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Archive for category Science

Astable Multivibrator: SMT LED Ballast Resistor

The original astable multivibrator ran from a dead CR123 primary lithium cell:

CR123A Astable - front

CR123A Astable – front

With a terminal voltage falling from barely 3 V, the LED drew about 3 mA (1 mA/div), tops, without a ballast resistor:

Astable - CR123A 2.8 V - 1 mA -green

Astable – CR123A 2.8 V – 1 mA -green

Hacking in a charged NP-BX1 secondary lithium cell boosted the supply to 4 V:

NP-BX1 Holder - SMT pogo pins

NP-BX1 Holder – SMT pogo pins

Which, diodes being the way they are, raised the LED current to nearly 400 mA (100 mA/div):

Astable - NP-BX1 4V - base V - 100mA-div

Astable – NP-BX1 4V – base V – 100mA-div

Somewhat to my surprise, a few weeks of abuse didn’t do any obvious damage to the LED, but I added a resistor while I was soldering up another holder:

Astable - 51 ohm SMD ballast

Astable – 51 ohm SMD ballast

There’s not quite enough room for a 1/8 W axial resistor, so why not blob in a surface-mount resistor?

Which cuts the current down to a mere 15 mA (10 mA/div) from a lithium battery at 4 V:

Astable - NP-BX1 - 51 ohm ballast - 10ma-div

Astable – NP-BX1 – 51 ohm ballast – 10ma-div

It’s still blindingly bright, but now I don’t feel bad about it.

 

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Six Gallon Can Lid Adapter to Platform Bird Feeder

A House Finch suffering from Finch Eye Disease prompted me to sterilize our feeder, which meant providing a temporary feeder to keep the birds flying. Having an abundance of lids from six gallon plastic cans / buckets, this made sense:

Can Lid Feeder - installed

Can Lid Feeder – installed

Which required an adapter betwixt pole and lid:

Can Lid Feeder - assembled

Can Lid Feeder – assembled

Which requires a bit of solid modeling:

Can Lid Platform Feeder Mount - solid model - bottom

Can Lid Platform Feeder Mount – solid model – bottom

The lids have a central boss, presumably for stiffening, so the model includes a suitable recess:

Can Lid Platform Feeder Mount - solid model - support structure

Can Lid Platform Feeder Mount – solid model – support structure

As usual, automatically generated support fills the entire recess, so I designed a minimal support structure into the model and cracked it out with very little effort:

Can Lid Feeder - support structure

Can Lid Feeder – support structure

The tangle off to the right comes from a bridge layer with a hole in the middle, which never works well even with support:

Can Lid Platform Feeder Mount - Slic3r - bridge layer

Can Lid Platform Feeder Mount – Slic3r – bridge layer

Didn’t bother the birds in the least, though, so it’s all good.

I loves me my 3D printer …

The OpenSCAD source code as a GitHub Gist:

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JYE Tech DSO150 Oscilloscope vs. Actual Signals

The DSO150 oscilloscope’s specs give a 200 kHz bandwidth, so a 50 kHz sine wave looks pretty good:

DSO150 - sine wave 50 kHz 10 us-div

DSO150 – sine wave 50 kHz 10 us-div

A 100 kHz sine wave looks chunky, with maybe 25 samples per cycle:

DSO150 - sine wave 100 kHz 10 us-div

DSO150 – sine wave 100 kHz 10 us-div

The DSO150 tops out at 10 µs/div, so you can’t expand the waveform more than you see; 25 samples in 10 µs seems to be 2.5 Msample/s, exceeding the nominal 1 Msample/s spec. I have no explanation.

A 10 kHz square wave shows a blip just before each transition that isn’t on the actual signal:

DSO150 - square wave 10 kHz 20 us-div

DSO150 – square wave 10 kHz 20 us-div

At 50 kHz, there’s not much square left in the wave:

DSO150 - square wave 50 kHz 10 us-div

DSO150 – square wave 50 kHz 10 us-div

And, just for completeness, a 200 kHz square wave completely loses its starch:

DSO150 - square wave 200 kHz 10 us-div

DSO150 – square wave 200 kHz 10 us-div

A 10% (-ish) duty cycle pulse at 25 kHz has frequency components well beyond the scope’s limits, so it’s more of a blip than a pulse:

DSO150 - pulse 25 kHz 10 us-div

DSO150 – pulse 25 kHz 10 us-div

The pulse repetition frequency beats with the scope sampling and sweep speeds to produce weird effects:

DSO150 - pulse 25 kHz 100 us-div

DSO150 – pulse 25 kHz 100 us-div

Tuning the pulse frequency for maximum weirdness:

DSO150 - pulse 15 kHz 200 us-div

DSO150 – pulse 15 kHz 200 us-div

None of this is unique to the DSO150, of course, as all digital scopes (heck, all sampled-data systems) have the same issues. The DSO150’s slow sampling rate just makes them more obvious at lower frequencies.

