The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Electronics Workbench

Electrical & Electronic gadgets

  • VGA-grade Video Cable Connector

    VGA-class Video Cable Connector
    VGA-class Video Cable Connector

    Setting up a new (well, new to me, it’s that old GX270) PC gave me reason to rummage in the Video Cable box and come up with this VGA-class cable. Half the connector shell had worked its way off, giving a nice view of the handiwork. Easily snapped back on with no permanent damage.

    The cable works OK at 1280×1024, although the image seems a bit soft, and higher screen resolutions are out of its reach. There are no obvious signal reflections visible on the screen, so the impedance bumps are not as bad as you might think.

    The VGA connector includes common returns for the Red, Green, and Blue signals, and the two wires for each color should be twisted together. That obviously hasn’t happened here, but crosstalk doesn’t seem to be much of a problem.

    To their credit, they did solder the cable shield to the connector shell, which is a really nice touch. Alas, the impedance of a one-inch pigtail pretty much chokes off the high-frequency stuff you really want to drain to the shell.

    Memo to Self: One of these days, run a bandwidth check with the spectrum analyzer. Use 6-dB pads to get nice 75-Ω terminations.

  • SPICE Crystal Model

    Linear Technology’s LTSpice generic capacitor model has all the parts you need to synthesize a crystal, which is pointed out in the help file and various spots around the web. What’s missing is the relation between all the parts and the values you have in hand for an arbitrary crystal.

    SPICE Capacitor Model
    SPICE Capacitor Model

    The crystal capacitor model looks like this…

    Cpar (usually C0) along the right edge is the inter-electrode capacitance, on the order of a few pF.

    Rpar (usually R0) along the left edge is the parasitic resistance across the case, on the order of hundreds of MΩ.

    The RCL string in the middle is the “motional” part of the crystal model, generally found with a subscript “m” in the specs.

    • Rser (Rm or ESR) is on the order of 100 Ω
    • Capacitance (Cm) is the motional capacitance, on the order of fF (that’s femtofarad: 10-15)
    • Lser (Lm) is tens to thousands of mH
    • RLshunt is something I haven’t seen in any other model and, in fact, it doesn’t appear in the properties panel.
    Crystal Properties
    Crystal Properties

    Now, the part I screwed up is that the capacitor’s value (the number appearing on the schematic) is Capacitance (in the angle brackets that royally screw up WordPress HTML), not Cpar. So the crystal capacitor properties panel looks like this…

    That models a 10 MHz crystal, taken directly from a sidebar in Refinements in Crystal Ladder Filter Design by the legendary Wes Hayward W7ZOI, in the June 1995 issue of QEX.

    Guess what? Plug it into a model of his crystal-measuring circuit and it works exactly like he says it should. No surprise there…

    SPICE has a bit of trouble simulating high-Q oscillators; they tend to not start up properly. If nothing seems to be happening, wait for a few tens-to-hundreds of milliseconds before despairing. Try chopping Rser down by a factor of two or four to see if that improves its disposition.

    You could try injecting a few (hundred thousand) cycles of a kickstart signal, but that’s fraught with peril: you’re simulating something even further from reality than usual.

    Memo to Self: You can rename the cap from C2 (or whatever) to X1 (or whatever) and everything still works fine.

  • D-Cell Corrosion: Prepare for Liftoff!

    Corroded Alkaline C Cells
    Corroded Alkaline C Cells

    Surprisingly, the flashlight holding these cells wasn’t damaged.

    Judging from the position of the switch, my mother tried to turn the thing on, it didn’t light up, and she just dumped it back in the drawer. Time passed, corrosion never sleeps, and the weak link (fortunately, between the two cells) let the alkaline nastiness out.

    I gotta collect all these pix in one big pile…

    (These are not the D-cells from the Maglite adventure.)

  • Tenergy AA NiMH Cell Self-Discharge

    Got a set of Tenergy 2600 mAh AA NiMH cells a while ago and ran some self-discharge tests.

    Tenergy 8 AA NiMH Pack Self-Discharge
    Tenergy 8 AA NiMH Pack Self-Discharge

    The three curves on the far right (two overlap pretty closely) are freshly charged. All three packs exhibit pretty much the same 1.8 Ah capacity, about 1.8 Ah or 70% of the advertised value.

    I must say that Tenergy cells aren’t exactly Tier 1 quality.

    The little stubby red curve on the far left is the as-received capacity. They’re not advertised as ready-to-use and, for sure, they’re not. My analysis of some RTU cells is there.

    The two curves on the left of the main group, after six and seven weeks, are down to 1.4 Ah. That’s 78% of the actual fresh-charge capacity, a mere 54% of nominal.

