The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Electronics Workbench

Electrical & Electronic gadgets

Raspberry Pi WiFi Adapters

One might be forgiven for thinking these two USB Wifi adapters are essentially identical:

USB Wifi adapters

Turns out the SunFounder RT5370 (on the top, with the stylin’ curved case) has better performance than the Wifi With Antenna (on the bottom, with full-frontal chunk goin’ on), by a not inconsiderable 5 to 10 dB. Boosting the received power level in the fringe areas of our house from -70 dBm to -63 dBm makes all the difference between not working and steady streaming.

The built-in WiFi antenna on a Raspberry Pi 3 ticks along 10 dB lower, with -80 dBm (10 pW!) at the receiver making for poor communication: a Pi 3 works perfectly within reasonable line-of-sight of the router (even through our wood floor) and wakes up blind in fringe areas. Hacking an external antenna probably helps, but definitely isn’t a net win compared to ten bucks worth of USB adapter.

The wavemon utility (it’s in the Raspbian repo) comes in handy for figuring that sort of thing.

There is, of course, no way to determine anything important about the adapters from their product descriptions, which are essentially identical, right down to the price. Neither have any product identification on their cases. The back of the package for the SunFounder gadget gives some specs, none of which may mean anything (clicky for more dots):

SunFounder RT5370 USB WiFi Adapter Specs

I ordered another SunFounder adapter, Just In Case it comes in handy, with the hope that both behave the same way.

2017-01-11
Raspberry Pi 3 Reset Switch

The (relatively) new Raspberry Pi 3 PCB layout puts the Run header in a different location than in the Pi 2, but a minute of filing gnaws a suitable opening:

Raspberry Pi 3 – Reset Switch

As before, a hot-melt glue blob holds the switch in place. I’d prefer a black case, if only to hide the blob, but clear-ish is what’s available right now.

Remember those orderly shutdowns, even at the cost of a keypad button!

2017-01-10
Raspberry Pi Streaming Radio Player: Ignoring a Missing Volume Knob
The Dell AC511 USB SoundBars have volume control knobs, which this udev rule turns into the /dev/input/volume device:
```
ATTRS{name}=="Dell Dell AC511 USB SoundBar", SYMLINK+="input/volume"
```
I recently wanted to use an ordinary USB “sound card” that did not, of course, have a volume knob:

Sabrent USB Audio Adapter

This hack skips the configuration that makes the knob’s events visible to the Python program:
```
import os.path

... snippage ...

# if volume control knob exists, then set up its events

VolumeDevice = '/dev/input/volume'

vp = select.poll()
if os.path.exists(VolumeDevice):
  v = InputDevice(VolumeDevice)
  v.grab()
  vp.register(v.fileno(),select.POLLIN + select.POLLPRI + select.POLLERR)
```
It turns out that if you never register a device with the event polling interface, then the interface never reports any events and the rest of the code remains blissfully undisturbed: the non-existent knob doesn’t do anything, while the volume control buttons on the keypad continue to function as usual.

The end result of this fiddling puts a Raspberry Pi 2 Model B to work as a streaming player on my Electronics Workbench, untethering the laptop from those powered speakers:

RPi 2 Streaming Player – USB sound gadget

It’s a shame that USB audio gadget is so big, because it crowds out standard USB plugs to the side.

The most satisfactory LED configuration for a translucent case with an external WiFi adapter seems to be:
```
dtparam=pwr_led_trigger=cpu0
dtparam=act_led_trigger=mmc0
```
The rest of the code remains unchanged as shown in that GitHub Gist.

Bomb the bass!
2017-01-03
ATX Lithium Ion 18650 Cell Capacity

The 2016-11A and 2016-11B cells produced the overlapping red and green curves, with the gritty section due to crappy battery pack connections:

Li-Ion 18650 cells – ATX prot – bare – Ah scale – 2016-12-17

The lower curve comes from an old unprotected cell harvested from a defunct media player and retrieved from the to-be-recycled pile.

I picked 1 A as a reasonable value for their intended use in flashlights and maybe a helmet camera. Unlike some other cells in the recent past, these deliver 3.0 A·h, reasonably close to their rated 3.4 A·h capacity at a (presumably) lower current.

Replotting the voltage vs. energy delivered doesn’t show any surprises:

Li-Ion 18650 cells – ATX prot – bare – Wh scale – 2016-12-17

The voltage declines more-or-less linearly, without the relatively flat discharge curve for smaller cells, which explains why the J5 V2 flashlight becomes seriously dim after a few hours. On the upside, that allows a reasonably accurate state-of-charge display.

