplifier 312

I think that there are several reasons that old blog posts dominate my views:

  • Most of my viewers (other than subscribers, whose loyalty I really appreciate) come via search engines. Of the 60,609 views, 33,202 were search-engine referrals (55%). Search engines will favor long-established pages that many people have clicked on in the past.  My next highest referrer is Facebook (which I don’t use) at 195 viewer—a tiny number in comparison.
  • My recent posts have been more specialized than older ones, so have a narrower audience.
  • I’ve been less active recently in calling attention to my recent posts on mailing lists and blog comments.

One good trend is that the most popular posts are now mostly contentful ones, rather than ones that are just links to other sites.

I don’t get any revenue from my blog (but I don’t pay anything for it either).  The clicks from my blog mostly go to other of my blog posts (1259), Digikey (1229), AliExpress (179), and LeanPub (175).  The average cost per click for advertising is 50¢–$2, so Digikey and AliExpress probably should be paying me, but they aren’t.  (Of course, the click rate would probably drop way down if I was being paid to push products, rather than just providing links to things I have bought and used or am thinking about buying.)

My top commenters (based on the last 1000 comments, so several years’ worth of comments) are

Commenter Comments
gasstationwithoutpumps 399
CCPhysicist 114
gflint 50
xykademiqz 31
mathproblemsolvingskills 26

My comments are mostly pingbacks caused by links to older posts for continuity, though some are replies to other commenters. I’d like for my comments to be about 25% of the total, rather than 40%, but I’ve not had much success in getting my lurking subscribers and followers to say anything. If each of my followers made just one comment a year, the number of comments would quadruple. Questions, corrections, and suggestions for blog posts are particularly welcome.

I admit to being somewhat envious of bloggers who have active discussions among their commenters—my readers don’t seem to have formed that sort of on-line community, perhaps because my posts are not open-ended enough or because I wander over many different topics rather than staying focused on a specific niche, so the readership may not share many interests with each other.

For those who have been commenting—thank you! It really helps me to know that people are reading my blog (and raw numbers don’t really do that—I can’t really tell whether viewers coming in from search engines are reading what I have to say, or just clicking on a link and deciding it was a mistake).

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