Gas station without pumps

2018 June 27

Three boxes this morning!

Filed under: Uncategorized — gasstationwithoutpumps @ 21:30
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I got three boxes this morning: two via FedEx and one via an unmarked van.

One of the FedEd boxes was from Digilent—I ordered the new Impedance Analyzer attachment for the Analog Discovery 2.  I plan to check it out later this week with some precision resistors.  The design looks a little strange to me (using latching relays with 100mΩ contact resistance rather than lower impedance FETs), and there is no mention of using precision resistors for the reference impedances, so I suspect that it is not quite as good as it should be.  I also ordered the Breadboard Breakout for the Analog Discovery 2, as the female headers on the flywires that it comes with are getting a little loose.  I’ll probably test that out in the next week or two also.

Another FedEx box was from eBay for a new toaster oven (the Breville BOV845SS).  Our old one still works, but the buttons on it are getting a bit unreliable, and I was unable to take the box apart to clean the contacts (one of the screws I needed to remove had its head stripped without my being able to get it out).  The new one does a better job of producing uniform toast, so I like it better already. (I had toast for lunch today, so that I could test the toaster oven.)

The third box was the most fun: a Monoprice Delta Mini 3D printer. I spent a chunk of the day trying to get it to work. The gcode file included on the micro SD card printed ok for about 20 minutes, then retracted the filament a long way and continued “printing” without laying down any more plastic. I tried it twice and it failed at about the same point each time, so I suspect a corrupted gcode file, but I don’t currently have any way to read the micro SD card to see. (I have an SD reader, but it is old enough that it doesn’t include a micro SD slot.)

I then downloaded Cura and Printrun-Mac-18Nov2017 to try driving the printer directly from my Macintosh. I downloaded the Make Magazine test files from Thingiverse and Cura profiles from https://www.mpminidelta.com/slicers/cura.  The profile files on that wiki only work with Cura 3.2 and 3.2.1, so I had to download version 3.2.1 of Cura also.

I had no problem getting Cura to load and slice the stl files, but I could not get my Mac to talk to the printer (it saw the USB device  as

Malyan 3D Printer:

Product ID: 0x0300
Vendor ID: 0x2e26
Version: 2.00
Speed: Up to 12 Mb/sec
Manufacturer: Malyan System

but did not create a serial port for it).

Several online sources also concluded that Mac OS X cannot talk to the printer via USB. This is generally believed to be a firmware bug in the printer.  The printer is running version 44.160.3 of the firmware, which seems to be a very recent version, so Malyan System has not fixed the USB bug yet.

I have one very low-speed HP laptop that runs Windows (which we refer to as the “Barbie” laptop, because of its bright color and toy-like capabilities), which was originally purchased (used @ $75) for testing PteroDAQ on Windows.

The Barbie laptop had no trouble talking with the printer using Printrun, and I tried printing the Make magazine 4_DimmensionalAccuracy.stl file (note: the double “m” in “dimension” is Make’s spelling error, not mine).  Cura estimated a 45-minute print time, but Printrun estimated 67 minutes, which was fairly accurate—there must be some speed setting in Cura that is wrong about what full speed is for the printer.

Make’s preview of 4_DimmensionalAccuracy (from their Thingiverse folder).

The MDM_4_DimmensionalAccuracy.gcode file printed ok, but I had trouble getting it off the build plate. A wrench to twist it off worked best (after putting a small gouge in the plate trying a technique with a screwdriver and mallet). The nominal 20mm dimension of the object turned out to be 19.35mm, which is rather smaller than desirable. The layers were 24.35 by 24.30mm, 19.35×19.35mm, 14.50×14.55mm, 9.70×9.75mm, which is fairly consistently 3% smaller than they are supposed to be.

Make’s 2_XY-test preview file, from their Thingiverse folder.

My second test was with Make’s 2_XY-test.stl file, which I sliced with 0.2mm layers, 10% infill, and a Cura-generated raft (“build plate adhesion” checkbox). The raft did seem to make popping the print off the build plate easy (though not having much area in contact with the plate probably helped also).  Removing the raft did delaminate a little of the bottom layer in one place.  The texture of the vertical walls changes rather abruptly each time there is a hole in one wall, probably due to a change in the way the head moves when a continuous circuit is possible and when it has to either reverse or skip the hole. (Sorry, no photos—it is now too dark out for natural-light photos, and I’ve never had much luck with flash on macrophotography.)

