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.

Tag: Improvements

Making the world a better place, one piece at a time

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

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

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

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.

2012-06-26
Dell 75UYF Battery Teardown: Cell Capacity

Putting that battery into the Dell 8100 laptop produced the dreaded blinky light of doom, so it has been on the shelf for maybe half a year. Having gutted the cells from the case, the next step was to discharge the cells completely, thereby producing the lower four curves in this plot:

Dell 8100 Laptop Battery Cells

I arbitrarily labeled the cell pairs 1 through 4. Pair 1 has the lowest remaining charge and the other three seem very closely matched.

I recharged the four cell pairs one-at-a-time from a bench power supply set to 4.2 V. Each pair started charging at about 2 A, somewhat lower than the pack’s 3.5 A limit, so the supply’s 3 A current limit didn’t come into play. You probably don’t want to do this at home, but …

The usual charge regime for lithium cells terminates when the charging current at 4.2 V drops below 3% of the rated current (other sources say 10%, take your pick). The pack’s dataplate sayeth the charging current = 3.5 A, so the termination current = 100 mA. I picked 3% of the initial 2 A current = 60 mA and stopped the charge there, so I think the cells were about as charged as they were ever going to get.

As nearly as I can tell, increasing the voltage enough to charge at a current-limited 3.5 A (a bit beyond my bench supply’s upper limit, but let’s pretend), then reducing the voltage to 4.2 V as the current drops would be perfectly OK and in accordance with accepted practice, but I’m not that interested in a faster charge.

Unlike the other three pairs, Pair 1 quickly became warm and I stopped the charge. Warming is not a nominal outcome of charging lithium-based cells, so those were most likely the cells that caused the PCB to pull the plug on the pack. The other pairs remained cool during the entire charge cycle, the way they’re supposed to behave.

However, even with that limited charge, Pack 1 had about the same capacity as the (presumably) fully charged Pack 2, showing that the cells get most of their charge early in the cycle. Pairs 3 and 4 had more capacity, but they’re not in the best of health.

The blue curve in this graph shows the discharge curve for the 1.1 A·h Canon NB-5L battery (actually, a cell) that came with the SX230HS camera:

Canon NB-5L – first tests

Notice that it remains above 3.4 V until it produces 1.1 A·h at 500 mA, which is roughly its rated capacity. The other traces come from those crap eBay NB-5L batteries.

The two best pairs of Dell cells can each produce about 1.3 A·h at 1 A before dropping below 3.4 V (the cursor & box mark that voltage in the top graph), so they’re in rather bad shape. Strapping the best two pairs together would give a hulking lump with perhaps three times the life of the minuscule NB-5L battery, so I think that’s probably not worth the effort.

Particularly when one can get a prismatic 3.7 V 5 A·h battery for about $30 delivered, complete with protective PCB and pigtail leads…

2012-06-22
Quilting Pin Caps: Recycled O-Ring

While doing something else, I rediscovered the fact that common 5 gallon plastic bucket lids have an O-ring gasket that seals against the top of the bucket. Some seals are hollow tubes, some are solid rods:

5 gallon can lid gaskets

The white O-ring has about the right consistency to serve as a quilting pin cap, along the lines of those 3D printed and silicone rubber filled cylinders. Although the rubber / plastic stuff isn’t quite as squishy as silicone snot, it holds the pin point firmly without much of a push.

Chopping the O-ring into 10 mm sections produced another small box of prototypes:

Lid gaskets as pin caps

Garden planting season remains in full effect, shoving all quilting projects to the back burner and delaying the evaluation phase of the project…

2012-06-16
Garden Knife Scabbard

Mary uses an ancient paring knife (that, back in the day, my father had sharpened beyond all reason) to harvest garden veggies, which called for a scabbard to protect the blade, the bike pack, and the fingers.

I snagged a random block of acrylic from the heap, straightened the long sides, milled a channel just wide and thick enough for the blade down the middle, then added small recesses at the right end for the knife’s haft:

Garden knife scabbard – main block

The cover is an acrylic sheet, solvent-glued and clamped in place:

Garden knife scabbard – clamping cover

The cover exposes about 1/4 inch of channel so she can lay the point in place, rather than precisely aligning the point with the slot. I suppose I should have used gray acrylic to provide some contrast; maybe we’ll add a snippet of tape.

