• Homebrew Computers & Transistors, and surely more...

    Eric Hertz04/16/2022 at 08:05 1 comment

    Mid-May, just came across this draft from about a month ago... Still pondering the idea off-n-on.


    I'm almost certainly in no position to be, right now, but the past two months have finally calmed a bit, and I desperately need to feel like I'm accomplishing something, which I don't get outside my realm... which basically includes everything I probably should be, or have been forced to be, doing.


    So, I've been revisiting old ideas, one of which is that of building *something* from old CPUs I have laying around, but haven't the experience with to just throw into a project.

    Before the crazy-shizzle started (crazier than normal, which has been crazy itself)  I'd spent months (more?) on my TI-86, learning about z80s and more... #Vintage Z80 palmtop compy hackery (TI-86) . (Really, I'd probably kill two birds with one stone if I'd just pick up where I left off on that... but my brain doesn't switch tasks like these easily, there's a huge process involved, this is probably that process). Anyhow, one of the results of that project thus far is a renewed interest and much better understanding in the make-use-of-processors realm.

    So, now, I have a pretty well-mulled-over idea of a sort of "homebrew computer" build-process that builds it in pieces, each step along the way doing /something/ blinky and interactive. And a sort-of general-path that should work with a wide variety of CPUs I've stumbled on in my collection; z80, 6800, 6502(?), 8088... even up to the 486. 

    E.G. The first step is an easily hand-wired board that merely shows LEDs for the address bus (maybe just 8 bits), and simply wiring the data-bus to NOP (jumper-disableable)... Oh, AND the circuitry necessary for single-stepping. This board will have a header for an IDC-Connector-cable (E.G. an old IDE cable?) connecting the necessary pins for 16bit addressing and 8 bits of data. Though, I'm half-tempted to make it even simpler by just using a chip-clip to breakout to the next boards.

    The second board will connect there, and add switches (and LEDs) for the data bus. Unlike other switch-driven systems like the Altair, this won't (alone/initially) be used to enter a program /into memory/ to be run later. Instead the idea is to have a small program which will be entered in "realtime." The processor requests an address, the user toggles the switches for the opcode, then hits "Step," repeat.

    Now, with two tiny boards we have 8 bits of the address bus visible on LEDs, 8 bits for data on switches and LEDs, and a step button.

    From there, I think the next board will contain a jumper-enableable diode-ROM (and couple/few-bit address demultiplexer) for a program maybe as simple as "jmp address 0". If I get this right, the Diode-ROM containing these instructions (this instruction?) could be mapped in various locations in the address-space via jumpers, and the default NOP from the previous board would be accessed otherwise. Now we have a binary counter (the program-counter) visible on LEDs and resetting at, say, a  selectable count of 4+3=7, 8+3=11, 16+3=19...128+3=131. Basically +3 because all the CPUs I've encountered so far have a 1-byte NOP and a 3-byte absolute-jump. And then, powers-of-two since its easiest to decode with one jumper.

    Anyhow, the idea is... I don't want to throw these efforts away, and they're too big to hang on the fridge... So, if Imma build a homebrew computer, maybe I can make each "step" along the way /useful/ even in the "end-product". Each step will introduce more features... When RAM's added, the switches/LEDs on the data bus from the second board will be used to load the memory. The RAM board then will probably add switches and 8 more LEDs for the address bus...

    One board early-on may add line-drivers/buffers, then break out to a new header with the same pinout...

    Another board down the line might add memory-mapping for more than 64k, So, those same boards from before could be moved /after/ this addition... same pinout....

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  • Elderly batteries, Lappies, and other devices

    Eric Hertz07/31/2020 at 22:31 16 comments

    I've two old laptops I've been trying to use with some old software necessarily connecting to old hardware via old ports... thus we're talking a 486 and a p150.

    The 486 is actually a great machine; quality. I used it last year for #OMNI 4 - a Kaypro 2x Logic Analyzer 

    BUT, after a year in storage, even though it worked great after the twenty years in storage prior, it started acting *really* weird; claiming it was running off the battery, though it was reporting 0% charged, and though it was plugged-in to AC... and not even attempting to charge the battery... and, if I recall [it's been *many* days fiddling with it, since], after just a few minutes' use, would briefly warn about a low battery and planning to drop into standby, but would actually shut off completely.

