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

LED Flashlight PCB Contacts

One of the myriad cheap LED flashlights around the house & shop stopped working. This one consists of an aluminum shell with a pushbutton switch in the screw-on rear cap; somewhat to my surprise, the switch worked fine.

Poking around the PCB in the front revealed the problem: only friction held it in place against the springs contacting the three AA cell battery container. Pushing a bit harder shoved the lens and the LED / reflector / PCB assembly out:

LED flashlight PCB

The spring in the middle contacts the positive battery terminal. Those three square pads pressing against a locating shoulder inside the shell, but two of the pads have a solder layer and one is bare. I don’t know if the long lead on the LED at about two o’clock is a deliberate attempt to form an additional contact.

Peering inside the shell reveals three teeny nubs on the locating shoulder that could, presumably, dig into the solder pads:

LED flashlight – shell contact points

If you’re having trouble spotting them, so did I. Running a fingernail around the shoulder helps: one is at the bottom, another about 10 o’clock, and the third at about 1 o’clock. They’re not evenly spaced at 120° to match up with the pads.

With only friction holding the PCB in place, I understand why the flashlight didn’t work; given enough of an impact, the battery would push the PCB just far enough forward to make the connection at least intermittent.

I aligned the two solder-coated pads with two nubs, shoved everything together, pressed the lens firmly in place, and we’ll see how long that lasts…

2012-10-02
Cutting Music Wire

It should go without saying, but you do not cut music wire with diagonal cutters intended for electrical wire or the low-carbon steel shears built into wire strippers. I use a bicycle cable cutter that easily slices through the hard wire used in brake cables and their housing:

Bicycle cable cutter

I’ve owned this one forever, but those cutters from Park should work just as well; the odd protrusions behind the pivot crimp aluminum caps on stranded cable. I also have diagonal cutters with hardened jaws, but they’re too bulky for fine work and tend to fire the stub ends across the Basement Laboratory.

Every now and again I touch up the jaws with a diamond file to get rid of small dings; despite being hardened, those fine points seem particularly prone to burrs.

When you see an ordinary wire cutter with matching half-moons in each blade, you know what happened…

2012-09-26
Dead-on-arrival Lithium Cell

DOA Energizer CR2032 cell

The display on Mary’s Cateye Astrale cyclocomputer (remember cyclocomputers?) faded to gray, which meant a new CR2032 lithium cell was in order. I grabbed one from the heap, popped out the old cell, inserted the new cell, and … the display stayed blank.

Quick like a bunny, I reinserted the old cell to save the odometer (15524 miles) and wheel circumference (1475 mm) data; the display returned to dim gray.

The “new” cell, which came from an unopened pack, read 0.45 V with no load…

The cell didn’t have a date code, but the package sports a cryptic MU that might encode the date of manufacture or the date of packaging or the copyright date or something; the various search results aren’t forthcoming and the Energizer site gives no explanation.

I’m pretty sure I haven’t owned that package for more than a few years and it’s been in a shirtsleeve environment (plus the occasional hot van) ever since.

Another Energizer cell from a more recent lot, bearing CA on the package and YA on the cell, worked fine.

Being that sort of bear, I wrote the date and mileage on the previous cell (a Newsun, whoever they are, with a 3Y code), because the last time around the odometer value didn’t survive the cell change. The current total works out to 277 miles/month = 3300 miles/year, including winter downtime, which is fine with us; we mostly ride the bikes around town on errands and take the occasional tour.

2012-09-21
Multimeter Range Switch Contacts: Whoops!
One of my multimeters began reporting bogus values that improved by working the range switch back-and-forth, which suggested the switch contacts need cleaning. Taking the meter apart was easy, right up to the point where I removed the range switch from the PCB by compressing the four locking tabs on the central shaft:

Multimeter range selector switch

Just before taking that picture, the switch launched half a dozen spring contacts across the bench, my shirt, and the floor… I recovered four for the picture and later found a fifth smashed on the floor, but the last contact remains AWOL.

The contact in the middle, the oddly shaped one with small tabs on the ends, is a prototype replacement conjured from 6 mil phosphor bronze stock:

Multimeter range switch contacts

The little domes ensure a good sliding surface, but require two bends in the middle of the contact and some way to shape the metal into a dome. After a few experiments, I filed the end of a nail into a rounded chisel that worked pretty well:

DMM switch contact punch

The original contacts came from 3.5 mil stock and have considerably more flex; 6 mil stock is what I have.

I think I should make half a dozen contact springs to replace the entire set, a task requiring more time than I have right now. For the record, the overall process goes like this:
- lay out overall shape, slightly longer than needed
- cut center opening with abrasive wheel
- cut out contact
- punch contact domes (from back = dimples)
- bend to shape
- trim ends to length (not done in picture)
- dress raw edges (not done in picture)
Given the number of parts and the fiddly accuracy required to make the slot, this might be a good job for the Sherline, although clamping each little proto-spring down while getting the abrasive wheel in there seems daunting.

