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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.

Twenty-ninth weight progress report

Filed under: Uncategorized — gasstationwithoutpumps @ 08:06
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This post is yet another weight progress report, continuing the previous one, part of a long series since I started in January 2015.

This past year has seen repeated attempts to recover from the weight gain of the previous two years.

I’m right on the upper edge of my self-imposed weight range. If I stop paying attention to my weight or start snacking to cope with the stress of grading, my weight shoots up rather quickly.

As I mentioned in Starting jogging, I started running over Memorial Day weekend. I’ve kept it up for 9 days, with only minor aches from stretching out muscles and tendons that don’t get used on my bicycle.  During March, April, and May I also averaged 4.66 miles a day of bicycling (mainly my bike commute to work).  The bicycling helps loosen up muscles that tighten from the running.

I started jogging over Memorial Day weekend, and have gradually been increasing my distance. (For those who prefer Imperial units, 12 km/h is an 8-minute, 3-second mile pace, and 2km is 1 mile 1282 feet.)

I plan to continue running throughout the summer, with distances gradually increasing. Once I get up to about 3km, I’ll start alternating short and long days, to give my muscles some time to heal between the longer runs.  When I get up to 10km, I’ll probably switch to a long-rest-short cycle, to allow more healing time.

I’m in better shape now than I was in summer 1970 when I couldn’t run a mile, but not in as good shape as summer 1971 when I could run a 6-minute mile (16 km/h) and could run up to 15km (though only at 12 km/h).  Although I’ve been keeping to around 12 km/h so far, I expect to slow down as the distances increase.  I’d like to keep above 10km/hr (a 9:40 mile pace), though, or the running will take up too much time.

My current goal is 12km runs by  Bike Santa Cruz County’s  12km run on 26 August 2018 and 15km runs by the end of October.  If I don’t injure myself, I’d like to get to the point where I run a marathon before the end of 2021.

So far, I’ve been running in my Teva sandals, which have more padding in the soles than any of my other shoes, and which fit comfortably.  I’ll be buying some running shoes in a couple of weeks, once I have some spare time.  I’ve been recommended Altra and Brooks Cascadia shoes, and both lines are carried at Santa Cruz Running Company (along with half a dozen others brands).  The method they describe for helping customers pick shoes sounds reasonable, and better than just picking a low-cost shoe randomly from Amazon (which is pretty much how I choose my bicycling shoes every 4 or 5 years).


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