# Gas station without pumps

## 2021 June 27

### Volunteer potatoes

Filed under: Uncategorized — gasstationwithoutpumps @ 13:59
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This Spring, when I was turning my compost from the compost bin, I came across a few potatoes that had sprouted, so I planted them in the garden behind the compost bin and pretty much ignored them (maybe remembering to water them once every couple of weeks, which is not really enough for potatoes in the California drought).  When the plants died, I dug them up, generally getting one or two potatoes per plant.  Today I dug up the biggest of the plants and found that I had gotten a pretty good yield from it:

The nineteen potatoes here are the ones that were big enough to be worth cooking—there were another dozen or so tiny ones that I’ll probably replant.

For volunteers from the compost bin, these potatoes look surprisingly good. I think I’ll try taking a number of the potatoes that are too small to cook and try starting more plants from them in a planter that is just deep enough. I should then have another crop around September.  I might even make a point of planting a few of the tiny potatoes at the end of each month so that I’ll have a continuous crop (at least until we get a hard frost, which probably won’t be until December).

## 2021 June 25

### Retirement present from my students

Filed under: Uncategorized — gasstationwithoutpumps @ 22:56
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Several of the students in my applied analog electronics course got together to get me a retirement present, which was very sweet of them.  They gave me two “foodie” books: The Bread Baker’s Apprentice by Peter Reinhart and Renaissance Recipes by Gillian Riley.  A dozen of the students wrote notes on the end papers of The Bread Baker’s Apprentice. It was a very sweet and thoughtful present. Somewhat surprisingly, my wife and I did not own either of these two books in our collection of about 140 cookbooks (which include over a dozen renaissance cookbooks and at least nine books on baking bread).

The students also gave me an apron with silk screening:

The Man   The Myth

THE

LEGEND

HAS RETIRED

I suppose I should pick one of the recipes (oops—formulæ) from The Bread Baker’s Apprentice for the next bread-and-tea event.

### Secret Walks

Filed under: Uncategorized — gasstationwithoutpumps @ 22:24
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I recently bought Secret walks & Staircases in Santa Cruz, by Debbie Bulger and Richard Stover.  It is a book idea that I had about a decade ago, that I had idly thought of as a possible retirement project, so I was both delighted and disappointed that Debbie and Richard beat me to it.  They did a better job of it than I would have, so it is probably better for Santa Cruz that they did it first.

My wife and I have decided to do all 28 walks at a rate of about one a week, starting with some of the closer and shorter ones, as we are both somewhat out of shape, and we’ll have to walk to the start of each walk and walk home from them. I think that it will take us more than 28 weeks for this project, though, as some weeks we’ll want to do other walks (like the trek out to Audrey Stanley Grove for Santa Cruz Shakespeare).

Today we did one of the shortest walks, the 1.5-mile Beach Hill loop, with the addition of a 1.4-mile walk to get there (via Laurel and Washington) and a 2-mile walk back (via Chestnut, Jenne, the upper half of the Neary Lagoon path, and Bay).  Some of the other short walks will not be short for us—for example, the 1.1-mile Schwann Lake loops would begin and end with 3.7-mile walks, making an 8.5-mile excursion for us.  For us right now, a 5-mile walk is about right.

I don’t think that any of the walks will be over 10 miles, though, even the longer ones that start on the Eastside, so if we get back into shape, losing the 15 pounds gained during the pandemic (speaking for myself—I have no idea what my wife weighs nor whether she put on weight in the past 15 months), we should be able to do them all.  We’ll probably start with walks closest to home, so that we get the highest ratio of “guided tour”  to walking to and from the start points.  We may also rotate some of loops, so that we join them at the closest point to our house, rather than at the designated start.

I did not think to bring my camera with me on today’s walk (not even my phone), so I took no pictures, but there were a few photo-worthy spots. In fact, the cover of their book is the stairs between Cliff St. and Laurel St Extension (though not identified in the book as such).  I thought that the stairs looked like the same construction and hand rails, but the surroundings in the photo were somewhat different. So I asked Debbie about it by email and she confirmed that they were the same steps, but that the photo on the cover was 14 years old.

On future walks, I’ll bring my camera, so that I can illustrate blog posts.

Update 2021 June 28:  On Sunday I had to go drop something off near Neary Lagoon, so I took my camera along to take a few photos of some of the highlights of the Beach Hill walk:

For pedestrians, the ceramic decorations around the roundabout at Center, Front, and Pacific are a treat that are missed by the car drivers baking in their grid-locked cars. Here are some sea stars.

Some sea jellies.

A squid.

Some sea anemones.

The very high door at the back of this house on Third St (viewed here from across Front Street) is a famous landmark for cars stuck in beach traffic.

