Archive for January, 2017

60 kHz Preamp: Board Holder

A cleaned up version of my trusty circuit board holder now keeps the 60 kHz preamp off what passes for a floor in the attic:

Preamp in attic

Preamp in attic

The solid model became slightly taller than before, due to a serious tangle of wiring below the board, with a narrower flange that fits just as well in the benchtop gripper:

Proto Board - 80x110

Proto Board – 80×110

Tidy brass inserts epoxied in the corners replace the previous raw screw holes in the plastic:

Proto Board Holder - 4-40 inserts and screws

Proto Board Holder – 4-40 inserts and screws

The screws standing on their heads have washers epoxied in place, although that’s certainly not necessary; the dab of left-over epoxy called out for something. The screws got cut down to 7 mm after curing.

The preamp attaches to a lumpy circle of loop antenna hung from the rafters and returns reasonable results:

WWVB - morning - 2017-01-16

WWVB – morning – 2017-01-16

The OpenSCAD source code as a GitHub Gist:

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60 kHz Preamp: First Pass

Encouraged by the simulation, the 60 kHz preamp hardware sprawls over a phenolic proto board:

60 kHz preamp board - fake antenna

60 kHz preamp board – fake antenna

The inductors and resistors hanging off the screw terminals produce more-or-less the same impedance  as the real loop antenna. The alligator clips connect a function generator to the secondary winding of a current transformer (used backwards), thus injecting a wee differential signal into the “antenna”.

The clump of parts in the lower left knock the 24 VDC wall wart down to 20 V and produce a 10 V virtual ground in the middle:

60 kHz Preamp - power supply - Kicad schematic

60 kHz Preamp – power supply – Kicad schematic

The LEDs give a cheerful indication that the power supplies have reported for duty, plus apply a minimum load to the LM317 while I was tinkering. The heatsink gets tolerably warm, so I should dial back or disconnect the LEDs to reduce the load.

The preamp hardware matches the simulated layout, with a few extra bits tossed in:

60 kHz Preamp - Kicad schematic

60 kHz Preamp – Kicad schematic

The weird values come from whatever 1% resistors and silver-mica caps emerged from the heap. The 27 V Zener diodes and 5 kΩ resistors may or may not protect the instrumentation amp inputs from lightning-induced transients.

Because the HP8591 analyzer’s tracking generator starts at 100 kHz, I fed the DDS function generator into the preamp, manually stepped the frequency in 250 Hz increments, and had the analyzer show the maximum response of 19 separate sweeps:

Preamp - max hold - 250 Hz steps

Preamp – max hold – 250 Hz steps

That was tedious and, no, it’s not a comb filter: the actual response skates across the peaks of all those bumps.

The marker shows the preamp bandwidth is 2 kHz, roughly what the simulation predicts; the extremely tight span of that plot makes it look a lot flatter that the usual presentation.

Tightening the span even more shows an unexpected effect:

Preamp - 120 Hz modulation

Preamp – 120 Hz modulation

Those sidebands at ±120 Hz (probably) come from power-line magnetic fields into the “antenna”, because the magnetic field strength depends on the absolute value of the voltage. If they came from the signal generator, they’d be at ±60 Hz: the waveform amplitude depends directly on the voltage.


Kenmore 158: Presser Foot Tweak

After watching Mary fiddle with the shrunken presser foot screw, I tapered the tip as a guide into the hole:

Presser Foot Screw - tapered tip

Presser Foot Screw – tapered tip

A hint-and-tip (which I cannot, alas, find again) suggested making bushings to simplify trimming screws in the lathe. A rim on the bushing aligns it with the front of the jaws, the screw threads into the central hole with a jam nut locking it in place, then you can turn / shape / file the end of the screw just beyond bushing with great support and a total lack of drama.

For the moment, I just aligned the screw in the tailstock drill chuck, crunched the three-jaw spindle chuck on the screw head, backed off the tailstock, took unsupported sissy cuts, and it was all good:

Presser Foot Screw - chuck alignment

Presser Foot Screw – chuck alignment

Gotta make those bushings!

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LED Filaments: Whoops

Five bucks delivered three sets of five warm-white LED filaments from halfway around the planet:

LED Filaments - 3x5 sets

LED Filaments – 3×5 sets

Unfortunately, the “Top Rated Plus” eBay seller just popped three ziplock baggies into an unpadded envelope and tossed it in the mail:

Unpadded LED Filament Envelope

Unpadded LED Filament Envelope

Which had pretty much the result you’d expect on the glass substrates within:

Broken LED Filament 1

Broken LED Filament 1

Turns out every single filament had at least one break:

Broken LED Filament 3

Broken LED Filament 3

Indeed, some seemed just as flexy as the silicone cylinder surrounding the pulverized substrate.

I reported this to the seller, with photographs, and got a classic response:

can you use?

No, I cannot imagine a use for broken LED filaments.

The seller proposed shipping replacements that would might arrive just after the eBay feedback window closed. I proposed refunding the five bucks. The seller ignored that and sent the replacements in an untracked package “as it is an economical shipping, we have to reduce our loss, so is it ok?”.

No, it’s not, but he / she / it didn’t actually intend that as a question.

