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Archive for category Science

Crispy Skin Slow Roasted Pork Shoulder: Whoops

The previous times we slow-roasted a pork shoulder, the smoke alarm went off well before the skin crisped. We’d drained the drippings from the pan before crisping the skin, but the residue still smoked up a storm; this time we we left the pool in place to see if it kept the surface cooler and reduced the smoke.

Well, no, it didn’t. This happened in the five minutes between one rotation and the next:

Roast Pork Shoulder - Smoked Kitchen

Roast Pork Shoulder – Smoked Kitchen

Knowing things would get at least a little smoky, I’d closed the pocket door (on the left) and hung a beach towel across the opening into the laundry room (to the right), which kept most of the smoke out of the rest of the house. The smoke detector in the laundry room didn’t go off until I walked through the towel, so my precautions worked pretty well.

Wow, was that skin crispy:

Roast Pork Shoulder - Crispy Skin

Roast Pork Shoulder – Crispy Skin

Plenty of smoke and no fire; the roasting pan has narrow slits for that very reason. Took a couple of hours to vent the house, during which the yard smelled downright yummy.

Next time, we’ll plunk the roast on a lined cookie sheet (with a rim!) and see what happens.

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WWVB Reception: 60 kHz Tuning Fork Resonator Filter

Some early morning data from the WWVB preamp with the 60 kHz tuning fork resonator filter in full effect (clicky for more dots):

WWVB - xtal filter - waterfall 5 fps RBW 109.9 Hz Res 0.02 s - gqrx window - 20171116_103542

WWVB – xtal filter – waterfall 5 fps RBW 109.9 Hz Res 0.02 s – gqrx window – 20171116_103542

The dotted line comes from WWVB’s 1 Hz PWM (-ish) modulation: yeah, it works!

The filter cuts out the extraneous RF around the WWVB signal, as compared with a previous waterfall and some truly ugly hash:

WWVB - 24 hr reception AGC - 2017-01-16 to 17 - cropped

WWVB – 24 hr reception AGC – 2017-01-16 to 17 – cropped

Well, not quite all the hash. Enabling the SDR’s hardware AGC and zooming out a bit reveals some strong birdies:

WWVB - xtal filter - waterfall - hardware AGC - 2017-11-16 0612 EST

WWVB – xtal filter – waterfall – hardware AGC – 2017-11-16 0612 EST

The big spike over on the left at 125.000 MHz comes from the Ham-It-Up local oscillator. A series of harmonics starting suspiciously close to 125.032768 kHz produces the one at 125.066 MHz, just to the right of the WWVB signal, which leads me to suspect a rogue RTC in the attic.

There is, in fact, a free running “Test Signal Source” on the Ham-It-Up board:

Ham-It-Up Test Signal source - schematic

Ham-It-Up Test Signal source – schematic

Although I have nary a clue about that bad boy’s frequency, measuring it and cutting the inverter’s power trace / grounding the cap may be in order.

The SDR’s AGC contributes about 30 dB of gain, compresses the hottest signals at -25 dB, and raises those harmonics out of the grass, so it’s not an unalloyed benefit. Manually cranking on 10 dB seems better:

WWVB - xtal filter - waterfall - 10 dB hardware preamp - 2017-11-16 0630 EST

WWVB – xtal filter – waterfall – 10 dB hardware preamp – 2017-11-16 0630 EST

The bump in the middle shows the WWVB preamp’s 2 kHz bandwidth around the 60 kHz filter output, so the receiver isn’t horribly compressed. The carrier rises 30 dB over that lump, in reasonable agreement with the manual measurements over a much narrower bandwidth:

60 kHz Preamp - Bandwidth - 1 Hz steps

60 kHz Preamp – Bandwidth – 1 Hz steps

With all that in mind, a bit of careful tweaking produces a nice picture:

WWVB - xtal filter - waterfall - 10 dB hardware preamp - 2017-11-16 0713 EST

WWVB – xtal filter – waterfall – 10 dB hardware preamp – 2017-11-16 0713 EST

I love it when a plan comes together …

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Monthly Science: 60 kHz Preamp Resonator Bandwidth

