Archive for category Science
With a terminal voltage falling from barely 3 V, the LED drew about 3 mA (1 mA/div), tops, without a ballast resistor:
Hacking in a charged NP-BX1 secondary lithium cell boosted the supply to 4 V:
Which, diodes being the way they are, raised the LED current to nearly 400 mA (100 mA/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:
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:
It’s still blindingly bright, but now I don’t feel bad about it.
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:
Which required an adapter betwixt pole and lid:
Which requires a bit of solid modeling:
The lids have a central boss, presumably for stiffening, so the model includes a suitable recess:
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:
The tangle off to the right comes from a bridge layer with a hole in the middle, which never works well even with support:
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:
The DSO150 oscilloscope’s specs give a 200 kHz bandwidth, so a 50 kHz sine wave looks pretty good:
A 100 kHz sine wave looks chunky, with maybe 25 samples per cycle:
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:
At 50 kHz, there’s not much square left in the wave:
And, just for completeness, a 200 kHz square wave completely loses its starch:
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:
The pulse repetition frequency beats with the scope sampling and sweep speeds to produce weird effects:
Tuning the pulse frequency for maximum weirdness:
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.
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:
Up on the second floor, it’s 90 psi:
With a different brand of gauge, it’s also 90 psi on the third floor:
Maybe 85 psi on the fourth:
Squinting at the parallax, call it 80 psi on the fifth:
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:
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:
Call it 0.42 psi/ft, which is pretty close to the right answer.
Hitching a charged, albeit worn, NP-BX1 lithium battery to the astable multivibrator produces a blinding flash:
The current pulse shows the wearable LED really takes a beating:
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:
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 …
Coverage of capacitors as charge-storage devices, rather than filters:
We avoided all the calculus and derivations, taking the exponential waveform as a given for RC circuits:
Discussions of dielectrics, plate spacing / area, and suchlike:
Some handwaving discussion of construction, electrolytic capacitor innards, and The Plague:
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!
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:
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:
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 …