Were the filaments intact, they’d pass 15 mA with 50 to 60 V applied in one direction or the other, for 1 W average dissipation. That’s probably too high for prolonged use in air (spendy bulbs with similar LEDs have argon / krypton fill for better heat transfer), but I can surely throttle them back a bit.

Perhaps the replacements will arrive before the feedback window closes?

I did order another batch from a different seller that might arrive intact before then. We shall see…


Raspberry Pi CPU Temperature Watcher

Having just put a headless Raspberry Pi in the attic, the chip temperature is of some interest. Doing this in an SSH session comes in handy:

watch 'echo "scale=1 ; d = $(cat /sys/class/thermal/thermal_zone0/temp) / 1000 ; print d , \" °C\n\" " | bc'
# blank line to ensure the underscore displays correctly

Raspbian doesn’t have the bc calculator by default, so do that first.

For whatever it’s worth, the Pi starts out at 10 °C and warms over 60 °C under heavy load:

Every 2.0s: echo "scale=1 ; d = $(cat /sys/class/thermal/thermal_zone0/temp) / 1000 ; print d , \" °...  Sat Jan 14 19:58:59 2017

61.7 °C

It ticks along in the mid 30s under light load.

You can run all that in one tab of a terminal window through VNC. If you’ve got that much GUI goin’ on, just add a thermal monitor in the panel and be done with it.



Pot Lid Repair

For reasons not relevant here, we (temporarily) have a set of pots with glass lids. One of lids had a remarkable amount of crud between the glass and the trim ring under the knob, which turned out to be corrosion falling off the screw. Trying to remove the screw produced the expected result:

CKC Pot Lid - broken screw in handle

CKC Pot Lid – broken screw in handle

For whatever reason, they used an ordinary, not stainless, steel screw:

CKC Pot Lid - corroded screw

CKC Pot Lid – corroded screw

I figured I could mill the stub flat, drill out the remainder, install a new insert, and be done with it. The knob has a convex surface and, even though this looked stupid, I tried clamping it atop a wood pad:

CKC Pot Lid - precarious clamping

CKC Pot Lid – precarious clamping

Two gentle cutter passes convinced me it was, in fact, a lethally stupid setup.

Soooo, I poured some ShapeLock pellets into a defunct (and very small) loaf pan, melted them in near-boiling water, and pressed the knob into the middle, atop some stretchy film to prevent gluing the knob in place:

CKC Pot Lid - ShapeLock bedding

CKC Pot Lid – ShapeLock bedding

That’s eyeballometrically level, which is good enough, and the knob sits mechanically locked into the room-temperature plastic slab. Clamping everything down again makes for a much more secure operation:

CKC Pot Lid - clamped ShapeLock fixture

CKC Pot Lid – clamped ShapeLock fixture

A few minutes of manual milling exposes the original brass insert molded into the knob, with the steel screw firmly corroded in the middle:

CKC Pot Lid - screw stub milled flat

CKC Pot Lid – screw stub milled flat

Center-drill, drill small-medium-large, and eventually the entire insert vanishes in  a maelstrom of chips and dust:

CKC Pot Lid - OEM insert removed

CKC Pot Lid – OEM insert removed

Run a 10-32 stud into an insert, grab in drill chuck, dab JB Kwik around the knurls, press in place while everything’s still aligned in the Sherline, pause for curing, re-melt the ShapeLock, and the insert looks like it grew there:

CKC Pot Lid - new insert installed

CKC Pot Lid – new insert installed

Wonder to tell, a 1 inch 10-32 screw fit perfectly through the pot lid into the knob, with a dab of low-strength Loctite securing it. Reassemble everything in reverse order, and it’s all good:

CKC Pot Lid - repaired knob

CKC Pot Lid – repaired knob

Well, apart from those cracks. I decided I will not borrow trouble from the future: we’ll let those problems surface on their own and, if I’m still in the loop, I can fix them.


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Raspberry Pi vs. Avahi

It turns out that the various Avahi daemons performing the magick between whatever.local names and dotted-quad addresses for Raspberry Pi descend into gibbering madness when confronted with:

  • One name corresponding to multiple IP addresses
  • One IP address used for multiple MAC addresses
  • Multiple names for one IP address
  • Multiple names for one MAC address
  • Multiple IP addresses for one MAC address
  • Multiple MAC addresses for one IP address
  • Any and all combinations of the above at various times

The least of the confusion involved an incorrect IP address linked to a familiar name pulled from deep history by a baffled daemon doing the best it can with what it thinks it knows. Despite what I concluded, rather early in the process, there’s no real error, other than my performing what amounted to a self-inflicted fast-flux nameserver attack.

Anyhow, I devoted the better part of an afternoon to sorting out the mess, which involved labeling all the streaming radio players with their MAC addresses and rebooting them one-by-one to allow all the daemons time to recognize the current situation:

Raspberry Pi 3 - WiFi MAC address

Raspberry Pi 3 – WiFi MAC address

That label corresponds to the Pi 3’s on-board WiFi adapter.

For Pi 2 boxen, the MAC address travels with the WiFi adapter jammed into a USB port:

SunFounder WiFi Adapter - MAC address

SunFounder WiFi Adapter – MAC address

I didn’t label the (unused) Ethernet jacks, figuring I’d solve that problem after it trips me up.