Posts Tagged Memo to Self

Pogo Pins

A Pogo Pin reference may be useful:

  • P.. and R.. refer to Pin and Receptacle (a.k.a. socket), respectively
  • Pxx  and Rxx = nominal pin diameter in 0.01 mm units: P50 = 0.48 mm

For pins, the suffix -hn indicates pin head shape, the most useful of which may be:

  • B1: 45° cone
  • J1: dome end
  • Dx: large dome, also 1D
  • Gx: cylinder
  • Ex: large 90° cone, sometimes 1E
  • T2 – large chisel

For sockets, the suffix -ntl gives:

  • n – entry shape: 1 = shaped entry, 2 = straight entry
  • t – termination: C = crimp, S = solder, W = wire
  • l – length of wire in 100 mm units: 7 = 700 mm

From what I can find on eBay, all pins have 6 mm travel with typically 75 / 100 / 180 g spring force.

A picture ripped from the reference to forestall link rot:

P75 Spring Test Probes

P75 Spring Test Probes

Memo to Self: US-based eBay sellers charge three times more than Chinese sellers, but deliver in one-third the time.



Scrap Metal Prices

Earlier this year, I finally hauled a pile o’ scrap metal to the recycler. For future reference, here’s what clattered down on the scale:

Scrap Metal Prices - 2017-04

Scrap Metal Prices – 2017-04

I think the IRONY tag means ferrous bits & pieces in the mix. There’s a powerful motivation to hand them clean copper scrap, although I stop just after cutting off soldered pipe fittings and before stripping insulation.

Memo to Self: Next time, ask about PCBs and gold-plated connectors.


Fordham FG-801 Function Generator Power Switch

The power switch in my trusty Fordham FG-801 Function Generator failed with an accumulation of oxidation / crud on the contacts. That’s fix-able, but the switch contained not one, but two powerful springs, and puked its guts all over the floor around the Squidwrench Operating Table. Even with (a preponderance of) the parts in hand, I couldn’t figure out how to reassemble the thing; the only way out was to replace the switch.

The OEM switch had a 0.360+ inch diameter pushbutton that fit into a ⅜ inch hole and, alas, my remaining stock of line-voltage switches had toggle levers and used ¼ inch holes. So I converted a bit of aluminum rod into a suitable bushing:

Fordham FG-801 Fn Gen - new switch hardware

Fordham FG-801 Fn Gen – new switch hardware

The lock washer in the middle started with a much wider tab that I filed down into a tooth for the dent from a #2 center drill. Protip: center drills don’t walk off like twist drills, even when you hand-hold the front panel at the drill press with all the electronics dangling below.

The bushing dimension doodle:

Fordham FG-801 Function Generator - Replacement Switch Bushing

Fordham FG-801 Function Generator – Replacement Switch Bushing

The internal wiring routes the 120 VAC line conductor to the switch, then to the fuse, then to the transformer. I don’t know whether it’s better to have an unfused switch or an unswitched fuse (surely there’s a UL spec for that), but I didn’t change anything. The new switch, being slightly smaller and mounting directly on the panel, required a new wire (the blue one) from the fuse:

Fordham FG-801 Fn Gen - power switch - installed

Fordham FG-801 Fn Gen – power switch – installed

The OEM switch mounted on two round brass standoffs and, wonder to tell, the new switch fit between them!

From the front, the new switch looks like it grew there:

Fordham FG-801 Fn Gen - switch in action

Fordham FG-801 Fn Gen – switch in action

The PCB mounts to the top of the case with one screw and four hexagonal brass standoffs. The standoffs have 6-32 tapped holes on one end and a 6-32 stud on the other; one of those stud had broken off. A 6-32 stainless steel screw secured in a clearance hole with a dab of epoxy solved that problem:

Fordham FG-801 Fn Gen - standoff stud

Fordham FG-801 Fn Gen – standoff stud

I stood it vertically and tweaked the screw to be perpendicular while the epoxy cured.

Memo to Self: The next time around, put a nut on the stud to make sure the answer comes out right. I didn’t do this time to avoid epoxying the nut to the standoff.


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Raspberry Pi: Forcing VNC Display Resolution

You can use VNC with a headless Raspberry Pi, but, absent a display with which to negotiate the screen resolution, X defaults something uselessly small: 720×480. To force a more reasonable resolution, edit /boot/config.txt and set the framebuffer size:


You can use a nonstandard resolutions, as with the 1920×1280 that fits neatly on my 2560×1440 landscape monitor, but getting too weird will surely bring its own reward. When you plug in a display, X will ought to negotiate as usual for the highest resolution the display can handle.

