Monthly Science: Small Praying Mantis

These Praying Mantis nymphs may have emerged from the ootheca I rescued from the grass trimming operation earlier this year:

Praying Mantises in grass - 2020-07-24
Praying Mantises in grass – 2020-07-24

The closest one was about 60 mm long, with plenty of growing ahead in the next few months:

Praying Mantis - 2020-07-24
Praying Mantis – 2020-07-24

A few days later, I spotted a smaller one, maybe 40 mm from eyes to cerci, hiding much deeper in the decorative grass clump. Given their overall ferocity, it was likely hiding from its larger sibs.

They have also been stilting their way across the window glass and screens in search of better hunting grounds. My affixing their oothecae to another bush may have disoriented them at first, but they definitely know where their next meal comes from!

Perhaps as a bonus, a Katydid appeared inside the garage, stuck to the side of a trash can that Came With The House™ long ago:


I deported it outside, in hopes of increasing the world’s net happiness.

The stickers covering the can say “WPDH: A Decade of Rock ‘n’ Roll”, suggesting they date back to 1986, ten years after (Wikipedia tells me) WPDH switched from country to rock. Neither genre did much for me, so I never noticed.

NPN RGB Astable Multivibrator Timing Adjustment

Back in the beginning of July, I replaced the NP-BX1 battery in the RGB Piranha astable multivibrator with a 18650 lithium cell and a USB charge controller, then watched it blink for the next two weeks on the first charge:

Astable - 10 11 12 uF tweak - 027
Astable – 10 11 12 uF tweak – 027

However, the blinks looked … odd and some poking around with a Tek current probe showed the red and blue astables had locked together, so they blinked in quick succession. Alas, I don’t have a scope shot to prove it.

I built all three astables with the same parts, figuring the normal tolerance of electrolytic caps would make the astables run at slightly different rates, which they did at first.

This being a prototype, I just soldered a 1 µF cap onto the blue channel’s existing 10 µF cap:

Astable - 11 uF cap - detail
Astable – 11 uF cap – detail

You can barely make out the top of the additional 2.2 µF cap on the red channel, through the maze of components; now, they definitely have different periods.

Aaaand the scope shot to prove it:

Astable NPN - 10 11 12 uF tweak - 10 mA-div
Astable NPN – 10 11 12 uF tweak – 10 mA-div

The bottom trace shows the battery current at 10 mA/div. The first pulse, over on the left, has the red and blue LEDs firing in quick succession with some overlap, but they separate cleanly for their next pulses.

You don’t want to build a battery-powered astable from NPN transistors, because the 8 mA current between blinks is murderously high. In round numbers, each of the three LEDs blinks twice a second for 30 ms at 20 mA, so they average 3.6 mA, less than half the current required to keep the astables running between blinks. Over the course of 14 days, the circuit drew 11.6 mA × 336 hr = 3900 mA·h until the protection circuit shut it down.

The lead photo shows a harvested 18650 cell, but I started with a known-good Samsung 18650 cell rated at 2600 mA·h at a 0.2C = 520 mA rate to 2.75 V. It’s comforting to see more energy trickling out at a 0.005C rate!

I must conjure a holder with contacts for an 18650 cell, support for a trio of 2N7000 MOSFET astables, and some kind of weird spider with the RGB Piranha LED on the top. Even a harvested 18650 cell should last a couple of months with a much longer blink period (500 ms is much too fast), less LED current (this one is shatteringly bright), and a lower average current.

And, yeah, I’ve been misspelling “Piranha” for a while.

Quilting Hexagon Template Generator

Mary took on the task of finishing a hexagonal quilt from pieced strips, only to discover she’ll need several more strips and the myriad triangles required to turn hexagons into strips. The as-built strips do not match any of the standard pattern sizes, which meant ordinary templates were unavailing. I offered to build a template matching the (average) as-built hexagons, plus a triangle template based on those dimensions.

Wikipedia has useful summaries of hexagon and equilateral triangle geometry and equations.

Quilters measure hexes based on their finished side length, so a “1 inch hex” has sides measuring 1 inch, with the seam allowance extending ¼ inch beyond the sides. It’s difficult to measure finished sides with sufficient accuracy, so we averaged the side-to-side distance across several hexes.

Some thrashing around produced a quick-and-dirty check piece that matched (most of) the stack of un-sewn hexes:

Quilting Hexagon Cutting Template
Quilting Hexagon Cutting Template

That one came from a knockoff of the circle template, after some cleanup & tweakage, but failed user testing for not withstanding the side force from the rotary cutter blade. The inside and outside dimensions were correct, however, so I could proceed with some confidence I understood the geometry.

Both the pattern width (the side-to-side distance across the inside of the hex) and the seam allowance appearing in the Customizer appear in inches, because that’s how things get measured outside the Basement Laboratory & Fabrication Facility:

FinishedWidthInch = 2.75;
FinishedWidth = FinishedWidthInch * inch;

SeamAllowanceInch = 0.25;
SeamAllowance = SeamAllowanceInch * inch;

You feed in one side-to-side measurement and all other hex dimensions get calculated from that number; quilters default to a ¼ inch seam allowance. Remember, standard quilt hexes are measured by their side length, so just buy some standard templates.

This is one of the few times I’ve needed triangle graph paper:

Hex Quilting Template - geometry doodles
Hex Quilting Template – geometry doodles

After I gave up trying to get it right on square-grid paper, of course.

Solidifying those relations:

Quilting Hex Template - build layout
Quilting Hex Template – build layout

Then math got real:

Hex Quilting Templates - on strips
Hex Quilting Templates – on strips

Both templates have non-skid strips to keep the fabric in place while cutting:

Hex Quilting Template - grip strips
Hex Quilting Template – grip strips

I should have embossed the size on each template, but this feels like a one-off project and YAGNI. Of course, that’s how I felt about the circle templates, so maybe next time I’ll get it right.

As it turned out, Mary realized she needed a template for the two half-triangles at the end of each row:

Quilting Hex Template - half-triangle
Quilting Hex Template – half-triangle

It’s half of the finished size of the equilateral triangle on the right, with seam allowance added all around. The test scrap of fabric on the left shows the stitching along the hypotenuse of the half-triangle, where it joins to the end-of-row hexagon. Ideally, you need two half-triangle templates, but Mary says it’s easier to cut the fabric from the back side than to keep track of two templates.

The family portrait now has three members:

Quilting Hex Template - family
Quilting Hex Template – family

The OpenSCAD source code as a GitHub Gist:

OXO Pepper Grinder: Inadvertent Abuse

Being that type of guy, I’m reasonably sure I would not have bought what’s now clearly labeled as an OXO Radial Pepper Grinder for use as a salt mill:

OXO Salt Mill - corrosion
OXO Salt Mill – corrosion

Mary recalls we got it at Target, back when one could go places and buy things, and I vaguely recall contemplating a wall of OXO gadgets. It’s been a while and I neglected to save the packaging for future reference.

Obviously not stainless steel, but not lethal, so we’ll continue abusing it.