Gas station without pumps

2021 November 1

Halloween 2021

Filed under: Uncategorized — gasstationwithoutpumps @ 20:09
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This year, we had a fairly modest Halloween, after not celebrating it all last year.  We had only 36 trick-or-treaters, about the pre-COVID normal number, but way down from the glory days 15–20 years ago, when we could get 120.  We harvested over 30 volunteer decorative gourds from the area around our compost heap, and we added two purchased pumpkins that we carved.  I also bought an anatomically correct PVC skeleton this year (if we’re going to have a skeleton in our closet, it should at least be a good one).  I did not make a spiderweb on the porch railing this year, nor did I get out the UV LED strip—just too lazy.

corner-gourds

Here is a small pile of a few of the gourds.

frowning-pumpkin

This frowning pumpkin is the one I carved. I went for a minimalist look—cutting off one side of the pumpkin, drilling 2 1″ holes with a brace and bit, and 5 cuts to make the mouth.

frowning-pumpkin-2

Another view of the frowning pumpkin.

frowning-pumpkin-lit

The frowning pumpkin with the stroboscope inside. The strobe worked quite well—it helped that I had sloped the drill holes for the eyes somewhat.

lemon-pumpkin

My wife carved a thinner-walled pumpkin. It turned out to have a soft spot, so she had to make the eyes quite large—the lemon slices made quite effective irises.

lemon-pumpkin-lit

A candle in the lemon pumpkin lit the eyes well.

harry-potter-pumpkin

I liked this gourd best—I called it the Harry Potter pumpkin.

porch

The porch showed off the two carved pumpkins and the gourds.

porch-lit

Once it got dark, the porch light lit up the skeleton and the gourds, without detracting from the strobe and candles.

mini-strobe

The mini-strobe that I used is the same hardware as in the 2016 Halloween strobe, but with only 4 of the LED boards, instead of 20. I bent some solid copper wire to hold the boards, with the wires just held by the screw connector.

kyle-hanging

The skeleton, which my wife named Kyle, hung in our breakfast room window for most of the week before Halloween, but was moved to the dining room for Halloween itself. The male name is appropriate, as Kyle’s pelvic bone has a male conformation.

kyle-side-full

Here is Kyle from the side on his rolling stand (rather than hanging by a thread). You can see the black USB cable coming out of the foramen magnum just behind the atlas (C1) and axis (C2) vertebrae.

kyle-side

A closer view of Kyle’s skull, with a KitKat between his teeth and the hanging cord and USB cable both visible.

kyle-front

Kyle on his stand in the dining room window—not very clear during the day because of reflection, though his red eyes are somewhat visible.

kyle-brains

Inside Kyle’s head, showing his “brains”: a Teensy 3.1 board, a couple of 150Ω resistors, and a board with two LEDs behind the eye sockets. The board was program to alternate blinking the eyes every couple of seconds, alternating that with a random action (winking left, winking right, blinking, … ).

behind-eyes

The LED board wedged in firmly in the sella turcica between the anterior and posterior clinoid processes.

LED-board

I had to cut away a lot of board to get the LEDs to sit directly behind the superior orbital fissures. The LEDs themselves are LTL307E LEDs, which take about 20mA at 2V, and the 150Ω resistors limited the current to about 9mA, which is what the Teensy board can supply on each pin.

Now that we have this fine skeleton, we’ll probably keep it on display in the living room, but only light up the eyes for Halloween.

2018 July 19

Dropping old LED lighting project

Filed under: Uncategorized — gasstationwithoutpumps @ 14:28
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About 4 years ago, I started a project to replace the breakfast room ceiling fan (which my wife has always hated) with an LED fixture of my own design.  In order to do this, I designed some custom LED boards that could work over a range of voltages with PWM control of the brightness and I made some desk and table lamps using the boards, as well as a stroboscope (Summer project, LED board I-vs-V curve, Summer project 2, Desk lamp, Newer dimmer software, LED strobe using dimmer boardHalloween 2016, … ).

Although I have had the electronics done for the breakfast room fixture for about 3 years, I’ve not been able to come up with a mechanical design that I think will look good.  Also, the task of cutting the conduit in the attic to install an outlet for the wall-wart power supply did not appeal to me, so I kept putting off the project.