Putting a small capacitor in series with the tuning fork resonator pulls the series resonant frequency upward and reduces the amplitude:

60 kHz Quartz TF Resonator - CX variations

60 kHz Quartz TF Resonator – CX variations

So something around 10 pF, net of stray capacitance and suchlike, should suffice. Plunk a small twiddlecap on the preamp board and tune for best picture:

LF Crystal Tester - resonator protection

LF Crystal Tester – resonator protection

Using the DDS generator as a manual signal source with 1.0 Hz step size shows the resonator tightens up the preamp’s response quite nicely:

60 kHz Preamp - Bandwidth - 1 Hz steps

60 kHz Preamp – Bandwidth – 1 Hz steps

I’m not convinced the preamp will have filter skirts that low farther away from the peak, but it’ll do for a start.

Zoom in on the peak with 0.1 Hz steps:

60 kHz Preamp - Bandwidth - 100 mHz steps

60 kHz Preamp – Bandwidth – 100 mHz steps

The bandwidth looks like 0.6 Hz, centered just slightly above 60.000 kHz, which should be fine for a first pass.

I’m tickled: all the hardware & firmware fell neatly into place to make those graphs possible!

Next step: install it in the attic and see whether the filter cuts back the RF clutter enough to stabilize the SDR’s AGC gain.

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Two-Wheel Plotter

While contemplating all the hocus-pocus and precision alignment involved in the DIY plotter project, it occurred to me you could conjure a plotter from a pair of steppers, two disks, a lifting mechanism, and not much else. The general idea resembles an Rθ plotter, with the paper glued to a turntable for the “theta” motion, but with the “radius” motion produced by pen(s) on another turntable:

Rotary Plotter - geometry 4

Rotary Plotter – geometry 4

The big circle is the turntable with radius R1, which might be a touch over 4.5 inches to fit an 8.5 inch octagon cut from ordinary Letter paper. The arc with radius R2 over on the right shows the pen path from the turntable’s center to its perimeter, centered at (R1/2,-R1) for convenience.

The grid paper represents the overall Cartesian grid containing the XY points you’d like to plot, like, for example, point Pxy in the upper right corner. The object of the game is to figure out how to rotate the turntable and pen holder to put Pxy directly under the pen at Ixy over near the right side, after which one might make a dot by lowering the pen. Drawing a continuous figure requires making very small motions between closely spaced points, using something like Bresenham’s line algorithm to generate the incremental coordinates or, for parametric curves like the SuperFormula, choosing a small parameter step size.

After flailing around for a while, I realized this requires finding the intersections of two circles after some coordinate transformations.

The offset between the two centers is (ΔX,ΔY) and the distance is R2 = sqrt(ΔX² + ΔY²).  The angle between the +X axis and the pen wheel is α = atan2(ΔY,ΔX), which will be negative for this layout.

Start by transforming Pxy to polar coordinates PRθ, which produces the circle containing both Pxy and Ixy. A pen positioned at radius R from the center of the turntable will trace that circle and Ixy sits at the intersection of that circle with the pen rotating around its wheel.

The small rectangle with sides a and b has R as its diagonal, which means a² + b² = R² and the pointy angle γ = atan a/b.

The large triangle below that has base (R2 – a), height b, and hypotenuse R2, so (R2 – a)² + b² = R2².

Some plug-and-chug action produces a quadratic equation that you can solve for a as shown, solve for b using the first equation, find γ from atan a/b, then subtract γ from θ to get β, the angle spearing point Ixy. You can convert Rβ back to the original grid coordinates with the usual x = R cos β and y = R sin β.

Rotate the turntable by (θ – β) to put Pxy on the arc of the pen at Ixy.

The angle δ lies between the center-to-center line and Ixy. Knowing all the sides of that triangle, find δ = arccos (R2 – a) / R2 and turn the pen wheel by δ to put the pen at Ixy.

