Posts Tagged Memo to Self
While washing our ancient electric crock pot (“slow cooker”), I wondered how corroded the inside of the steel shell had become. A simple nut secured the base plate and unscrewed easily enough, whereupon what I thought was a stud vanished inside the shell.
The shell wasn’t rusty enough to worry about, but the stud turned out to be a crudely chopped-off thumbscrew on a springy rod pulling the base toward the ceramic pot:
Evidently, they pulled the thumbscrew through the base, tightened the nut, then cut off the thumbscrew flush with the nut.
I desperately wanted to drill a hole in a new thumbscrew and repeat the process, but I no longer have a small drawer full of assorted thumbscrews. So I must either lengthen the existing thread just enough to complete the mission or build a screw from scratch.
The thumbscrew is threaded 10-24, I have a bunch of 10-32 threaded inserts, so pretend they have the same thread diameter and tap one end to 10-24:
Jam the new threads on the thumbscrew and jam a 10-32 setscrew into the un-wrecked end:
You can see the surface rust in there, right?
Make a Delrin bushing to fit around the insert poking through the base:
Reassemble the internal bits with permanent Loctite, top with a nyloc nut, and it’s only a little taller than the original nut:
The setscrew let me hold the new “stud” in place while torquing the nut, plus it looks spiffy.
Memo to Self: If it ain’t broke, don’t look inside. Hah!
Surprisingly, both Amazon and eBay lack useful thumbscrew assortments …
A recent Amazon purchase of three 3 lb bags of walnuts from a known-good seller arrived with many damaged nuts:
The damage matches what I read about Walnut Husk Fly infestations: shriveled kernels and terrible taste.
In round numbers, I found 8 oz of damaged nuts in each 3 lb bag, enough to ruin the entire batch. The seller immediately refunded the purchase price for all three bags, so there’s that.
It’s definitely not one of the counterfeit products plaguing Amazon, but I wonder why that lot didn’t fail incoming inspection.
I’m loathe to buy more walnuts for a while, though.
Memo to Self: Always inspect incoming purchases, even from reputable sellers!
Mixed decay mode begins as fast decay, but at a fixed period of time (75% of the PWM cycle) switches to slow decay mode for the remainder of the fixed PWM period. This occurs only if the current through the winding is decreasing (per the indexer step table); if the current is increasing, then slow decay is used.
The 24 V supply on the CNC 3018-Pro provides too much voltage for the motors, because slow decay mode can’t handle those rising slopes:
Note that “rising” means the current increases with either polarity from 0 A at the midline. The DRV8825 uses a MOSFET H-bridge to drive winding current in either direction from the +24 V motor supply voltage.
Both traces show motor winding current at 1 A/div, with the XY axes creeping along at 10 mm/min (thus, 7.1 mm/min each). The upper trace is the X axis, with a stock DRV8825 module in mixed decay mode. The lower trace is the Y axis, with its DRV8825 hacked into fast decay mode.
The basic problem, about which more later, comes from the current rising too fast during each PWM cycle:
V = L di/dt di/dt = 24 V / 3 mH = 8 kA/s
The first 1:32 microstep away from 0 calls for 5% of max current = 50 mA at a 1 A peak. The DRV8825 datasheet says the PWM typically runs at 30 kHz = 33 µs/cycle, during which the current will change by 270 mA:
267 mA = 8 kA/s × 33.3 µs
Some preliminary measurements suggest these (probably counterfeit) DRV8825 chips actually run at 16 kHz = 66 µs/cycle:
During those cycles the current can increase by more than 500 mA. The first scope picture shows an abrupt increase to maybe 700 mA, so, yeah, that’s about right.
Having the wrong current in one winding means the motor isn’t positioned correctly during those microsteps. The 3018-Pro runs at (an absurd) 1600 µstep/mm, so being off by even a full step isn’t big deal in terms of positioning.
The real problem comes from running nearly 1 A through both windings. Those little motors run really hot: they’re dissipating twice what they should be.