I’m now convinced that I can get the printer to work, so I need to pick a tool for building models in STL format and pick some project(s) to work on.  My son thinks that I should use OpenSCAD, which is a “programmer’s CAD tool”, providing easy ways to create shapes using programming language and view the results, but not edit them interactively.  Given how very frustrating I found the SolidWorks GUI last fall, I think he may be right—I’ll look into OpenSCAD.

One of the first things I’ll print, though, is a design by someone else—extension legs to raise the printer and improve the air flow underneath.  Lengthening the legs by just 1cm will greatly reduce the noise the printer makes.

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2018 June 22

Repairs: kitchen sink and lawnmower

Filed under: Uncategorized — gasstationwithoutpumps @ 15:48
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Now that my grading is done for the year and all my grades are filed, I finally have time to take care of some chores around the house.

One of the first chores was to fix a slow leak under the kitchen sink.  It has been there for a year, and I believed that the leak was coming from the adapter between the faucet (which had ⅛” female pipe thread) and the sprayer hose (which had ¼” female pipe thread).

The first thing I did was to try to shut off the water to the faucet (not that I really needed to, since the faucet valves were still working). The quarter-turn shutoff valve seemed a little stubborn, and when I pulled hard on the lever, the whole pipe broke, spraying water all over the kitchen.  I ran out to the whole-house shutoff and managed to shut the water down with only about 2 gallons (8 liters) of water to mop up.

The pipe snapped right next to the body of the shutoff valve.

Some idiot (most likely me) had attached the brass shutoff valve directly to the steel pipe, with no intervening galvanic break, so there was a lot of corrosion due to galvanic currents.

Inside the valve and the pipe the corrosion was rather extreme.

So I went down to the hardware store and got a new shutoff valve, a CPVC nipple to replace the steel one (thus getting the necessary galvanic break), and a replacement for the adapter.  The hardware store did not have ⅛” MPT to ¼” MPT, so I ended up getting ⅛” MPT to ¼” FPT and  ¼” MPT to  ¼” MPT.

I put in the new shutoff valve and reassembled the faucet-to-sprayer connection.  The new shutoff worked fine, but the sprayer hose connection leaked worse than before.  It was now clear, however, that the leak was coming from the ¼” MPT-to-hose connection, and not earlier in the system.

I went to the hardware store again to get a new washer for the hose.  I was sold a 00 faucet washer, though I was bit dubious that it would work.  Sure enough, when I assembled hose connection it just squeezed the washer into the pipe, and the connection leaked as badly as before.

So I went back to the hardware store again and bought a whole new sprayer with hose.  I would have replaced just the hose, but the sprayer I had did not have a detachable hose—or rather, the hose was detachable, but neither end of it would pass through the sprayer hose guide, so I needed to replace the hose and the hose guide, at which point it was cheaper to replace the whole thing.

I replaced the hose guide and the sprayer, tightened up all the connections that I had just made, and the leak seems to have stopped.  One chore down!

My next chore was to fix the lawnmower again (see Electric lawnmower repair and Electric lawnmower repaired again).  There were two problems this time: the extension cord was not making good contact with the plug for the mower and the lawnmower blade was very dull.

Sharpening the lawnmower blade was pretty easy: I took the blade off with a crescent wrench, and brought it inside to grind on my wet wheel.  I could not get the curved parts of the blade that way, so I clamped the blade in a vise and used a half-round file to do those parts of the blade.  The mower blade is a fairly soft steel, to keep from chipping or shattering when it hits stones or other hard objects, so it sharpens quickly but doesn’t take a very sharp edge. I did manage to make it sharper than the rather rounded, dented edge it had before.

I determined that the problem with connection to the extension cord was with the cord, not the lawnmower, by the simple expedient of trying a different (shorter) extension cord, so I went to the hardware store (again!) to get a 15A replacement socket for the end of the extension cord.  I cut off the old socket, stripped the wires, and attached the new socket.  After verifying that I had connected everything correctly (using a standard 3-neon bulb socket tester), I checked out that the lawnmower worked with the fixed cord—it seems to be fine.