Last weekend, my wife, my son, and I went to the local Habitat for Humanity ReStore, ostensibly to look for cabinet handles and doors, but mainly just to browse. While there, we saw a used drum light ceiling fixture that was on sale for only about $16 (it is one that normally sells for around $120 new from Home Depot).  Since I had pretty much given up on coming up with a design using my LED boards that would look good, and my wife liked the drum fixture, we bought it and my son helped me install it.

The hardest part of the installation was probably pulling new wire through the conduit, since I did not want to use the old cloth-covered solid copper wire that was there.  I also added a grounding screw to the junction box for the lamp, since the old ceiling lamp had no ground connection.

Here is the “new” ceiling lamp in daylight.

Here is the ceiling lamp with 3 2700K LED bulbs.

Lining up the screw holes of the fixture with the screw holes of the junction box was also a bit tricky—my son’s holding the fixture while I fussed with that was a big help.

The whole project cost under $30: $16 for the fixture, <$3 for screws and wire, $7.50 for 3 LED bulbs. The new lamp is not dimmable (because of the cheap bulbs I chose), but that feature was not really needed for the breakfast room anyway.

So I now have a lot of spare LED boards that I need to find a project for—I’ve used a few of the boards for the desk lamp and stroboscope projects, but I don’t really need another desk lamp or stroboscope.  Anyone have any ideas?

2016 November 2

Halloween 2016

Filed under: Uncategorized — gasstationwithoutpumps @ 16:47
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Halloween was a bit of an impromptu arrangement this year. I was at the iGEM Jamboree all weekend, and my wife and I only got home from Boston a little before 7 p.m. on Halloween, so we did not have time to carve a pumpkin this year.  Instead, I decided to improvise a very fast pumpkin surrogate:

First, I found a cardboard box large enough to hold the LED stroboscope that I had made for the mini Maker Faire.

First, I found a cardboard box large enough to hold the LED stroboscope that I had made for the mini Maker Faire.

Then I cut out a stencil pattern from a piece of heavy paper.

Then I cut out a stencil pattern from a piece of heavy paper.

I made one mistake when cutting the stencil, cutting away the opaque center for the lower part of the B. I just used blue painter’s tape on the back of the paper to stick the piece back on, and recut the B.

Initially, I tried taping the stencil over the opening in the box, but the light was not diffuse enough—the individual LEDs were visible and the stencil pattern unclear. I then tried taping the stencil to the cutting mat and taping the cutting mat to the front of the box. It wasn’t very secure (painter’s tape is not very sticky—by design), but it worked for the evening.

The final result looked much better in the window than a 20-minute project had any right to.

The final result looked much better in the window than a 20-minute project had any right to.

2016 April 4

First week’s grading done

Filed under: Circuits course — gasstationwithoutpumps @ 22:44
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I spent all day Sunday grading the first set of lab reports.  I was expecting 24 reports of about 3 pages each, but I got 25 averaging about 5 pages each.  I think that the reports were a bit better this year than at corresponding times in previous years, but I did not get my grading done until almost midnight Sunday night, keeping me from getting much else done this weekend.

(I did manage to get my hair cut and to build a new strobe stand with room for 20 of my LED boards, which should give 1800 lumens during the flash. With a duty cycle of only 1/65, I don’t think that I need heat sinks on the boards for the strobe, as the average current should be only 40mA, though the peak current will be about 2.6A.)

In class on Monday, I gave students some group feedback on their writing, plus a couple of \LaTeX pointers, then took questions, some of which were about writing, but most were about the optimization of the fixed resistor in the voltage divider for the resistance-to-voltage converter in the thermistor lab.  I showed them how to set that up, but did not try to solve it in class.

After class, when I was making up the key (redoing all the problems—I don’t like just looking up results—refreshing my memory on how to solve the problems by resolving them is best), I ran into a little trouble doing the optimization. I used to be able to just ask Wolfram Alpha to solve the differential equation, but their newer parser seems to be much harder to convince to do anything.  I eventually gave up and used a cruder tool to just take the second derivative and solved for the resistance by hand.  That was faster than the time I wasted trying to get Wolfram Alpha to do anything useful.  (I suspect that they have deliberately crippled it, to make people pay for Mathematica.)

Monday afternoon and evening (from about 1:30 to 7:45) was spent grading the first pre-lab homework.  Again the results are a little better than previous years, but there were 9 prelabs fewer than I expected (3 students have dropped already and 6 did not do the prelab).  I hope that those who did not do the prelab were just confused about when it was due, and not starting a trend towards coming to class and lab unprepared. I also hope that no more students drop—this class is not a weed-out class, though it is a lot of work.