    The p150, as I recall, was doing similar, which is why I reverted to the 486... since I'd used it more recently.

    Long story short: It appears that:

    * upon losing BIOS-settings due to a dead [actually, removed, so-as to avoid leakage] CMOS battery, the default option for the power-button is standby, rather'n power-off. Thus, apparently the machine patiently waited to be awoken, for a year, sitting at the "now safe to shut off" screen, until sipping both the main battery and the standby battery dry...

    * the system apparently does not recognize that the main battery is connected if it detects 0.14V where /at worst/ the 7.2V battery shouldn't go below something like 6V... maybe it can't even measure that low... [interesting; two diode-drops?] Thus, won't even attempt to charge a battery it doesn't think is there.

    * I still can't explain the "Power source: battery" when plugged into AC; but

    * I *think* the reason it'd run for only a few minutes at a time is due to the *standby* battery; which is three NiCd[?] heatshrunk coin-cells, deeply hidden inside. AC would charge them while the unit was powered-off, then the machine would run off three friggin' coin-cells for a few minutes before they ran too low.



    It's not that I /need/ the thing to run off the battery, but that it's near /useless/ if it can only run a few minutes at a time... and... 

    In many cases such an issue might wind-up an otherwise fully-functional machine in eWaste, presuming something internal like a faulty charging circuit... 

    Well, I looked-er-over inside and out [which was not easy, mind you!], and besides a long-broken screen hinge and a couple stupid plastic tabs I broke off trying to open it, etc. this thing is near-pristine inside, going-on 25years old!

    So, then, I broke apart the battery pack and found 6 18650 li-ion cells [li-ion existed 25 years ago?!]... three parallelled, in series with another three parallelled. And, each parallel-combination measuring 0.07V. Yikes.

    Now, if you believe the hype... a li-ion lower than something like 2V is "useless" and unusable, some even say "dangerous," and must be immediately disposed and replaced... Something about dendrites shorting plates, thus never being able hold a charge. Something else about big-enough dendrite-shorts causing huge currents and flames?

    While those theories may have merit in some extreme cases, I'm sure there are varying-degrees of "dangerous" and "useless." And I beg to differ, regarding their uselessness; I think the scenario I've described above is a *perfect* use-case.

    [Similarly applicable, I imagine, and have unwittingly encountered, with many cell-phones, tablets, netbooks, etc.--portable devices of all sorts!--which literally won't turn on without a battery, regardless of a decent charger/power-supply]


    The question then becomes, how to get my laptop [or other device] running... and /now/?

    After a bit of digging, I found that there are many rebels and dare-devils out there [thank goodness] who've had and reported various successes [and failures] trying various methods to revive Li-ions which've been depleted *far* below 2V, some...

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  • Starter battery, wee!

    Eric Hertz07/14/2020 at 00:36 0 comments

    forgot to turn off my headlights, and listening to the radio... guess what happened.

    So, I figured it was time to run an experiment brewing in the ol' noggin' since I saw here of someone charging their car starter battery from an 18V cordless drill battery, and not long after seeing that, @Ted Yapo tried [nearly successfully] to get enough charge outta a coin-cell...

    Well, I've got a *bunch* of USB power-packs... three in series gives 15V.

    Nice thing about these over, say, a regular ol' battery [even a cordless drill battery] is they have boost-converters built-in... so as they discharge, they still put out 5V. It's a bit like a built-in joule-thief. Also, built-in current-limiting, which could be a mixed blessing depending on how it's implemented... oh yeah, and thermal-protection!

    So here's a weird culmination of coincidences:

    the way I wired 'em up was via bulky usb power cables I built a while-back... their housing hot-glued to the connector.

    Then I inserted my ammeter between it and the cigarette lighter outlet...

    2.4A --Perfect.

    The batts are rated for 2.1, but I looked up the chips once, and they're set to 3A.

    I thought maybe it was actually limiting, 'cause 15V -> 12 would mean for a lotta wire resistance to only be 2.5A...