Anyhow, the pin layout shows the DRV8825 DECAY mode selection on pin 19:
Which sits on an inconveniently skinny little PCB pad, fifth from the left on the bottom:
Memo to Self: Don’t make that mistake when you lay out a PCB. Always put a little pad or via on a disconnected pin, so as to have a hand-soldering target big enough to work with.
The objective is to pull the pin high:
Pin 15, in the lower left corner, provides the output of a 3.3 V linear regulator, with its PCB trace connected to the left side of the ceramic cap:
Those are two different PCBs. The crappy TI logos, not easily visible in those low-res pix, on both ICs suggest they’re by-now-typical counterfeits, so seeing a factor-of-two difference in PWM frequency isn’t surprising.
I’ve finally had it beaten into my head: any public exhibition requires paper handouts, if only for younger folks who are too shy to ask questions. Paper may seem obsolete, but it serves as a physical reminder long after the sensory overload of a busy event fades away.
Hence, I made up cards describing my exhibits at the HV Open Mad Science Fair, each sporting a QR code aimed at far more background information than anybody should care about:
The QR codes come from one-liners:
qrencode https://softsolder.com/?s=dso150 -s 5 -d 300 -o dso150.png
So, go ahead, shoot ’em with your phone:
Memo to Self: put the cards in the Big Box o’ Stuff the night before.
Why you shouldn’t use antistatic foam for long-term storage:
The lump emerged from Mad Phil’s parts stash, now residing under a bench at Squidwrench. The
952 date code on the HEP802 JFET suggests he tucked it in around 1980; you’re looking at nigh onto four decades of corrosion.
Memo to Self: use it or lose it!
On the drive side, of course:
I’d noticed some brake drag on our last few rides, but forgot to check until I saw the rim wobble while extracting images from the rear camera.
It’s a lot easier to fix in the Basement Shop than on the road. After nigh onto a decade since replacing the last broken spoke, perhaps this is a harbinger of doom to come.
Memo to Self: spoke tension is now 20-ish on the drive side, 15-ish on the left.
With the wrecked 5U4GB safely in the trash, I popped a smaller, somewhat less stately triode from the Big Box o’ Hollow-State Electronics and wired it up with a pair of SK6812 RGBW LEDs:
The tube’s markings have long since vanished, but, at this late date, all that matters is an intact glass envelope!
After two years, the ordinary white foam tape holding the knockoff Arduino Nano lost most of its sticktivity and easily popped off the 3D printed base:
Two layers of 3M outdoor-rated foam tape clear the bottom-side components and, based on current evidence, its stickiness should stick forever more:
The alert reader will notice the mis-soldered 1 kΩ SMT resistor above-and-right of the CH340 USB interface chip. I think those two resistors are the isolators between the 328P microcontroller and the CH340, letting you use the TX and RX lines as ordinary I/O without killing either chip.
Despite the mis-soldering, it evidently passed their QC and works fine. Seeing as how I didn’t notice it until just now, it’ll remain in place until I must open the lamp base for some other reason, which may never happen.
The data output is now on pin A5, to match the rest of the glowing widgetry:
Blobs of hot melt glue affix the SK6812 and wiring to the socket:
The original “plate cap” wiring ran directly through a hole in the hard drive platter, which I embiggened for a 3.5 mm panel-mount headphone jack. The knurled metal plug looms next to this smaller tube, but it looks better (in a techie sense) than the raw hole:
Octal tubes have an opaque Bakelite base, so I devoted some Quality Shop Time™ to the post:
Although I’d made a shell drill for 5U4’s base, this base was so crumbly I simply joysticked the spinning cutter around to knock off the rest of the post:
The shell drill would open the bottom to admit a bit more light. I may do that to see if it makes any visible difference.
I didn’t expect the serrations in the top mica plate to cast interesting patterns around the platter:
Memo to Self: use the shell drill to avoid nicking the evacuation tip!