Back in January, Mike wanted to know where I ended up doing my grading. Sunday I did my grading in my breakfast room, with the laptop on the floor where I could get to it if I really needed to look something up, but where it was not a constant temptation to goof off.  On Monday, I worked in my office on campus, where the e-mail was a minor distraction that I checked between problems.  (For the prelabs, I graded the entire stack for problem 1, then the entire stack for problem 2, and so forth.  This makes for more consistent and faster grading than grading a student at a time, but it would be faster still if the students didn’t put their answers in random order on what they turned in.) I’ll probably continue with weekend grading in the breakfast room and prelab grading in my office until the distractions get to be too much—then I’ll look for a coffeeshop to grade in.

2016 March 30

Class topic not what was planned

Filed under: Circuits course — gasstationwithoutpumps @ 21:27
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In my Applied Electronics for Bioengineers course, I had planned to spend the lecture time today talking about sampling and aliasing, but that is not what ended up happening.

I am making it a point to answer student questions (unless they are irrelevant) first, before doing whatever I prepared. The point of the lectures is to help students understand the reading and do the design work for the labs, and anything I have prepared is just a best guess at what the students need. Their questions address more directly what they perceive as their need.  Most of the prepared lecture material is in the book (I wrote the book based on what I have covered in lectures), so answering questions from students who have read the book and are still confused is going to be better than my repeating what is in the book.

Today students had some logistic questions about what to write up for Lab 1 (not much, it was just soldering headers onto the Teensy boards and setting up PteroDAQ—I just asked for a description of what they did, whether anything went wrong, and what they did to fix the problem) and about prelab homework for Lab 2 (do it, but don’t turn it in, it is just setting up gnuplot so that they can use it for the lab).  Those only took a couple of minutes.

The big question that diverted the entire flow of the lecture was a request for an explanation of the high-pass filter in Lab 2 that is used for recentering the function generator output at 1.65V. This lead to several things:

  • Description of block diagrams as functional blocks connected by interfaces, and why this was an important concept in engineering. Frequency and voltage information was put on the block diagram  connections.
  • Capacitor symbol and DC-blocking property of capacitors.
  • Resistor to Vref and why that would cause the output to become Vref, if there was no current through the output.
  • Back to the block diagram to add the constraint that the analog-to-digital converter on the Teensy board couldn’t take any current from its input.
  • Definition of “gain” as \frac{dV_{out}}{dV_{in}}.
  • Showing the high-pass filter Bode plot as two lines meeting at the corner frequency, and giving the corner frequency as \frac{1}{2\pi R C}, without derivation.  I promised the students that we would derive that result in a few weeks, once we’ve had complex impedance.
  • Replacing the resistor to Vref with a pair of resistors to 3.3V and Gnd.
  • Introduction of the triangular ground symbol, and rejection of the chassis ground and earth ground symbols as not relevant for the class.
  • Derivation of the voltage-divider formula from Ohm’s Law, using the important constraint that no current is taken from the output node of the voltage divider, so that the two resistors have identical currents. I had the students help with this, in order to elicit the most common mistake
  • Assertion, without derivation or explanation, that the RC time constant for the high-pass filter should treat the two resistors as being “2R” rather than “R”.

For the last couple of minutes of class, I finally got to do the demo with the homemade stroboscope and pendulum of aliasing, but it was not very effective. Even with the lights off in the classroom, there was enough light through the windows to wash out the strobe. I could not easily keep the pendulum swinging with one hand and adjust the strobe with the other.  If I do this again next year, I should make a panel with about 20 of the LED boards, for around 2.35A during the flash.  At 1.64ms for the longest flash, that’s 3.85mC, which would drain 8.2V from the 470µF capacitor, if the power supply weren’t capable of delivering that much current (but I have a 6A 9V supply, so there should be no problem delivering full power).  Hmm, maybe I should make up that panel for the Mini Maker Faire, instead of the wimpy 4-LED strobe I now have.

I’m actually pleased that I didn’t give the lecture I had planned—my book, which was based on my lectures, already covers the material adequately, and I’d much rather spend precious class time explaining the things that aren’t clear in the book.  The only way I can know what the students need to hear is for them to ask for clarification where they are confused.

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