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Posts Tagged Memo to Self

Walnut Husk Fly Damage

A recent Amazon purchase of three 3 lb bags of walnuts from a known-good seller arrived with many damaged nuts:

Damaged walnuts - detail
Damaged walnuts – detail

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!

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DRV8825 Stepper Driver: Forcing Fast Decay Mode in a (Likely) Counterfeit Chip

The DRV8825 stepper driver chip defaults to mixed decay mode, which TI defines thusly:

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:

3018 XY - Mixed Fast - 24V - 10mm-min 12V 1A-div
3018 XY – Mixed Fast – 24V – 10mm-min 12V 1A-div

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:

3018 X - ripple 1 step - 18V - A0 B-90 500mA-div
3018 X – ripple 1 step – 18V – A0 B-90 500mA-div

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:

DRV8825 pinout
DRV8825 pinout

Which sits on an inconveniently skinny little PCB pad, fifth from the left on the bottom:

DRV8825 PCB - open Decay pin
DRV8825 PCB – open Decay pin

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:

DRV8825 DECAY pin settings
DRV8825 DECAY pin settings

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:

DRV8825 PCB - Decay pin wired low
DRV8825 PCB – Decay pin wired low

On the scale of TSSOP packages, even 30 AWG Wire-Wrap wire looks like a bus bar!

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.

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Exhibit Hand-Out Cards: QR Version

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:

Mad Science Fair - handout cards
Mad Science Fair – handout cards

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:

  • Blog search QR code: astable
  • Blog search QR code: bowl-of-fire
  • Blog search QR code: dso150
  • Blog search QR code: halogen
  • Blog search QR code: hp7475a
  • Blog search QR code: tubes

Memo to Self: put the cards in the Big Box o’ Stuff the night before.

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Transistor Leads vs. Antistatic Foam

Why you shouldn’t use antistatic foam for long-term storage:

Anti-static foam - decades of corrosion
Anti-static foam – decades of corrosion

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!

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Broken Spoke

On the drive side, of course:

Tour Easy - broken rear spoke
Tour Easy – broken rear spoke

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.

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Vacuum Tube Lights: Triode

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:

Triode - Purple-green phase
Triode – Purple-green phase

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:

Triode - Nano PCB - white strips
Triode – Nano PCB – white strips

Two layers of 3M outdoor-rated foam tape clear the bottom-side components and, based on current evidence, its stickiness should stick forever more:

Triode - Nano PCB - 3M strips
Triode – Nano PCB – 3M strips

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:

Triode - Nano installed
Triode – Nano installed

Blobs of hot melt glue affix the SK6812 and wiring to the socket:

Triode - socket wiring
Triode – socket wiring

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:

Triode - plate cap plug
Triode – plate cap plug

Octal tubes have an opaque Bakelite base, so I devoted some Quality Shop Time™ to the post:

Triode - base tip exposed
Triode – base tip exposed

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:

Triode - finished base
Triode – finished base

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:

Triode - cyan-purple phase
Triode – cyan-purple phase

Memo to Self: use the shell drill to avoid nicking the evacuation tip!

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Injured Arm Support Table: Narrow Version

For reasons not relevant here, I recently conjured a pair of tables to support an injured arm (ours are OK!) in the bathroom: one table fitting in the narrow space adjacent to a toilet and the other across the threshold of a walk-in / sit-down shower.

The raw material came from a plastic side table intended for outdoor use:

Arm Supports - OEM Patio table
Arm Supports – OEM Patio table

That’s the Patriotic Blue version, which seemed the least offensive of the colors on offer at the local store.

The plastic pieces unsnap easily enough:

Arm Supports - top panel disassembly
Arm Supports – top panel disassembly

The legs also come apart by pulling outward at the crossover points. You may need to clean the flashing from all the joints, as they’re only as finished as absolutely necessary.

A table about half the width seemed about right, so I sawed the two top plates off their struts, then angled the strut ends to match the new leg angle:

Arm Supports - trimming table struts
Arm Supports – trimming table struts

Because it’s now completely floppy, I drilled holes for 5 mm screws through the struts:

Arm Supports - cross-drilling struts
Arm Supports – cross-drilling struts

In the process, I discovered stainless steel nyloc nuts tend to gall on stainless steel screws:

Galled stainless steel cap screw and nyloc nut
Galled stainless steel cap screw and nyloc nut

I lost a pair of screws + nuts before I got a clue and began adding a drop of machine oil to each screw before tightening the nuts. Haven’t had that problem with the 3 mm SS screws, so there’s always something new to learn.

With all the screws in place, the half-table becomes a rigid contraption:

Arm Supports - narrow table - bottom view
Arm Supports – narrow table – bottom view

The top looks like it’s suffering from severe barrel distortion, but it really started out looking that way:

Arm Supports - narrow table - overview
Arm Supports – narrow table – overview

The slat sides are all curved, except the far edge that was once in the middle of the table and now fits against the wall.

It may be slightly too short, but we can stack foam slabs on the top, probably held in place with cable ties.

Memo to Self: lube all the stainless steel screws!

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