Archive for category Science
OK, it’s not as exciting as a Strava KOM:
We’re at the top of an uphill section and, riding together, we’re not sprinting for town line signs.
Ex post facto notes from the third Squidwrench Electronics Workshop.
Exhibit various 50 Ω resistors, including my all-time favorite, a 600 W 3 GHz dummy load:
… down to a 1/8 Ω metal film resistor.
The dummy load’s N connector triggered a regrettable digression into RF, belatedly squelched because I wasn’t prepared to extemporize on AC concepts like reactance which we haven’t covered yet.
Discussion of resistor applications, power handling, power derating with temperature, etc:
Why you generally won’t find 50 Ω load resistors in Raspberry Pi circuits. Cartridge heaters for 3D printers, not aluminum power resistors, although everyone agrees they look great:
Discussion of voltage vs. current sources, why voltage sources want low internal resistances and current sources want high resistances. Bungled discussion of current sources by putting diodes in parallel; they should go in series to show how added voltage doesn’t change current (much!) in sources driven from higher voltages through higher resistances:
Use Siglent SDM3045X DMM in diode test mode to measure forward drop of power / signal / colored LEDs, discuss voltage variation with color / photon energy. Measure 1.000 mA test current for all forward voltages.
Compute series resistor (500 Ω) to convert adjustable power supply (the digital tattoo box, a lesson in itself) into reasonable current source; roughly 10 V → 20 mA. Find suitable resistor (560 Ω) in SqWr
junk box parts assortment, digression into color band reading.
Wire circuit with meters to measure diode current (series!) and voltage (parallel!), measure same hulking power diode (after discovering insulating washers now in full effect) as before in 1 mA steps to 10 mA, then 15 and 20 mA, tabulate & plot results:
Discover warm resistor, compute power at 20 mA, introduce cautionary tales.
Lesson learned about never returning parts to inventory, with 560 Ω resistor appearing in diode drawer. Cautionary tales about having benchtop can of used parts as front-end cache for inventory backing store.
Another intense day of bench work!
Mary harvested a great bunch of spearmint from a place where it wouldn’t be missed and, after rinsing, plucking, and chopping, we now have a liter of Mint Extract in the making:
The big jars got 3 oz of coarse-chopped leaves apiece, the smaller jar 1 oz, and the (removed) stems added up to 3.5 oz, so call it 1/3 waste. Not that this is an exact science, but I’d say 3/4 pound of just-picked mint, packed slightly tighter than those jars, would produce a liter of extract.
A day later, the leaves definitely look dehydrated:
Those bottles are lying on their sides with the camera above, looking through the air bubble to the leaves. Unlike commercial mint extract, this stuff is green!
It’ll be finished after a month of daily agitation, but surely it’s an exponential process: a few hundred μl already pep up a mug o’ cocoa just fine.
In very round numbers, I get 10 drops / 0.1 ml, so 1 drop = 10 μl.
Bonus: the cutting board smells wonderful.
(*) It may be Olde White Guy Privilege, but clerks don’t even blink when I stagger up to the counter clutching a bottle of high-octane hooch; they don’t even card my age!
The O’Neill Center at WCSU has two sets of drinking fountains:
The bottle shape on the back of each fountain marks the sensor for its water bottle refill spout. The small rectangular block above and right of the sensor is a virtue signalling display giving the number of disposable bottles allegedly not consigned to a landfill.
The left fountain:
The center fountain:
The right fountain:
Which looked exactly like either a test pattern or a failed display, until I waved my hand over the senor and watched it increment to
00008889. Timing is everything!
The other trio of fountains had the same progression, so it must be a chirality thing.
I can’t say whether you should use the left fountain to avoid some germs or the right fountain for the freshest water. Not having to maneuver our bottle under the usual arch from a drinking nozzle was a big win, though, so mad props to ’em.
The first white LED fixture built to illuminate one of Mary’s Kenmore 158 sewing machines has been in regular use for the last four years:
We never found a good time to rip-and-replace the “prototype” with brighter SMD LEDs and one of the LEDs finally gave up.
They’re 10 mm white LEDs with five chips wired in parallel, which is obvious when you look into the remaining LED running at 1 mA:
The center chip is just dimmer than the others, which means their QC doesn’t tightly control the forward voltage spec.
The wire bonds on the anode terminal of the failed LED look a bit sketchy:
Fortunately, I hadn’t removed the 120 VAC wiring for the original bulb and I have two OEM bulbs from other machines, so I just removed my LED gimcrackery, installed a good old incandescent bulb, and she’s back to sewing with a pleasantly warm machine.
The fixture holding the LEDs broke apart as I extracted it, but it’ll never be used again:
The LEDs are rated at 3.5 V and 200 mA (!), but were reasonably bright in series from a 6 V unregulated supply. Perhaps a power glitch killed the poor thing? We’ll never know.
LEDs are reputed to have lifetimes in the multiple tens of thousands of hours, but I’ve seen plenty of failed automotive LEDs and fancy new LED streetlights out there, not to mention many dead and dying traffic signals. Seeing as how they’re in (presumably) well-engineered fixtures with good power supplies and are at most only a few years old, there shouldn’t be any failures yet.
Nothing lasts forever, not even concrete:
The downspouts are obviously more recent than the splash blocks, but the whole shopping center wasn’t more than a few decades old. Rain isn’t nearly as acid as it used to be, but it still eats away at concrete.
After about two decades, though, even high-quality concrete goes bad:
That’s the upper surface of the Rt 376 bridge at Red Oaks Mill, with a fragment of the corroded lower edge still dangling over the Wappinger Creek:
Mostly, we manage to bike around the decayed infrastructure.
Some ex post facto notes from the second SquidWrench Electronics Workshop. This turned out much more intense than the first session, with plenty of hands-on measurement and extemporized explanations.
Measure voltage across and current through 4.7 kΩ 5 W resistor from 0.5 V to 30 V. Note importance of writing down what you intend to measure, voltage values, units. Plot data, find slope, calculate 1/slope.
Introduce parallel resistors: 1/R = 1/R1 + 1/R2. Derive by adding branch currents, compute overall resistance, factor & reciprocal.
Review metric prefixes and units!
Introduce power equation (P = E I) and variations (P = I² R, P = E²/R)
Measure voltage across and current through incandescent bulb (6 V flashlight) at 0.1 through 6 V, note difference between voltage at power supply and voltage across bulb. Plot data, find slopes at 1 V and 5 V, calculate 1/slopes.
Measure voltage across ammeter with bulb at 6 V, compute meter internal resistance, measure meter resistance. Note on ammeter resistance trimming.
Measure voltage across and current through hulking power diode from 50 mV – 850 mV. Note large difference between power supply voltage and diode voltage above 750-ish mV. Note power supply current limit at 3 A. Plot, find slopes at 100 mV and 800 mV, calculate 1/slopes. Compare diode resistance with ammeter resistance.
Review prefixes and units!
The final whiteboard:
Hand-measured data & crude plots FTW!