Perhaps cutting the slots and punching the dimples would work better before cutting out the contacts, with a sheet clamped on four sides? The center will be floppy, what with all the slots, but grinding slots on the middle contacts first might be helpful. Would adhesive under the sheet to hold down the middle gunk up the abrasive wheel?

So many projects …

Memo to self: Springs! Always expect springs!
2012-09-20
Black and Decker Pocket Power: Battery State

A Black & Decker Pocket Power emerged from the heap and refused to take a charge, which obviously calls for a teardown. The case has three screws, one lurking behind the label:

Pocket Power – case screws

The sticker over on the right says it’s five years old, which explains the whole problem right there; you can evidently buy new-old-stock units from the usual low-dollar sources that will arrive with a similarly dead battery.

Peeling off the rubber bumpers and prying the case open reveals the innards:

Pocket Power – internal layout

The battery pack looks to be an octet of ordinary NiMH cells; the label on the other side of the shrink wrap reports 9.6 V @ 1200 mA·h, which is about what you’d expect, with a date in mid-2007 that matches the sticker on the case.

The upper left corner of the main label has some interesting information:

Pocket Power – label specs

The tiny wall wart that came with the unit produces 12 VDC at 300 mA, which doesn’t match the INPUT spec at all. Perhaps the maximum current from the internal pack made its way to the label by mistake?

The label also shows the reason I got this thing: it can produce just enough 120 VAC power to run an arbitrary wall wart charger for a gadget that doesn’t charge from a 12 VDC source. Upconverting 9.6 VDC to 120 VAC, then downconverting it to, say, 14.4 VDC makes no sense whatsoever, unless that’s the only way to charge that particular gadget. Which has, I’m sorry to say, been the case every now and again.

I think the Model name has a typo: everything else suggests this is a CP120B. So it goes.

Unsoldering the leads and perpetrating the obvious tests produces these curves:

B&D Pocket Power

The black curve is the initial “won’t accept a charge” state with the wall wart and internal circuitry; the pack obviously has two weak cells. The curves in the lower left correspond to individual cells and series pairs that I discharged to 0.9 V/cell after the top curve ended.

The tiny stroke between the sets, way over on the Y axis, is cell pair BC (my arbitrary labels) that probably accounts for the sudden drop in the black curve. However, the orange curve also came from pair BC after charging for about 18 hours at 120 mA, so they’re not completely dead. Their capacity has dropped to about 700 mA·h, though, which isn’t good.

Soldering the pack back together and charging for another 18 hours at 120 mA produced the green curve at the top, which shows the same sudden dropoff at about 700 mA·h.

So I’ll put it back together again and let it charge for a while, but new cells will definitely brighten its disposition.

2012-09-19
Three-way Lamp Socket: Fuse Test
After un-bending the top of a pole lamp that suffered an untimely collision with the floor, I discovered that the entire stock of three-way bulbs in the heap had at least one burned-out filament each; I’d acquired them when Mom moved out of the Ancestral House, so they dated back a long time. So I figured I’d insert a decently sized single-filament bulb and be done with it.

Three-way lamp sockets have an additional tab contact between the usual central contact and the outer shell:

Interior of 3 way lamp socket

The shell forms the common contact for the filaments and the switch counts in binary: off / off, off / on, on / off, on / on. In principle, the tab sits low enough to not contact the shell of an ordinary bulb.

I was doing this in the Basement Laboratory Workshop Wing, with the lamp plugged into the outlet strip along the front edge of the bench; that way, I simply poked the power strip button to remove line voltage from the lamp while swapping bulbs. So I:
- turned the power strip off
- unscrewed the last dead three-way bulb
- threw it away
- screwed in an ordinary bulb
- turned the strip on
At which point all the fluorescent overhead lights in the Laboratory went dim, the shop resounded with a deep resonant groan, and the acrid smell of electrical death filled the air. Elapsed time less than a second, tops.

Come to find out that the socket’s contact tab stuck up a little bit further than it should, producing a dead short across the line:

Melted bulb base

Of interest: the branch circuit breaker didn’t trip, the GFI on the circuit didn’t trip, and the pop-out breaker in the power strip didn’t trip.

Huh.

I harvested the pole sections, the base counterweight, and the line cord. The rest of the corpse joined the bulbs in the trash…
2012-09-18
Current-Sense Resistors: Mind the Power

The bench supplies I use have current limiting, but the 10 mA meter resolution leaves a lot to be desired, so I conjured up a simpleminded 200 mA meter from a panel-mount meter and a 1 Ω sense resistor. That means it’s good for only 200 mA, so I insert it in series with the supply only when it’s needed. Lately it’s been reading more than a little bit high and I took it apart to find this obvious evidence of abuse:

Homebrew millammeter with burned sense resistor

The loose resistor sitting atop the chip shows what the burned resistor soldered in the circuit should look like.

The power supply has a 3 A current limit. No surprise: 9 W is more than the unfortunate 5 W resistor can handle.

It’s all better now …

2012-09-13