Another two chores done!

Tomorrow, when electricity is cheaper, I’ll try mowing the front lawn, which has gotten a little shaggy.  The back lawn is probably not mowable (the grass is over 3 feet high), and will need chopping down with a weed whacker before I can mow.

In between the faucet repair and the mower repair, I tried replacing the wheels on my son’s rolling luggage.  The wheels appear to be 76mm diameter wheels with standard skateboard bearings.  I went over to Skateworks on Soquel Ave, but they said that the wheels were too narrow for skateboard wheels, and recommended trying the wheels for inline skates (which they do not sell).  Rather than wander all over town looking for rollerblade wheels, I ordered a pair of cheap ones from Amazon (hard ones for outdoor use—durometer 89A), which should arrive on Monday.  With any luck, I’ll be able to cross another chore off my list then.

2018 June 17

Taking a break from jogging

Filed under: Uncategorized — gasstationwithoutpumps @ 14:42
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As I mentioned in Starting jogging and Twenty-ninth weight progress report, I started jogging again after about a 40-year hiatus. Initially, things were going pretty well, and I was increasing the distance gradually, with only minor aches as I stretched out muscles that had not been used much lately.

As expected, my speed dropped as my distance increased, until I was running at a pace of about 9:30 per mile.

Astute readers may note that the running log ends on June 13, but today is June 17, and they may wonder why the long break.

The answer is simple—pain near the left knee. The minor aches in the muscles subsided in the first week, but a tender spot developed on my tibia, just below the left knee on the inside, and started getting worse rather than better. For a couple of days I also had exercise-induced edema in left leg (my foot swelled up and my weight went up by 4 pounds in two days), but that resolved itself in a day or two. I’ve done a little web research, and I think that what I’ve got is pes aserine bursitis or pes aserine tendinitis, based on where the tender spots are.

The standard treatment for any of the knee injuries I might have is rest, ice, elevation, and NSAIDs. So I’ve been resting the leg, grading on the couch with my foot propped up, and taking ibuprofen. I am not willing to ice the knee, though, as there is no swelling near the tender spot (and hasn’t been) and I have always found icing tendinitis to be acutely painful with no residual benefits for me. (Many years ago I had tendinitis or ulnar tunnel syndrome from a bad typing position, which took a long time to heal.)

This week I’ve found walking a bit uncomfortable, but not acutely painful, but bicycling on the recumbent bike seems to reduce the discomfort (probably from improved circulation without stressing the tendon).

I’m going to wait until the tenderness is gone before running again, which may take a week or a month. I’ll also start over with short distances, better shoes, and a softer running surface. This means it is unlikely that I’ll be up to Bike Santa Cruz County’s  12km run on 26 August 2018, but it is more important to me that I don’t make the minor injury worse.

2018 June 13

Romeo and Juliet

This year seems to be the time for Romeo and Juliet.  UCSC Shakes-to-go did it as their touring production to the local schools—I did not get to see that production this year, but my wife did.

UCSB’s Shakespeare in the Park class (THTR 194A) did it as their production, which I saw twice.  My son was in it, playing the role of Tybalt.  I took still photos of the Saturday production and video of the Sunday production.  I’ve not had time to select, crop, color-correct, and reduce resolution on the still photos yet, but my son processed the video and we put it up on Youtube:

There was a camera glitch at the end of the party scene, and I did not record a minute or two of the play while rebooting the camera.

Futuristic Lights provided the gloving lights for the party scene (donated to the cast) and loaned lights used for indicating the life of the actors (a rather futuristic way to handle the fight scenes that may not have been clear to the audience).  Many of the roles are cast cross-gender (a necessity with only 3 female roles and many actresses), including the part of Lady Capulet, played by a male actor.

Later this summer we’ll be seeing Santa Cruz Shakespeare doing Romeo and Juliet with professional actors, which will probably be the best production.

