Archive for June, 2012

Monthly Subconscious: Dysfunctional Father

This may be a universal truth, as seen from both sides of the divide:

Dysfunctional Father

Dysfunctional Father

In text:

dysfunctional father worry love
protect awkward girl child


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Fundamental 3D Printing Patents

DIY 3D printing seems surrounded by Good Ideas that don’t happen, which led me to look up some of the early patents in the field. As nearly as I can tell, any bright idea one might have has already been patented; although you can usually get away with tinkering it up in your basement (because you’re not worth enough to interest the patent holder’s attorneys), anything beyond that will darken your skies with lawsuits.

The granddaddy of all 3D extrusion machines seems to be US5121329 (Crump → Stratasys 1992-06-09): Apparatus and method for creating three-dimensional objects

Exploring the patents referencing that one as a foundation should keep you busy for a while; the PDF has clicky links.

Some fine tuning on the theme:

US6085957 (Zinniel/Batchelder → Stratasys): Volumetric feed control for flexible filament

US5303141 (Batchelder/et al → IBM): Model generation system having closed-loop extrusion nozzle positioning

Congealing 3D objects in a vat of goo probably starts with 4575330 (Hull → MVP 1986-03-11): Apparatus for production of three-dimensional objects by stereolithography

Remember: I’m not a patent attorney and my opinion is worthless…

US5121329 - Figure 1

US5121329 – Figure 1

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A Fork In the Path

The original path curved away from the new Nutt MECS Center at Trinity, but even engineering bears won’t follow a path that leads in the wrong direction:

Fork in the path at Trinity

Fork in the path at Trinity

An old story has it that [name of administrator] at [name of new college] had the architect remove all but the most obvious walking paths from the new campus plans. After the first year passed, then they paved the routes that people actually used.

Vassar College has a good example of that design in the residential quad:

Vassar Paths - Paved Quad

Vassar Paths – Paved Quad

But even they won’t slash diagonals across a lawn just for students:

Vassar Paths - Grass

Vassar Paths – Grass


Tektronix P6401 Logic Probe

Mad Phil gave me his EMI Go-Kit, which contained a Tek P6401 logic probe (along with a short ton of ferrite cores):

Tek P6401 Logic Probe - kit

Tek P6401 Logic Probe – kit

It’s slightly younger than dirt (copyright 1974, please forgive me) and still works fine on TTL-level logic. The red & green indicators use tiny grain-of-wheat incandescent bulbs, of course, so the thing draws nigh onto a quarter of an amp with both lights on.

The front of the instruction card shows what the blinky lights mean and the back gives the specs; it’s doubled up so you can pass one along to a friend:

Tek P6401 Logic Probe - Specs and Usage Card

Tek P6401 Logic Probe – Specs and Usage Card

If you have one that doesn’t seem to work, check the internal thermal fuse: tack it back down with a hot dry soldering iron and it’ll probably outlive you…


K-26 Metal Detector: Sensor Coil Rewinding

There ought to be a survey marker pin at the front corner of the lot where it’d come in handy for locating the edge of the yet-to-be-contracted driveway paving, but if it’s there it’s been pushed below ground level. So I mooched a homebrew metal detector based on the Elenco K-26 PCB

K-26 Metal Detector PCB

K-26 Metal Detector PCB

The kit included 45 feet of  22 AWG enamel wire that should have become a 5 inch diameter coil with 30 turns, but the as-built detector had a coil wrapped around a 1 foot diameter cardboard form. The coil inductance sets the oscillation frequency, which turned out to be around 300 kHz: far below the nominal 1000 kHz. So I wound 40 turns of 22 AWG magnet wire around an old CD-ROM spindle case (which is, quite coincidentally, just over 5 inches in diameter), and taped it atop the cardboard form.

The datasheet recommends a nonmetallic handle, so I swapped in a plastic umbrella support for the original metal mop (?) handle.

Rewound homebrew metal detector

Rewound homebrew metal detector

The K-26 schematic looks like a common-base Colpitts oscillator, with only the most utterly absolutely vital essential components:

K-26 Schematic

K-26 Schematic

In round numbers, the oscillation frequency varies inversely with the number of turns:

F = 1/(2π√(LC)) (for a simple tank)

L = stuff × N2 (stuff = various constants & sizes)

F = stuff / N

The rewound coil oscillated at 350 kHz, so I spilled off a few turns at a time to produce these results and a tangle of wire on the floor:

L – µH Freq – kHz
330 350
186 535
107 711
65 840
42 1140

For the record, the coil in the photo corresponds to the last line and has 12 turns.