Lower the pen to make a dot.

Done!

Some marginal thinking …

I’m sure there’s a fancy way to do this with, surely, matrices or quaternions, but I can handle trig.

You could drive the steppers with a Marlin / RAMPS controller mapping between angles and linear G-Code coordinates, perhaps by choosing suitable steps-per-unit values to make the degrees (or some convenient decimal multiple / fraction thereof) correspond directly to linear distances.

You could generate points from an equation in, say, Python on a Raspberry Pi, apply all the transformations, convert the angles to G-Code, and fire them at a Marlin controller over USB.

Applying 16:1 microstepping to a stock 200 step/rev motor gives 0.113°/step, so at a 5 inch radius each step covers 0.01 inch. However, not all microsteps are moved equally and I expect the absolute per-step accuracy would be somewhere between OK and marginal. Most likely, given the application, even marginal accuracy wouldn’t matter in the least.

The pen wheel uses only 60-ish degrees of the motor’s rotation, but you could mount four-ish pens around a complete wheel, apply suitable pen lift-and-lower action and get multicolor plots.

You could gear down the steppers to get more steps per turntable revolution and way more steps per pen arc, perhaps using cheap & readily available RepRap printer GT2 pulleys / belts / shafts / bearings from the usual eBay sellers. A 16 tooth motor pulley driving a 60 tooth turntable pulley would improve the resolution by a factor of 3.75: more microsteps per commanded motion should make the actual motion come out better.

Tucking the paper atop the turntable and under the pen wheel could be a challenge. Perhaps mounting the whole pen assembly on a tilting plate would help?

Make all the workings visible FTW!

Some doodles leading up to the top diagram, complete with Bad Ideas and goofs …

Centering the pen wheel at a corner makes R2 = R1 * sqrt(2), which seems attractive, but seems overly large in retrospect:

Rotary Plotter - geometry 1

Rotary Plotter – geometry 1

Centering the pen wheel at (-R1,R1/2) with a radius of R1 obviously doesn’t work out, because the arc doesn’t reach the turntable pivot, so you can’t draw anything close to the center. At least I got to work out some step sizes.

A first attempt at coordinate transformation went nowhere:

Rotary Plotter - geometry 2

Rotary Plotter – geometry 2

After perusing the geometric / triangle solution, this came closer:

Rotary Plotter - geometry 3

Rotary Plotter – geometry 3

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Glass vs. Epoxy: Divot!

The WS2812 RGB LED atop the Bowl of Fire Floodlight

Reflector floodlight - purple phase

Reflector floodlight – purple phase

failed in the usual way after a bit over a year of constant use.

I’d done an unusually good job of epoxying the ersatz heatsink in place:

Reflector floodlight - finned LED holder

Reflector floodlight – finned LED holder

I wrapped the bulb in a towel with only the heatsink sticking out, whacked the side of the heatsink parallel to the glass with a plastic-face hammer, and it popped right off:

Epoxy vs glass - divot

Epoxy vs glass – divot

The missing piece of the epoxy ring turned out to be a divot ripped out of the glass, which I didn’t notice until I’d chipped the fragment off the aluminum, so no pictures.

Given the relative strengths of epoxy and glass, pulling a divot seems impossible, but folks doing 3D printing on glass platforms have been reporting exactly that failure for years. The prevailing theory seems to involve small scratches and defects in the glass surface, with subsequent weakening, and stresses applied to the epoxy perpendicular to the glass surface pulling the cracks apart.

Replacing the RGB LED will require drilling it out and probably a complete rewiring, because I seem to have made liberal use of epoxy inside the heatsink and brass tube.

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Pure Matte Black the Hard Way

The best way to get a pure, non-reflective black uses optics, not pigments:

Matte black blade edges

Matte black blade edges

The shiny steel blades reflect light just fine, but the reflections have no way back out of the gap between adjacent edges: the angle of reflection always points away from you and the incoming light.