    Condensing the endpoints into a table, we have:

    Pack Charge Date Discharge Date Storage Time Capacity
    A 09/04/09 09/04/09 0 1.82
    A 09/04/09 09/11/09 7 1.64
    B 09/04/09 09/24/09 20 1.58
    C 09/04/09 10/25/09 51 1.41
    A 09/11/09 10/25/09 44 1.41
    B 09/24/09 10/25/09 31 1.48

    A picture being worth a kiloword, a quick-and-dirty graph is in order:

    Tenergy AA NiMH Self-Discharge
    Tenergy AA NiMH Self-Discharge

    Ignoring the first and last points, the slope is more-or-less constant at 6 mAh/day.

    Call it 0.3%/day after the first week: store the cells for 50 days and you’re down 15%.

    Remember, that’s measured from the actual as-charged and as-measured capacity, not the nominal as-read-about-in-the-spec-sheet capacity.

  • Alpha-Geek Clock

    You’ve probably seen “ultimate geek clocks” floating around on the web, which seem to be Nixie tubes, binary readouts, or analog clocks with lightly encoded markings.

    Poseurs, all of them!

    If you’re an alpha geek, this is how you tell time…

    Absolute Geek Clock
    Absolute Geek Clock

    It’s a WWVB receiver wired up to a CR123A primary lithium cell. The time display is a single red LED, driven by a low-threshold FET. Yeah, you can package it up in a cute little box (which is the picture on hackaday.com), but this is the essence of the thing.

    Over the course of a minute, the LED blinks out the hour, minute, year, day-of-year, Daylight Saving Time, leap year, leap second, and some other stuff in binary-coded decimal.

    The key to the format is there and the bit format is straightforward:

    • Long = frame marker
    • Medium = binary 1
    • Short = binary 0

    You just watch the LED, catch the frame marker, decode BCD data on the fly, convert from UTC to local time, and that’s all there is to it.

    Sheesh, it’s only one bit a second: anybody can handle that, right?

    Truth to tell, I can hang on long enough to get the minute, but I taper off pretty quickly after that.

    Tech detail…

    Basically, you get the receiver and CR123 cell holder from DigiKey for maybe fifteen bucks. Wire up a FET (ZVNL110A or some such) to the receiver’s inverted-polarity output, so the LED is ON during the data bit’s active time (carrier drops 10 dB). I blobbed on a 300 ohm SMD resistor, so the total current is maybe 250 µA with the LED on. If you’re going crude, you can probably wire the LED & resistor directly to the receiver’s positive-polarity output.

    A primary CR123A is good for 1500 mAh and the average current is maybe 150 µA, so the clock will run for nearly a year. The LED is pretty dim, but perfect for late-night viewing.

    Reception is iffy during the day here in the Hudson Valley. At night it’s just fine. Interference from LCD panels with near-60-kHz refresh is a real problem, so it doesn’t play well near PCs.

    I put the clock on a shelf where I can watch it when I wake up in the middle of the night: it knocks me out again pretty quickly.

    In real life, I put this together to verify my WWVB simulator… but I might just box up a spare for the shelf, too.

  • Still More Alkaline Cell Corrosion

    This is depressing …

    Alkaline Cell Corrosion in Boom Box
    Alkaline Cell Corrosion in Boom Box

    We got a boom box so Mom could have background music; the Olde Family Tube Radio was far beyond its Best Used By date.

    Prompted by recent events around here, I checked it on a recent visit and, yup, more corrosion. In all fairness, the cells suggest “Best If Installed By Jan 99”, so they’re well past their date, too.

    This used to be a whole lot less of a problem when flashlights and radios (without clocks!) were the only things using “dry cells”: when the battery went dead, the thing didn’t work and you replaced the cells.

    Nowadays, we expect alkaline cells to supply keep-alive trickle current for memory backup; even after the cell corrodes, it still supplies that tiny current and we never notice what’s happening inside.

    I’m beginning to loathe alkaline cells just like I loathe the small internal combustion engines in yard equipment.

  • More Alkaline Cell Corrosion

    Must be something going around…

    Corroded clock-thermometer cell
    Corroded clock-thermometer cell

    The outdoor thermometer over my desk (which also displays UTC so I don’t have to reset the mumble clock twice a year) started blinking. That’s the usual sign of a dead battery and, yup, when I opened it up, that “leakproof” Eveready was pretty far gone.

    Surprisingly, at least to me, the cell hovered around 1.1 V open-circuit and 800 mV under the meter’s “battery test” load. Given the amount of corrosion, I thought it would be flat dead.

    The corrosion had crawled out of the compartment along the negative terminal and coated the entire metal tab with bluish-green crystals. Some protracted dabbing with vinegar, rinsing with wet cotton swabs, and drying put things pretty much back in order.

    I usually scrawl the date on each cell when I install it, but either I didn’t do that here or the corrosion ate the ink. All I know is that it’s been up there for quite a few years; look at the discoloration where it faces the sun through the window!

    The thing was a surplus freebie to begin with and has long since been fully depreciated…