Then mill four sides flat, break the edges & corners with a file, and it’s all good (in a blocky kind of way):

Garden knife scabbard

The blade has become sufficiently bent over the years that simple friction holds it in the slot. It’s open on both ends so she can flush out the inevitable dirt.

I was going to engrave her name on the back, but came to my senses just in time…

2012-06-13
Forsythia Clearing

It was decided, in that place where what is decided must be, that the time had come to hack back the giant forsythia stand encroaching from the neighbor’s yard. The stuff tip-roots, so anything that stands in its way gets assimilated, and the only way to make headway is to tear it out by the roots.

We eventually clearcut a section about 15 feet wide and 40 feet long by the simple expedient of lopping off everything that stuck up:

Cleared Forsythia

Removing the roots required prying with a 7 foot length of 1.5 inch octagonal steel bar braced on a chunk of 4×4 inch lumber rammed up against the roots. With my full weight on a 6 foot lever arm, the roots would just barely break free.

A dozen wheelbarrow loads like this went atop the branches on the heap:

Forsythia root balls

That’s my kind of outdoor work: kill them all…

Mary raked and seeded the debris field just before the next rainfall. It ought to be good for another few years.

2012-06-04
NIC Backpanel Shortening

Actually, that NIC didn’t slip right into place, because its backpanel plate was sized for a full-height PC case. Unlike the cheap stamped steel you find these days, NetGear used much thicker metal that required an attack with the bandsaw, a hammer, and some files to clean up the raw edges.

But it fit pretty well after all that:

Shortened NIC backplate

You can just barely see the NetGear logo wrapped around the right-angle bend…

2012-06-03
Upstart vs. rc.local
In the process of figuring out how to set up the isolated WiFi Internet link on the file server, I discovered that the /etc/rc.local file runs before the eth0 interface that connects to the outside world comes up. As a result, my DynDNS host address hadn’t been updated in quite some time.

Worse, trying to set up eth1 failed, apparently because there’s a bunch of other network infrastructure that doesn’t start until eth0 comes online. Part of that infrastructure involves iptables; the added rules simply vanished.

The solution seems to require writing an upstart script that waits for whatever events it needs, does what needs to be done, and then goes away. The whole upstart mechanism and its event list seems, um, lightly documented, as I discovered there, but the custom setup formerly in /etc/rc.local now lives in /etc/init/local.conf:
```
description "Stuff that used to be in /etc/rc.local"
author "Ed Nisley - KE4ZNU"

start on (local-filesystems and net-device-up IFACE=eth0)
stop on shutdown

script

logger Starting local init...

logger Updating dyndns
ddclient -force

logger Bringing up eth1
ifconfig eth1 192.168.3.1 netmask 255.255.255.0 up

logger Setting iptables
iptables -A FORWARD -i eth1 --destination 192.168.0.0/16 -j REJECT
iptables -A INPUT -i eth1 --destination 192.168.0.0/16 -j REJECT
iptables -A POSTROUTING -t nat -j MASQUERADE

logger Ending local init

end script
```
That code assumes the outbound network interface will be eth0, which won’t work on a system using a pure wireless connection on, say, wlan0 or anything more exotic. I haven’t a clue how to parameterize that selection. Most likely, one would write another upstart script that would emit a custom signal based on the usual suspect …

It also assumes the networking infrastructure triggered by eth0 lighting up has hauled itself to its feet and is ready to roll. That seems to be true, although I’ll admit the script is, at best, lightly tested.

With the eth1 NIC up and iptables rules added, I think this script will restart eth1 when it goes down, but it’s not clear where the requisite network-device-down event comes from (certainly not from any script in /etc/init/*conf):
```
description "Restart eth1 when it dies"
author "Ed Nisley - KE4ZNU"

start on net-device-down IFACE=eth1
stop on net-device-up IFACE=eth1

script

logger Restarting eth1...
ifconfig eth1 192.168.3.1 netmask 255.255.255.0 up

logger Ending eth1 setup

end script
```
But, eh, at least the isolated interface comes up and packets go where they should (and not where they shouldn’t). Early results are encouraging…
2012-06-02