## 2021 June 21

### Controlling current

Filed under: Circuits course — gasstationwithoutpumps @ 11:06
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In the electrode lab this year, students had even more trouble than usual in understanding that the the goal was to provide a constant current to the silver-wire electrodes for a measured time period, in order to produce a known amount of AgCl on the anode.  I will have to rewrite that section of the book for greater clarity.  I also plan to add a circuit that does the constant-current control for them, so that they don’t have to adjust the voltage to get the desired current (a concept that seems to have eluded many of them).

Here is a possible circuit:

This circuit provides a current from Ip to Im of Vri/Rsense, as long as the voltage and current limitations of the op amp are not exceeded.

The negative-feedback loop tries to bring the $I_m$ output to the voltage of the $V_{\rm ri}$ input, which is only possible if the current through the sense resistor is $I=V_{\rm ri}/R_{\rm sense}$.  Let’s say that we want 1mA from Ip to Im—then we would set $V_{\rm ri}= (1\,mA) R_{\rm sense} = (1\,mA)(100\Omega) = 100\,mV$.  If $V_{\rm rail}$ is 5V and the op amp is a rail-to-rail op amp, then we could get the desired 1ma of output as long as the load resistance from Ip to Im is less than 4900Ω (well, 4650Ω really, because of the internal resistance of the op amp).  With a higher load resistance, the voltage at Ip would hit the top rail and still not provide the desired current.  There is no lower limit to the load resistance—even with a short circuit the current would be the desired 1mA.

I chose 100Ω for the sense resistor, so that the control voltages do not get too close to the bottom rail, while leaving enough voltage range for fairly large load resistances.  By using 100Ω, it is possible to specify currents up to 50mA, which is beyond the capability of the op amp to supply.  Since the MCP6004 op amps have a short-circuit current of about 20mA with a 5V supply, about the most we can deliver is 14mA for a short-circuit load, because of the internal resistance of the op amp.

Using a 1kΩ resistor might also be reasonable, since the input voltage in volts would then specify the current in mA, but a 1mA output current would limit the voltage across the output ports to $V_{\rm rail} -1\,V$ (which is probably still fine for the electrode lab). With a 1kΩ resistor and a 5V supply, the maximum specifiable current would be 5mA, and the maximum obtainable is about 4mA.  If you needed 2V across the load, then you could not specify more than 2.4mA (still plenty for the electrode lab).

For the electrode lab, the currents required are low enough that this circuit is adequate, but what if we needed more current?  Here are a couple of circuits that can provide that:

By using a pFET, we can have the voltage output of the op amp control the current. No current is needed from the op amp, and we just need that Vrail is large enough that the pFET can be fully turned on.

If we use a PNP transistor, then we need to turn the voltage output of the op amp into a current for the base.  That current is about 1/50th or 1/100th of the collector current being controlled (depending on the transistor).

Both these designs have the positive and negative inputs of the op amp reversed from the low-current design, because the pFET or PNP transistor provides a negation—the voltage at Im rises as the voltage at the output of the op amp falls.  I reduced to the sense resistor to 10Ω, to allow specifying higher currents (up to 500mA for a 5V supply).  The main limitations on this design are the thermal limitations of the transistor and the resistor—there may be both a large voltage drop and a large current.  The worst case for the transistor is when the load is a short circuit and the voltage at Im is half the power-supply voltage—then the power dissipated in the transistor (and in the sense resistor) is $(V_{\rm rail}/2)^2/10\Omega$.  For a 400mW limitation on the transistor, we would want to limit $V_{\rm rail}$ to 4V.  For a ¼W resistor, we would want to limit $V_{\rm ri}$ to 1.58V (specifying 158mA), or up the resistor to 100Ω for a 5V limit (but then we could only specify up to 50mA).  We really need a 2.5W resistor if we want to have 10Ω and a 5V supply and use the full range.

For the book, I think I’ll just present the low-current version of the current control—we don’t need the high-current version, and students are likely to request too much current for the electroplating if they have it available (errors in computing the area of the electrodes that are off by a factor of 100 are pretty common—mixing up $({\rm mm})^2$ and $({\rm cm})^2$, for example).

## 2021 June 19

### Electric lawnmower finally failed totally

Filed under: Uncategorized — gasstationwithoutpumps @ 14:30
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In Electric lawnmower repaired again and again, I reported on the latest fix to the electric lawnmower,  which was the replacement of the on-off switch in the handle.  It turns out that the fix was not worth the money and time, as the lawnmower has now failed in a major way—the ceramic permanent magnet in the motor has crumbled and is jamming the motor.

I think that the problem may have been caused by overheating, as the fan that is supposed to circulate air through the motor had only 2 of its original 8 blades, and many of the vent holes in the deck were plugged with solidly packed  grass residue.

In any case, I am now declaring this lawnmower to be trash (though I may salvage the switch and the bridge rectifier).  I’ll have to start looking at reviews of electric lawn mowers, to figure out what one to buy.