2018 June 3

VCO (voltage-controlled oscillator)

Filed under: Circuits course — gasstationwithoutpumps @ 11:01
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My students requested that I talk about voltage-controlled oscillators (VCO) and low-frequency oscillators (LFO) for audio work in my applied electronics course.  (We’re in the last week, and they have everything they need to know for the final EKG lab.)

I spent some time Friday night and this morning designing, building, and testing a couple of VCO circuits—one for which frequency is linear with voltage and one for which frequency is exponential with voltage.  Both can easily be turned into low-frequency oscillators by increasing the size of one capacitor.

The oscillators have two outputs: a square wave and a triangle wave. I chose a triangle-wave oscillator, because the design is simpler than for sine waves, and the students have all the concepts they need to understand the design.

The oscillator consists of two parts: an integrator to convert a constant current input into a constant slope: dV/dt = I/C, and a Schmitt trigger to change the current from positive to negative.  Rather than using a Schmitt-trigger inverter chip, I made the Schmitt trigger from an op amp (a comparator would give crisper transitions, but that is not important at the low speeds we’re dealing with).

This design has the frequency linear with the voltage.

The op amp in the upper-left corner is a unity-gain buffer to isolate the input from the rest of the circuit.  It isn’t really needed in this design, and it limits the input voltage range to the power-supply voltage (I used 3.3V).

The next op amp is the integrator, which turns an input current into a constant slope on the output voltage.  The current through the capacitor is 10nF dV/dt.

When the FET is turned off, the current flows through R6+R7, and I= (Vin/2 – Vin)/ (R6+R7), so dV/dt = – Vin/(2* 10nF * 6.6kΩ) = -Vin / 132µs.

When the FET is turned on, the current is the difference between the current through R8 and through R6+R7, and so is (Vin/2)/R8+(Vin/2 – Vin)/ (R6+R7), which simplifies to Vin/(2*6.6kΩ), and dV/dt = Vin/132µs.

The lower-right op amp is an inverting Schmitt trigger with thresholds at 1/3 and 2/3 of Vdd.  It turns on the nFET when the voltage of the triangle wave has dropped below the lower threshold and turns it off again when the voltage has risen above the upper threshold.

The upper-right op amp is just a unity-gain buffer to isolate the output from the oscillator.

My first attempt at this design used larger resistors for R6=R7=R8 and a smaller capacitor, but it had problems when the nFET turned off—the voltage continued to rise for a little while.  The problem was that the drain of the FET had to be charged through R6+R7 before the current through the capacitor was reversed, and this took too long.  Shrinking the resistors made the capacitance on the drain of the FET much less important.

This design has the frequency exponential with voltage (about one octave per 70mV).

The oscillator design is the same as for the linear one, but the diodes provide an exponential current from the input voltage. The input unity-gain buffer is now important, as it provides current limiting to prevent damage to the diodes.  To get double the current through the FET when it is turned on, the diodes are put in parallel on the lower leg (a series arrangement on the upper leg like for the resistors would not halve the current).

I tested both circuits and they seem to work ok, with duty cycles close to 50% for the square wave.  There are slight high-frequency glitches at the peaks of the triangle wave, so it may be worth replacing the output unity-gain buffer with a 40kHz low pass filter.

This is a fairly high frequency from the diode-controlled oscillator (15.431kHz) and the duty cycle is a bit off (59.36%), because the current has gotten large enough that the on-resistance of the nFET matters.

At 500mV, the frequency is 25.77Hz, the duty cycle is about 48%, and the glitches are not visible at this time scale.

I measured and plotted the frequency as a function of voltage for VCO with diodes:

The fit is done here omitting the lowest and highest points. I believe that the highest point has hit the current limits of the input unity-gain buffer, and so is not on the straight line.

The frequency scaling of the diode-based VCO could be changed by replacing the input unity-gain buffer with an amplifier with a different gain.

(2018 June 23: I feel compelled to point out that this circuit does not have any thermal compensation, and the exponential conversion of voltage to current in the diode junctions is highly temperature sensitive.  I’ll try this summer to redesign the circuit to have temperature compensation, though the use of 3 diodes here makes that difficult—a sawtooth oscillator, with only diode would be somewhat easier.)

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