Key takeaway: use the DSO150 for analog signals in the audio range, up through maybe 50 kHz, and it’ll produce reasonable results.

Using it for digital signals, even at audio frequencies, isn’t appropriate, because the DSO150’s low bandwidth will produce baffling displays.

 

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Monthly Science: Motel Water Pressure vs. Height

Being a sucker for infrastructure and numbers, the fire sprinkler system pressure gauges in the motel stairwell proved irresistible.

The first floor gauge shows a nice round 100 psi:

Hotel water pressure - floor 1

Hotel water pressure – floor 1

Up on the second floor, it’s 90 psi:

Hotel water pressure - floor 2

Hotel water pressure – floor 2

With a different brand of gauge, it’s also 90 psi on the third floor:

Hotel water pressure - floor 3

Hotel water pressure – floor 3

Maybe 85 psi on the fourth:

Hotel water pressure - floor 4

Hotel water pressure – floor 4

Squinting at the parallax, call it 80 psi on the fifth:

Hotel water pressure - floor 5

Hotel water pressure – floor 5

At the top of the vertical pipe on the fifth, on the other side of a valve, we return to the original valve company at 78 psi:

Hotel water pressure - floor 5 - top

Hotel water pressure – floor 5 – top

Water weighs just over 62 lb/ft³ at room temperature, which works out to 0.43 lb/in² per vertical foot. Not having packed my laser distance widget, I’ll guesstimate 12 feet and 5 psi per floor.

A quick graph with an eyeballometric straight-line fit:

Motel sprinkler water pressures

Motel sprinkler water pressures

Call it 0.42 psi/ft, which is pretty close to the right answer.

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Astable Multivibrator vs. Charged NP-BX1 Lithium Battery

Hitching a charged, albeit worn, NP-BX1 lithium battery to the astable multivibrator produces a blinding flash:

NP-BX1 Holder - SMT pogo pins

NP-BX1 Holder – SMT pogo pins

The current pulse shows the wearable LED really takes a beating:

Astable - NP-BX1 4V - 100mA-div

Astable – NP-BX1 4V – 100mA-div

The current trace is at 100 mA/div: the pulse starts at 400 mA, which seems excessive even to me, and tapers down to 200 mA. It’s still an order of magnitude too high at the end of the pulse.

On the other paw, maybe a 14% duty cycle helps:

Astable - NP-BX1 4V - base V - 100mA-div

Astable – NP-BX1 4V – base V – 100mA-div

The top trace shows the base drive voltage dropping slightly, although I suspect the poor little transistor can’t take the strain.

The LED really does need a ballast resistor …

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Squidwrench Electronics Workshop: Session 6 Whiteboards

Coverage of capacitors as charge-storage devices, rather than filters:

Session 6 - Whiteboard 1 - overview

Session 6 – Whiteboard 1 – overview

We avoided all the calculus and derivations, taking the exponential waveform as a given for RC circuits:

Session 6 - Whiteboard 1 - exponential detail

Session 6 – Whiteboard 1 – exponential detail

Discussions of dielectrics, plate spacing / area, and suchlike:

Session 6 - Whiteboard 1 - dielectric permittivity

Session 6 – Whiteboard 1 – dielectric permittivity

Some handwaving discussion of construction, electrolytic capacitor innards, and The Plague:

Session 6 - Whiteboard 1 - cap construction

Session 6 – Whiteboard 1 – cap construction

A 1 F cap charged through a 1.8 kΩ resistor during most of the session to show what an 1800 s time constant looked like. Nope, it never quite got to the 3.5 V from the power supply, even when we all decided it was time to shut down!

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Monthly Image: AMP Plug Board

Around 1960, somebody my father knew at the Harrisburg AMP factory gave me a chunk of plugboard bandsawed from a scrapped computer or industrial controller, because he knew I’d enjoy it:

AMP Plug Board

AMP Plug Board

He was right.

I spent months rearranging those little cubes (some with cryptic legends!) into meaningful (to me) patterns, plugging cables between vital spots, and imagining how the whole thing worked:

AMP Plug Board - detail

AMP Plug Board – detail

Long springs ran through the notches under the top of the blocks to connect the plug shells to circuit ground. The ends of the steel rails (still!) have raw bandsaw cuts, some of the blocks were sliced in two, the tip contact array behind the panel wasn’t included, and none of that mattered in the least.

Only a fraction of the original treasure trove survives. It was absolutely my favorite “toy” ever.

Quite some years ago, our Larval Engineer assembled the pattern you see; the hardware still had some attraction.

I’ve asked Mary to toss it in the hole with whatever’s left of me, when that day arrives …

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