I always admire the blackness when I open that box.

Yes, I’m a member of the Society of the Easily Amused.

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Monthly Science: Raising a Monarch Butterfly

A Monarch butterfly laid eggs in late July. On the 29th of July they looked like this:

Monarch Egg - focus stacked

Monarch Egg – focus stacked

By August 2, a pair of caterpillars had hatched and grew to 3 mm:

Monarch caterpillar - 3 mm - 2017-08-02

Monarch caterpillar – 3 mm – 2017-08-02

A day later, they were 4 mm long:

Monarch caterpillars - 4 mm - 2017-08-03

Monarch caterpillars – 4 mm – 2017-08-03

They really were sort of blue-ish with green hints:

Monarch caterpillar 1 - 4 mm - 2017-08-03

Monarch caterpillar 1 – 4 mm – 2017-08-03

And:

Monarch caterpillar 2 - 4 mm - 2017-08-03

Monarch caterpillar 2 – 4 mm – 2017-08-03

By August 9, one had had more mature coloration:

Monarch caterpillar - 2017-08-09

Monarch caterpillar – 2017-08-09

The other caterpillar had vanished; we assume it got out of the aquarium and wandered off.

Apparently, the front end of the caterpillar (at the bottom of the picture) has a hard windshield reflecting the ring of LEDs around the camera lens. The caterpillar eats its skin after each molting, except for the windshield:

Monarch Windshield - 2017-08-09

Monarch Windshield – 2017-08-09

We kept fresh milkweed branches in a vase and the caterpillar ate almost continuously:

Monarch caterpillar - 2017-08-13

Monarch caterpillar – 2017-08-13

By August 15, the caterpillar was ready for the next stage in its life. At 10 in the morning it had attached itself to the screen covering the aquarium and assumed the position:

Monarch caterpillar - starting chrysalis - 2017-08-15

Monarch caterpillar – starting chrysalis – 2017-08-15

It transformed into a chrysalis by 5:30 PM:

Monarch Chrysalis - with skin

Monarch Chrysalis – with skin

The discarded skin remained loosely attached until I carefully removed it.

What look like small yellow spots are actually a striking metallic gold color.

Eleven days later, on August 26 at 9 AM, the chrysalis suddenly became transparent:

Monarch chrysalis - ready - left

Monarch chrysalis – ready – left

And:

Monarch chrysalis - ready - right

Monarch chrysalis – ready – right

The shape of the butterfly becomes visible in reflected light:

Monarch chrysalis - ready - ventral detail

Monarch chrysalis – ready – ventral detail

The gold dots and line remained visible.

The magic happened at 3 PM:

Monarch chrysalis - emerging - unfolding

Monarch chrysalis – emerging – unfolding

The compacted wings emerge intense orange on the top and lighter orange on the bottom:

Monarch unfolding - left

Monarch unfolding – left

The butterfly took most of the day to unfurl and stiffen its wings into flat plates:

Monarch unfolding - dorsal

Monarch unfolding – dorsal

And:

Monarch unfolding - right

Monarch unfolding – right

By 8 PM it began exploring the aquarium:

Monarch unfolded - right

Monarch unfolded – right

As adults, they sip nectar from flowers, but don’t feed for the first day, so we left it in the aquarium overnight.

At 10 AM on August 27, we transported it to the goldenrod in the garden, where it immediately began tanking operations:

Monarch on Milkweed - left

Monarch on Milkweed – left

A few minutes later, it began sun-warming operations:

Monarch on Milkweed - dorsal

Monarch on Milkweed – dorsal

Mary watched it while she was tending the garden and, an hour or so later, saw it take off and fly over the house in a generally southwest direction. It will cross half the continent under a geas prohibiting any other action, eventually overwinter in Mexico with far too few of its compadres, then die after producing the eggs for a generation beginning the northward journey next year.

Godspeed, little butterfly, godspeed …

In the spirit of “video or it didn’t happen”, there’s a 15 fps movie of the emergence taken at 5 s/image.

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