    So I figured the limiters doing their job and plugged it in straight. Nope. One of the batts quit outputting. Grabbed another, tried again with the meter, 2.4A... again trying direct, again kaput batt. 

    OK, so, apparently, again in the last few completely unrelated projects, my ammeter's resistance had a big effect; this time quite handy. 

    I estimated ~1ohm between the car batt and my usb batts, to give that 2.4A... so, yahknow, whatever tiny resistance is in my meter was *just* right.

    There's a coinkidink!

    These things got *hot*... enough to melt the glue, still, not so hot as to shut down... though i watched pretty closely.

    I estimate about 8W lost in the hookup, and 24W, then, into the car batt... 24Wh, actually; it happened to take almost exactly an hour to drain the batts 'till one went kaput.

    Weird thing, someone walked-up needing a jump of his own... and just when my charging completed he got his started. I tried mine, and... yeah! I guess it worked! Nice to've had the backup of his services-offered, anyhow.

    Other interesting observations: the one holding the least charge [as indicated by its bargraph] seemed to drain fastest [makes some sense, lower voltage, higher amperage to boost]. Though, oddly, the one with the most charge, seemingly draining the slowest, [and, oddly, with the most airflow for cooling] went kaput a couple times. Can't quite wrap my head around it, but I guess it was prb for the better, as I bet the others probably didn't mind a break. ...and let that glue cool/harden.

    Interesting metaphors of man-hours, differing-abilities [and stubbornness], John Henry, and managers insisting on coffee-breaks...


    Random-ish thought also revealing the coinkidinkness of it all... that if I'd tried four batteries at 20V? Well, if my setup was otherwise unchanged, we'da been at ~7A... and... it just wouldn'ta worked. So, I guess some amount of luck or coincidence or something that put pretty much all those variables pretty much randomly thrown together not only at a functional point, but also near-maximal.

  • DC grounding, real-world measurements

    Eric Hertz07/04/2020 at 03:02 0 comments


    12VDC [car battery]

    a computer [Raspberry Pi Zero, + LCD + hub/keyboard, etc.] with internal DC-converters... ~1A, 12V

    a USB-attached external hard drive, ~1A, 12V

    each with identical five foot heavy-gauge power cables powered from the same point.

    devices not yet connected via USB



    ~20-30mV difference at negative input to computer vs. negative input to powered-off drive [at the devices]

    Thus ~20m ohm [0.02 ohms] on the negative power wires...


    Both devices powered, the mV difference between their negative inputs varies between ~+15mV and -30mV depending on what each device is doing.

    No biggy, right?

    Connect them via USB!


    Here's a resettable fuse on the drive's negative input:

    <todo, grab picture from old phone with good macro mode>

    Description: got so hot it looks nothing like a fuse anymore, more like a piece of burnt charcoal, complete with burn marks all around on PCB.


    Now, I'm not certain that came from this setup, or the fact that my stupid DC Barrel Jack on compy was all-round metal, allowing one to touch tip-to-shell on insertion.

    BUT: continue with me regarding the previously-described setup/measurements.


    ~20mV difference at negative terminals

    ~0.02ohms on power cables' negative wire

    ~1A, 12V to each device

    Devices NOT connected to each other via USB cable [yet]

    Connect ammeter between USB-A port's shell at compy, and USB-B port's shell at drive.



    "OK, so a couple LEDs, big deal! USB ports handle 500mA!"


    How does Ammeter measure current? 

    Puts a small resistor in series with probes and measures voltage across the resistor.

    [a resistor whose value has to be greater than probes, which are roughly the same gauge, and together about the same length as the power cords... so they're prb ~0.02ohms, then per 10-to-1 rule-of-thumb series resistor is probably ~0.2 ohms, which would make sense being that the minimum voltage-setting is 200mV and the amp-setting is 10A "max" but displays 15A if pushed to do-so]

    So, it's not measuring the current that would go through the shield/VBus-, but some *fraction* of it... plausibly 1/10th!

    700mA might surge through that when the drive starts writing/seeking!

    AND: that may go *either* way, the port may handle 700mA, but what about -700mA?