Assuming the Sony HDR-AS30V camera burns 0.1 W·h/min while recording (which is a fancy way of saying it dissipates 6 W), then it should run for (10 W·h)/(0.1W·h/min) = 100 min from one of these cells fitted as an outrigger. The best of the NP-BX1 cells for the camera delivers something like 90 minutes from a measured capacity of 4 A·h at 500 mA; I don’t know what to make of those numbers. Perhaps the camera runs the NP-BX1 cells below the 2.8 V cutoff I’ve been assuming?

2017-01-02
Fordham FG-801 Function Generator Recalibration

While I had the case open, I checked the FG-801’s calibration:

Fordham FG-801 Fn Gen – circuit board

Look at all those parts!

The raw filtered DC power supplies run a bit high and the output voltages & frequencies were off by a little, but not too much after all these years.

Page 11 of the instruction manual gives the setup and calibration adjustments (clicky for more dots):

Fordham FG-801 Manual – Page 11

Page 12 gives some values that should be true:

Fordham FG-801 Manual – Page 12

For whatever reason, the manual isn’t available on The InterWebs, so here it is for your amusement:

Fordham FG-801 Sweep Function Generator – Instruction Manual.pdf

2016-12-29
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

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

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

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

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

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.

Done!

2016-12-28

Loop Antenna Splice Reinforcement

Those solder joints and finicky little wires seem much too fragile on their own:

LF Loop Antenna - complete joint — LF Loop Antenna – complete joint

This should help:

Loop Antenna Splice - assembled — Loop Antenna Splice – assembled

Foam blocks hold the ribbon cable in place and provide a bit of strain relief around the hard plastic edge:

Loop Antenna Splice - hardware — Loop Antenna Splice – hardware

The brass inserts in the bottom block (on the left) got epoxied in place, because they must provide quite a bit of force to clamp the foam. Their larger knurled end sits flush with the outside surface and the smaller end has one thread thickness of clearance below the inner surface.

A last look at the wiring:

Loop Antenna Splice - wiring — Loop Antenna Splice – wiring

I think the preamp must sit at some distance from the antenna to prevent feedback, but that remains to be seen.

The M2’s nozzle accumulated a huge blob of PETG that turned into a giant smear:

Loop Antenna Splice - PETG booger — Loop Antenna Splice – PETG booger

Fortunately, it’s on the inside where nobody will ever see it. If you know where to look, it’s barely visible from the outside.

The solid model shows off the structure a bit better:

Loop Antenna Splice - show view — Loop Antenna Splice – show view

The inside view:

Loop Antenna Splice - bottom — Loop Antenna Splice – bottom

The OpenSCAD source code as a GitHub Gist:

	// Ribbon cable loop antenna splice
	// Ed Nisley KE4ZNU December 2016

	Layout = "Text";

	//- Extrusion parameters must match reality!

	ThreadThick = 0.25;
	ThreadWidth = 0.40;

	HoleWindage = 0.2;

	Protrusion = 0.1; // make holes end cleanly

	inch = 25.4;

	function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);

	//———-
	// Dimensions

	Cable = [200,48.0,1.5]; // X = longer than anything else

	Splice = [15.0,53.0,5.0]; // epoxy blob around joints

	Foam = [15.0,Splice[1],2.0];
	CornerRadius = 5.0;

	ID = 0;
	OD = 1;
	LENGTH = 2;

	Insert = [3.9,4.6 – 0.1,5.8]; // 4-40 knurled brass insert

	Screw = [2.7,5.5,2.0]; // OD = head LENGTH = head thickness
	Washer = [3.0,8.0,0.8];

	BlockOA = [60.0, // convenient length
	Splice[1] + 4*Washer[OD], // clearance around washer on top
	2(Insert[LENGTH] + 2ThreadThick)]; // insert sets both thicknesses

	NumScrews = 2; // screws along each side of cable
	ScrewOC = [BlockOA[0] / NumScrews,
	BlockOA[1] – 2*Washer[OD],
	2*BlockOA[2] // ensure complete holes
	];

	TextThick = 3*ThreadThick; // depth of text into surface
	TextFit = HoleWindage/2; // clearance around text polygons

	//———————-
	// Useful routines

	module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes
	Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);
	FixDia = Dia / cos(180/Sides);
	cylinder(d=(FixDia + HoleWindage),h=Height,$fn=Sides);
	}

	//—–
	// Blocky model of cable + splice + wire tap for subtraction

	module Antenna() {
	union() {
	cube(Cable,center=true);
	cube(Splice,center=true);
	for (i=[-1,1])
	translate([0,-Splice[1]/2,0])
	cube([Splice[0]/2,Splice[1],2*Foam[2]],center=true);
	}
	}