The System Configuration dialog has a “Resolution” button offering standard resolutions:

RPi display resolution configuration

RPi display resolution configuration

The shiny RPi Pixel UI bakes the RealVNC server directly into whatever handles the startup process these days, rendering all previous recommendations about forcing VNC resolutions inoperative. I found the trick of editing the config file on StackExchange after the usual flailing around.

Memo to Self: Remmina (the VNC client I use in XFCE on my desktop PC) doesn’t respond well to having the VNC server shut down while it’s connected. Fire up a command prompt, enter this:

sleep 10 ; sudo reboot

Then, quick like a bunny, disconnect the VNC session.

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Blog Backup

Recent news about Dropbox removing its Public folder feature reminded me to do my every-other-month blog backup. Wordpress provides a method to “export” the blog’s text and metadata in their XML-ish format, so you can (presumably) import your blog into another WordPress instance on the server of your choice. However, the XML file (actually, ten of ’em, all tucked into a paltry 8 MB ZIP file) does not include the media files referenced in the posts, which makes sense.

Now, being that type of guy, I have the original media files (mostly pictures) tucked away in a wide variety of directories on the file server. The problem is that there’s no easy way to match the original file to the WordPress instance; I do not want to produce a table by hand.

Fortunately, the entry for each blog post labels the URL of each media file with a distinct XML tag:


Note the two leading tabs: it’s prettyprinted XML. (Also, should you see escaped characters instead of < and >, then WordPress has chewed on the source code again.)

While I could gimmick up a script (likely in Python) to process those files, this is simple enough to succumb to a Bash-style BFH:

grep attachment_url *xml > attach.txt
sed 's/^.*http/http/' attach.txt | sed 's/<\/wp.*//' > download.txt
wget --no-verbose --wait=5 --random-wait --force-directories --directory-prefix=/where/I/put/WordPress/Backups/Media/ -i download.txt

That fetches 6747 media files = 1.3 GB, tucks them into directories corresponding to their WordPress layout, and maintains their original file dates. I rate-limited the download to an average of 5 s/file in the hope of not being banned as a pest, so the whole backup takes the better part of ten hours.

So I wind up blowing an extra gig of disk space on a neatly arranged set of media files that can (presumably) be readily restored to another WordPress instance, should the occasion arise.

Memo to Self: investigate applying the -r option to the base URL, with the -N option to make it incremental, for future updates.


Respooling Stainless Steel Thread

For various reasons, I needed a smaller quantity of that stainless steel thread / yarn, so I mooched an empty spool from Mary, ran a bolt through it with washers + nut on the far end, chucked the bolt in the lathe, and ran the spindle backwards at the slowest speed:

Stainless steel thread - smaller spool

Stainless steel thread – smaller spool

I started by letting the big spool unroll from the side, but that produced horrible twists in the slack thread. Remembering the lesson from our previous thread spool adventure, I put it on the floor and let the thread pull from the top:

Stainless steel thread - unwinding spool

Stainless steel thread – unwinding spool

It still accumulated a huge twist between the two spools, even while guiding it hand-over-hand onto the rotating spool. Either the factory lays the thread on the large spool with a built-in twist or, more likely, a multi-strand steel thread behaves like a spring, no matter what anybody wants, and comes off the spool with a nasty case of inherent vice.

Memo to Self: don’t let stainless steel thread slide through your hands under power, because some of the fuzz visible in the top picture will stay with you.

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Monthly Science: Bottled Water Evaporation

These emerged from a hidden corner of a basement shelf, where they’ve been sitting undisturbed for far too long:

Bottled Water Evaporation

Bottled Water Evaporation

I’ve known for a while that the PETE plastic used for nearly all bottles isn’t completely waterproof, but never had occasion to measure the results.

The laser-etched date code  on the bottles says they “expired” in late August 2012, so, assuming one year of shelf life, they’ve been quietly evaporating for five years.

Sampling a few bottles shows a nearly uniform weight of 459 g. A drained bottles weighs 13 g, so let’s say the bottles now contain 445 g of water. They should start out with 500 g, although I’d be mildly surprised if it wasn’t a bit over that to prevent some dork from complaining about getting only 498 g.

Rounding in all the right directions, losing 60 g during five years works out to a tidy 1 g/month in a basement room at 60% RH.

The surface area of those wonderfully convoluted bottles might be 300 cm², so they lose 3 mg/cm²·month.

They’re near enough to 0.10 mm thick, which I’m sure is a compromise between reducing weight (and, thus, plastic cost) and incurring messy failures during normal handling. The evaporation rate surely varies as an inverse exponential of thickness, but I’m not going there.

I’m certain water bottlers know those numbers to several decimal places and can plot them versus all the interesting variables.

Memo to Self: don’t lose track of the water bottles!