    That's just the tip of the iceburg.


    But, continuing that experiment, imagine the shield braid is roughly the same gauge as the power cords [when not wrapped around the USB signals] and the cable about 5ft... ~0.02ohms, just like the power cords' negative wire... so, now, if drive is idle/off and compy's running full-tilt at 1.5A, 1A will go through compy's power-cord's negative [0.02ohms], and 0.5A will go through the usb shield into the drive, and out its' power cord... [0.04ohms]. not so great. And, anyhow, 1/3rd of compy's current won't return down compy's power cord, when the drive is off/idle! And vice-versa!


    Think that's not a problem? Still OK with a self-powered USB device sinking and sourcing 500mA whenever?

    OK, how 'bout this:

    Compy has a DC-DC converter which was designed with a far underrated input capacitor... [I figured this out later. Bear with me]...

    So, imagine a DC-DC buck converter...

    How does it work?

    it has an SPDT switch and an inductor. 

    In one position the inductor current "charges" from the input-voltage [12V], and flows into your device/load.

    In the other position the inductor current "discharges" into the load.

    It switches *really fast* so that inductor barely has a chance to "discharge" before getting "recharged."

    Thus, in both switch positions the current through the inductor and into the device/load remains roughly constant, and so then does the voltage.


    "OK, what's the problem?"

    When the inductor is charging, the full device current [say 1A] flows from the source and back to it. So, half the time the battery supplies 1A at 12V [for...

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  • Data, still, and DC grounding

    Eric Hertz06/29/2020 at 06:13 0 comments

    Another setback, rather major, being the connecting of the external drive... eventually not only completely blew out a USB port, but damaged the entire hub it was connected to wnough to cause frequent I/O errors.

    It would seem there's quite a bit to be considered in DC power distribution.

    Imagine a 5 foot long power cable to the computer and another to the external hard drives, both connected to my 12V car battery... say that's 1A@12V for a lowly PiZero+display and 3A@12V for two hard drives... say there's one ohm in those cables... now "ground" at the computer is 1V above battery-ground, and "ground" at the drives is at 3V! 

    Now, imagine what happens when that USB cable is connected!

    ... worse, still, I did some falstad circuit simulations today to see about the best place to put a ground when using buck converters... and I was reminded of another factor I hadn't been considering; powering up a buck converter causes a huge power surge.
    So, Usb drive has a power switch, let's ground 'em through the USB cable *before* powering up.... compy-on, no biggy... maybe draws a little current through the usb shield/gnd wires, but certainly most through its power cable... now, switch on the 3A drive... heh. Huge surge spinning up, of course, but also huge surge charging the buck converter's input capacitor, huge surge charging up the inductor, huge surge charging up the output capacitor, and the motors aren't even spinning yet!

    So... I replaced the hub, and *thankfully* that seems to have been the only damage.

    But, I need to do some serious grounding thoughts before continuing.


    Stupidly, I had forseen some of this; I was building a power-bank last year[?] which has numerous floating-output DC converters for pretty much *exactly* the purpose of preventing such things... but, ultimately, it was such a hassle, and I guess I *really* needed to get some actual compy-time in... feel like I was actually progressing toward my original goals... and, well, I think I was more cautious about it in the beginning, then kinda forgot about it... and I guess it finally came 'round to bite me in the ass.


    Thankfully, again, I think the only damage was to the hub. Although, I ran some extensive tests [copying a large file 100 times] and found that, in fact, *somehow*, despite *numerous* checksums and retry-mechanisms all along the process [sending the command over USB to read the sector, reading the sector, transferring it over USB, transferring it *back* over usb, writing the sector, nevermind the process reading it back for comparison], somehow it did the job once or twice *without* 'cp' complaining, and *with* erroneous data! I don't understand how that's possible! There should've been a checksum failure somewhere, wherever that was it should've automatically reattempted until there wasn't, or /errored/... but no error, and still bad data. HUH.

    So, now, no idea whether all those millions of files transferred to my backup drive, and from there to my main drive, are accurate... am I really going to 'diff' them all? Especially after the reorganizing? UGH. I was *SO* ready to wipe those original drives so I could finally put this *years*-long process behind me.