Contrary to what the instructions imply, trimpot P1 does not adjust the oscillation frequency. It tweaks the transistor bias for best oscillation, so it’s more of an amplitude control than anything else. I adjusted P1 while watching an oscilloscope connected across the negative battery terminal and the emitter of Q1, but you could probably use a small sniffer loop to good effect.

It draws about 2 mA, so the battery should last quite a while; labeling the switch positions should help a lot.

The oscillator produces an unmodulated carrier, so I tuned a Kenwood TH-F6A HT in LSB mode for maximum squeal. Any variation in L changes the carrier frequency and thus the pitch of the demodulated audio; an earbud just barely in one ear makes this almost tolerable.

As you should expect from the picture, that metal detector lashup is mightily microphonic, to the extent that touching a blade of grass wobbles the audio pitch and bumping the cardboard plate against an object can detune the whole affair. A bit more attention to rigid coil mounting would certainly help, but this isn’t the most stable of designs to begin with and I doubt anything will help very much at all.

The coil can detect a chunk of rebar sticking out of the ground at a range of maybe half a foot, but it’s not clear how well it will cope with buried treasures (like, oh, let’s say a survey marker pin). In any event, I must mow the grass down there before going prospecting.


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Canon NB-5L Battery Teardown, Cheater, and Voltages

The motivation for gutting that Dell laptop battery was to find out if the cells could become a higher-capacity external battery for the Canon SX230HS camera. Those discharge curves suggest they can’t, but I also want to know what voltage levels correspond to the various battery status icons, which means I must feed an adjustable power supply into the camera… so I need a fake NB-5L battery with a cheater cord.

The first step: crack the case of the worst of the eBay junkers. I squeezed it in the bench vise to no avail, then worked a small chisel / scraper (*) into the joint. The lid was firmly bonded to the case, but it eventually came free:

NB-5L Battery - opened

NB-5L Battery – opened

The protective PCB sits at one end of the cell, with a strip of black foam insulating the components from the nickel strips:

NB-5L - protective PCB

NB-5L – protective PCB

It turns out that the cell’s metal shell is the positive contact, which I didn’t expect.

The component side of the PCB has a 10 kΩ resistor connected between the center and negative contacts. That should be a thermistor, but it’s a cheap eBay knockoff and I suppose I should be delighted that there’s not a gaping hole where that contact should be. The PCB fits against the small notch in the case and is held in place by small features on the top and bottom. The negative contact is on the far left:

NB-5L - PCB interior view

NB-5L – PCB interior view

Canon sells an AC adapter for the camera that includes an empty battery with a coaxial jack that aligns with a hole in the battery compartment cover. I soldered a pair of wires to the PCB, drilled a hole in the appropriate spot, added some closed-cell foam and hot-melt glue to anchor the PCB, and made a cheater adapter. For the record, the orange wire is positive:

NB-5L - gutted case with pigtail

NB-5L – gutted case with pigtail

It turns out that the camera battery cover must be closed and latched before the camera will turn on, but the sliding latch mechanism occludes the hole. This cannot be an inadvertent design feature, but I managed to snake the wire out anyway.

Connecting that up to a bench supply (with a meter having 0.1 V resolution) produces the following results:

Voltage Result
3.8 Full charge
3.7 2/3 charge
3.6 Blinking orange
3.5 “Charge the battery”

The camera draws about 500 mA in picture-taking mode, about 300 mA in display mode, and peaks at around 1 A while zooming.

The Genuine Canon NB-5L is good for 800 mA·h to 3.6 V, as are the two best pairs of the Dell cells. The latter remain over 3.7 V for 500 mA·h, which suggests one pair would run for about an hour before starting to blink. Maybe that’s Good Enough, but … a new prismatic battery is looking better all the time.

(*) Made by my father, many years ago, with a simple wood handle that eventually disintegrated. I squished some epoxy putty around the haft and covered it with heatshrink tubing, but (now that I have a 3D printer) I really should print up a spiffy replacement. I’ve been using it to pry objects off the printer’s build platform, so that’d be only fitting…


Choosing SSID Names

It turns out that WiFi SSIDs can be quite long and contain blanks:

SSID Names

SSID Names

Now, if I wanted to capture your private bits, I’d put up a public access point with a friendly name and no security at all…

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