	// Outside shape of splice Block, less screw clearance

	module SpliceBlock() {
	difference() {
	hull()
	for (i=[-1,1], j=[-1,1])
	translate([i(BlockOA[0]/2 – CornerRadius),j(BlockOA[1]/2 – CornerRadius),-BlockOA[2]/2])
	cylinder(r=CornerRadius,h=BlockOA[2],$fn=4*8);
	for (i = [0:NumScrews – 1], j=[-1,1])
	translate([-BlockOA[0]/2 + ScrewOC[0]/2 + iScrewOC[0],jScrewOC[1]/2,-(BlockOA[2]/2 + Protrusion)])
	PolyCyl(Screw[ID],BlockOA[2] + 2*Protrusion,6);
	}
	}

	// Splice block less cable

	module ShapedBlock() {
	difference() {
	SpliceBlock();
	Antenna();
	}
	}

	// Bottom

	module BottomPlate() {
	difference() {
	ShapedBlock();
	translate([0,0,BlockOA[2]/2])
	cube(BlockOA + 2*[Protrusion,Protrusion,0],center=true);
	Antenna(Splice);
	for (i = [0:NumScrews – 1], j=[-1,1])
	translate([-BlockOA[0]/2 + ScrewOC[0]/2 + iScrewOC[0],jScrewOC[1]/2,-(BlockOA[2]/2 + Protrusion)])
	PolyCyl(Insert[OD],2*Insert[LENGTH],6);
	for (i=[-1,1])
	translate([i*((BlockOA[0] – Foam[0] + Protrusion)/2),0,(BlockOA[2]/2 – Cable[2]/2 – Foam[2])])
	cube([Foam[0] + Protrusion,Foam[1],BlockOA[2]],center=true);
	}
	}

	// Top

	module TopPlate() {
	difference() {
	ShapedBlock();
	translate([0,0,-BlockOA[2]/2])
	cube(BlockOA + 2*[Protrusion,Protrusion,0],center=true);
	Antenna(Splice);
	for (i=[-1,1])
	translate([i*((BlockOA[0] – Foam[0] + Protrusion)/2),0,-(BlockOA[2]/2 – Cable[2]/2 – Foam[2])])
	cube([Foam[0] + Protrusion,Foam[1],BlockOA[2]],center=true);
	rotate(90) {
	translate([0,6,BlockOA[2]/2 – TextThick])
	TextHack("KE4ZNU",8,0.0,1.15,TextThick + Protrusion);
	translate([0,-6,BlockOA[2]/2 – TextThick])
	TextHack("2016·12",6,0.0,1.20,TextThick + Protrusion);
	}
	}
	}

	module TextHack(Text="sample",Size=10,Offset=0.0,Space=1.0,Thick=ThreadThick) {
	linear_extrude(height=Thick,convexity=10)
	offset(r=Offset)
	text(Text,font=":bold",size=Size,spacing=Space,halign="center",valign="center");
	}


	//———-
	// Build them

	if (Layout == "Antenna")
	Antenna();

	if (Layout == "SpliceBlock")
	SpliceBlock();

	if (Layout == "ShapedBlock")
	ShapedBlock();

	if (Layout == "Bottom")
	BottomPlate();

	if (Layout == "Top")
	TopPlate();

	if (Layout == "Text") {
	translate([0,6,0])
	TextHack("KE4ZNU",8,-TextFit,1.15,TextThick);
	translate([0,-6,0])
	TextHack("2016·12",6,-TextFit,1.20,TextThick);
	}

	if (Layout == "Show") {
	translate([0,0,5])
	TopPlate();
	translate([0,0,-5])
	BottomPlate();
	color("Orange",0.2)
	Antenna();
	}

	if (Layout == "Build") {
	translate([0,-0.6*BlockOA[1],BlockOA[2]/2])
	rotate([180,0,0])
	TopPlate();
	translate([0,0.6*BlockOA[1],BlockOA[2]/2])
	BottomPlate();
	}

view raw Loop Antenna Splice.scad hosted with ❤ by GitHub

2016-12-27

Category: Electronics Workbench

Raspberry Pi WiFi Adapters

Raspberry Pi 3 Reset Switch

Raspberry Pi Streaming Radio Player: Ignoring a Missing Volume Knob

ATX Lithium Ion 18650 Cell Capacity

Fordham FG-801 Function Generator Recalibration

Fordham FG-801 Function Generator Power Switch

Loop Antenna Splice Reinforcement