    And I still haven't figured out what to do about grounding, short of dangling a wire off each component and installing a bunch of friggin' banana-jacks in the van's wall.


    And, I'd much rather be working on #The Artist--Printze, [surprised I didn't have grounding issues with that!] or finally getting to #Floppy-bird, or several others... but... not without a compy... a /properly-grounded/ compy...

  • Data

    Eric Hertz06/14/2020 at 06:26 0 comments

    There've been *big projects* that maybe deserve their own page... but tonight I'mma rant about a "little" [160 friggin' Gigabytes little] discovery...

    I'd fought for literally *months* to first consolidate all my old machines' data onto one drive, then slowly organize it better... this was a big process, requiring going through each drive in reverse-order [newest to oldest], making sure they were organized similarly for comparison/merging/deleting duplicates/keeping the oldest dated duplicate and so-forth via home-brew script, yet retaining the original directory structures /as well/ should I ever have one of those "I remember where it /was/, now where did I put that?" moments... so, now two copies of everything [well, hardlinked] from three different machines, then a copy of the newest machine... then merging/comparison, etc. Then *finally* [literally months later], when that was finished, I could move all that stuff from that latest machine's directory-structure to one that makes more sense now. Doing all this with hardlinks is key, as it means each machine only occupies a little more space than original, despite there being two+ copies on the new machine [and far more on the backup drive] for some time. 

    Anyhow... I *finally* finished all that merging after months, then finally got rid of those old dir-structures... and finally started on, and got a ways in, reorganizing how I'd rather it be organized... despite the nagging feeling something was missing... just couldn't place my finger on what. And, surely that *couldn't* be the case, because I was *very* methodical and meticulous in my process, been literally working-out how I was going to go about it *for years*. I backed-up each machine's *actual* drive to my backup drive before starting. So nothing could be missing... right?

    Well, shit... some brat reminded me of an old project I kinda intentionally forgot... [as best I could, anyhow, being that it affects me still, every day]... which, I guess, is to say it's getting near the time the trauma's worn off enough that I might take a look at what happened... 


    It's GONE.

    Nowhere to be found... like, except for my HaD.io project-page for it, and a way-outdated github upload, like it never friggin' existed.

    Which, frankly, I was /kinda/ OK with, not wanting to revisit the trauma... but if *that* was missing, what else might be...? And so I went looking, and sure enough my favorite project of all-time was missing, as well. So then I was panicked... and *thankfully* eventually found that somehow I just forgot to update its version number, so it was stored in a previous project's folder...


    but... still that nagging feeling... so tonight I got out an old backup drive... and... Yeah.

    Somehow, I can't even begin to recall, the drive from my newest machine, prior to my current one, is *very* *very* different than the last backup made of that machine. I mean *very*. 

    And... the last backup...? 

    Well, it looks like about 160GB of sorting, merging, looking for duplicates, etc. again... only this time I've got two *very* different directory-structures to deal with. WEE!

    But, yeah, there's a *lot* of stuff in there to confirm that "something's missing" feeling I had.

    Lessee... I think I topped-out at 10MB/s between drives before... so... 16,000sec... about 4-5 hours? to transfer to the new backup drive... 

    The backup drive and computer drain the car battery in about that, but here with two drives... Can't start the engine to recharge midway; the starter drops the batt voltage too low to keep the inverter running = unhappy drives... so, I've gotta do this in steps. I *think* rsync, as setup in my backup script, can resume where it left-off. Though I had a problem with CTRL-Cing /something/ like this, then resuming... I think that was 'cp', which left a half-written file whose date was newer than the original, so didn't "update" it when I tried again... yeah, I think my rsync...

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  • Your goop ruined my vias!

    Eric Hertz05/21/2020 at 08:00 3 comments

    This was sitting as a draft, a bit outdated; the printer now works [except for a few dead nozzles], but I'll write about that adventure later....


    been trying to get an old printer running... it's a portable doodad which probably cost some executive a small fortune back in '91. But it's got a sticker on it for $3.50, I'm guessing an old buddy grabbed it from a thriftstore back before he decided guitars were better at picking up chicks than robots, and gave everything to me... I held onto the things that could plausibly one day be useful to <i>someone</i>. 

    This printer I never planned to find a use for, and actually put almost immediately in a box marked "To Donate" [to a thriftstore]. I'd done quite a bit of research buying my desktop photo-quality printer, I'd planned to hold onto for decadeS, and invested nearly $300 to do-so. I didn't even have a drivers' license for another fifteen years after, so I really saw no need for a portable inkjet... Sufficed to say, this printer's been in *my* storage for nearing 20 years... but these days I actually have a use [if not a need] for a *portable* printer... 

    so, cool-beans!

    This thing's actually EXTREMELY well-documented... right down to schematics *and* functional descriptions of the subcircuits. And the service manual's even kinda funny in some places. Its ink cartridges are well-documented, and even still available nearly 30 years later; which actually isn't a huge surprise, because at one time they actually made multiple printers, over several years, even from different brands, which used the same cartridges.

    [This thing shoots out droplets at <b>12 Meters Per Second!!!</b> I wonder how/if that could be used?]

    So... this thing might be a useful tool for some time, if not, frankly, a piece of test equipment which again might be useful for quite some time.

    Here're some thoughts, in the "hacker" realm... old printers like these were *designed* to be compatible with even older printers... plausibly all the way back to the days when one might literally log on to a system with only a keyboard and [dot-matrix? Teletype?] printer... no monitor!

    Simple commands can be used to print one line [maybe even one character?] at a time. E.G. in linux: 'echo "hello" > /dev/lp0' And maybe even one /character/ at a time E.G.2 'echo -n "a" >/dev/lp0'. Nevermind the fact they'll work with *countless* machines of the era, without special drivers!

    Now... imagine how friggin' useful such a thing could be in *this* era... e.g. attached to an arduino... true, it's a little more difficult than using printf [or cout?] statements, because it has a parallel, rather'n serial, interface. But only slightly. in fact, serial to parallel adapters existed for exactly that purpose, and even modifying printf to output parallel isn't particularly difficult. Oh, sure... debug-info may be best-suited to a console/terminal window, but having a physical printout of whatever you want at your fingertips?! How is it this ain't "a thing"?! 

    I dunno what newer printers do, but older ones like this were actually capable of *both* "text-only" <i>and</i> graphics... and that was no small feat on their part. In fact, it was so difficult, printers of the time had processors *on par* with the computers' CPUs of the time, just to handle e.g. rendering inbuilt fonts... Similarly; modems, and even some [e.g. Commodore] floppy drives were full-on computers in themselves, again, on-par with the systems that used them... because those seemingly-simple tasks--many of which we've exiled to "obsolete," such as converting a single byte of ASCII input into Hundreds of pixels on a piece of paper--were once handled by the devices themselves, rather than by the computer. In a way, many older peripherals are *way* smarter than the newer, seemingly more sophisticated ones, which require the *computer* and drivers to do all the work. [See also "winmodems," wavetable MIDI, and "framebuffer console"]...

    So, yahknow......

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  • Projects!

    Eric Hertz05/06/2020 at 19:48 5 comments

    been working on my computer... for months... Long friggin' story involving trying to put it in a sturdy housing so as not to damage anything... and friggin' destroyed the mobo in the process.

    Thankfully, I remembered I had a Raspberry Pi Zero I only turned on once... and, I managed to get that working. Then I powered up the display before powering up the Pi, and my stupid DC barrel plug shorted out something through the HDMI cable. No more HDMI output. But, the activity indicator on the Pi was still blinking(!). So I wired up the composite output, and finally got my old composite LCD running... and it booted! 

    But, barely legible.

    So, long ago I found a destroyed portable DVD player... the screen looked OK and oddly the board had labelled testpoints for all the LCD signals. Eventually got it running off the Pi's "Display Parallel Interface"

    Then I *finally* got to my original goal of packing it into a sturdy housing... two or three months in.

    Then the original original goal of setting up a working system I can actually use for projects...

    Which means backing up four previous dev-systems' drives to this one, and slowly merging everything into one directory structure... which has been a weeks-long process, as well. But, once that's done, I'll finally not have to carry around four old hard drives and try to remember which one has what I'm looking for, etc.

    So, midway through the process I changed a setting so I could legibly fit two terminal windows... and X broke. Really.

    Thankfully, I've been doing a lot of intermediate backups, so it only took a day to figure out which files were modified [aside from the slew of reorganizing I'd done prior], and put them back the way they were. [Note: be *very* careful using the Pi configuration buttons for "small screens" combined with the "Desktop Pager"! It seems one of them changed some settings for/to a display manager which wasn't installed?!]

    Anyhow, *finally* I've got almost all my files from decades[!] of previous systems all in one location that I can finally get to my original original original plan to actually work on some projects; revisiting old ones, building on them, starting new ones with my old dev-environment/libraries/etc. 

    And, another original original original goal of having all my photos [and music] accessible [and sortable] from one location.

    Whew! MONTHS.

  • Sad Scopey

    Eric Hertz04/24/2020 at 06:50 2 comments

    Preface: I've done a lotta diagnoses/troubleshooting since shooting this vid. I've got some pretty solid theories and a lot of interesting and confirming observations *after* this vid was filmed...

    Key factors:

    I'm running it off a cheapo car inverter, which appears to output a bipolar square-wave.

    The "reversing" of the trace seems to occur when the inverter pulse rises, when it falls, it reverses again [back to normal].

    This seems confirmed by the fact that when I load the inverter more [e.g. turning up the scale illumination lights] the width of the "reversal" increases; thus, the inverter is changing its pulse-width to compensate for the load.

    This is further confirmed by the fact that when a probe is floating, it picks up Sharp Spikes at each edge of the reversal.

    Interestingly, everything acts *almost* normal when the engine's running; the output voltage, as measured by a multimeter on AC, is roughly the same [around 119V] but the PWM[?] output duty-cycle varies, causing the "reversal" to lengthen or shrink.

    Despite roughly the same measured output voltage of the inverter, the input-voltage [engine running=~14V, engine off=~11.5V after long wires] also seems to effect voltages internal to the 'scope: when the engine's *not* running, the 'scope screen seems shifted up and left, the "reversal" is large. When the engine's running, things seem *almost* normal, with a *slight* "rreversal"

    Also, interestingly, when switched to digital mode, obviously, the "reversal" *can't* happen, as the samples won't reverse in index-numbers. Instead, it shows as a varying sinusoidal-ish voltage, even when the input is set to GND.

    This suggests to me that the 'scope's internal grounding may not be adequate for cases like this where the input power is non-sinusoidal. There may be some inadequate power-filtration in different locations; e.g. the digital circuitry may be well-regulated, but the ADC's connection to the analog circuitry may be less-so. It wouldn't be apparent with a sine-wave power-supply input, because filtration to the analog measurement circuitry may be aimed at 60Hz. Yet a square-wave filtered *to* 60Hz would result in a 60Hz wave. I dunno, exactly.

    I do know I like this 'scope a lot, so I may ponder a switching power-supply upgrade... There may be some caveats; some functions seem to look at the power source zero-crossings. Especially, e.g. the "Line trigger." 

    There also seem to be *multiple* AC taps off the transformer, so I've yet to determine if high-voltage stuff [like the CRT anodes] are derived from DC or the AC input.

    In the now... it *might* work well-enough on this inverter while the engine's running to do the measurements I set-out to do [rough-estimate timings for an LCD], if I keep in mind the present functional-oddities.

  • Turn your tv into a shadow camera

    Eric Hertz02/28/2020 at 02:04 0 comments

    sorta thing coulda been in MIMMs' books, Boy's Life, Popular Electronics, etc... Hard to goog, though.

    Heh, wire up Hue and Sat, and a tiny bit more circuitry on Value/Intensity, and yah got a shadow-puppet blue/green-screen from the DVD!

    Ah, CRT only... duh. *maybe* STN LCDs.

    Also, record in realtime with yer vcr ;)

    Inspired by:


    Apparently, there is a scanner made from picture tubes [rather'n camera tubes] "Flying Spot Scanner" [see wikipedia].