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Archive for February, 2014

Sewing Machine Lights: LED Strips

Mary wants more light on her free-motion quilting, right where the needle meets the fabric. I proposed LED strip lights on the machine arm, in addition to rebooting the end cap lamp into the current millennium, which requires:

  • No snagging on a bulky quilt shoved through the machine
  • Not completely butt-ugly
  • Reasonably durable

I picked up reels of cool-white and warm-white waterproof LED strips (12 V, 3528-size chips, 5 m, 600 LED, 25 mm segments) from the usual eBay supplier, who promptly charged for both and shipped only the warm-white reel. Cool-white LEDs will be a better color match to daylight from the window and the little Ottlite she uses for detail work, but I ran some prototypes while we wait for the replacement.

The Chinese New Year really comes in handy as an excuse for screwing things up and not responding for a week or two. ‘Nuff said.

They’re similar to the RGB LEDs from a while ago, with even gummier “waterproof” encapsulation. I got double-density 600 LED strips to put more light emitters across the arm:

Various LED strip lights

Various LED strip lights

The smaller 3528 SMD LEDs (vs. 5050 chips in the others) allow a narrower strip and the double-density layout means each three-LED segment is half as long long. The as-measured dimensions work out to:

  • 25.0 mm segment length
  • 8.2 mm strip width
  • 2.5 mm thickness

The sealant thickness varies considerably, so I’d allow 3.0 mm for that in case it mattered. It slobbers over the edge of the strip here and there; allowing at least 9.0 mm would be wise.

The SMD resistor in each segment is 150 Ω. A 5 segment length drew 85 mA @ 12 V = 17 mA/segment. Boosting the voltage to 12.8 V got the current to the expected 100 mA = 20 mA/segment.

The LEDs are noticeably less bright than the 5050 LEDs, even at 20 mA/segment, but I think they’ll suffice for the task.

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Desiccant Adsorption vs. Relative Humidity Chart

This chart, shamelessly ripped from the Interwebs because the links keep rotting out, may prove useful in the future:

Desiccant absorption vs humidity

Desiccant absorption vs humidity

In round numbers: between 10% and 40%RH, silica gel equilibrates at 1.8%RH for each percent of weight gain. If you toss 100 g of dry silica gel into a container with some filament, when it weighs 120 g (20% weight gain) the air inside the container will be at about 36%RH.

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Casio EX-Z850 Backup Battery Replacement

When our Larval Engineer repaired the Casio EX-Z850 camera’s buttons, we noticed that the memory backup battery was on its last legs:

EX-Z850 internal battery corrosion

EX-Z850 internal battery corrosion

The camera has returned home, where I’ll put it to good use on the microscope, but I’m the type of guy who swaps batteries every now and again, soooo that needs fixing. Wikipedia says the battery isn’t replaceable, but you can’t believe everything you read on Wikipedia, right?

Removing the camera’s front cover (stick the screws to a length of masking tape!) reveals the backup battery hasn’t magically healed itself:

Casio EX-Z850 backup battery - corrosion

Casio EX-Z850 backup battery – corrosion

The main battery applies 3.2 V with the top terminal negative; it’s marked to help me remember that fact.

I snipped both legs of the top contact bracket, which promptly fell off, and then pushed the battery off its bottom contact. The condition of those two pads suggests a pair of cold solder joints (clicky for more dots):

Casio EX-Z850 backup battery - contact pads

Casio EX-Z850 backup battery – contact pads

I wanted to replace it with a polyacene supercap, but there’s just not enough room in there. The biggest cap that fit was a 33 μF 16 V SMD electrolytic cap, so I soldered one in place:

Casio EX-Z850 backup battery - capacitor replacement

Casio EX-Z850 backup battery – capacitor replacement

I had to flip the camera around to get the soldering iron in between the cap and what looks to be an intrusion monitoring switch just to its left. No lie, that shiny metal thing seems to be a tab that presses against the front cover; it could be a static discharge / grounding point, but the base looks more complex than that.

Now, a capacitor isn’t a battery, but memory backup doesn’t require much of a battery, either. I guesstimated the memory (or whatever) would draw a few microamps, at most, giving me a few seconds, at least, to swap batteries. A quick measurement shows that I’ll have plenty of time:

Casio EX-X850 backup capacitor - voltage vs time

Casio EX-X850 backup capacitor – voltage vs time

The camera started up fine after that adventure, so the memory stays valid with the backup voltage down around 1 V.

The cap measured 34 μF, so a voltage decline of 24 mV/s works out to:

IC = C (dV/dT) = 34 μF x 24 mV/s = 820 nA

So, at least at room temperature, the memory draws less than a microamp.

I love it when a plan comes together!

With any luck, that capacitor should outlast the rest of the camera. It’ll definitely outlast a lithium battery, even if I could find one to fit in that spot.

I did those measurements by sampling the capacitor, rather than holding the meter probes in place, because the 300 nA of current drawn by a 10 MΩ input resistance would cause a pretty large measurement error…

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Thing-O-Matic Y-axis Idler Support Bracket: Oops

The STL file from CampbellsBot’s Y-Axis Idler Support Bracket printed without incident (admittedly, on the M2):

Thing-O-Matic Y-axis Idler Support Bracket

Thing-O-Matic Y-axis Idler Support Bracket

Come to find out that Makerbot changed the spacing between the Y-axis rod and the idler bolt, so it doesn’t fit the TOM286. I could fire up the Token Windows Box, install Sketchup, modify the model, rebuild and clean up the STL, and try again, but it’s easier to just give up. The TOM286 has worked fine so far, so maybe this isn’t really needed.

Ah, well, it’s another show-n-tell doodad…

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Radio Shack Sound Level Meter: Switch Repair

My trusty Radio Shack Sound Level Meter recently began misbehaving: switching to the most sensitive two ranges (-60 and -70 dB) caused it to turn off. Finessing the switch got it back in operation, so I completed the mission (a string quartet in Vassar’s Skinner Recital Hall topped out around 90 dB) and laid it out for repair:

Radio Shack Sound Level Meter - PCB solder side

Radio Shack Sound Level Meter – PCB solder side

After cleaning the already pristine gold-plated (!) contact pads and putting it back together, the switch failed the same way.

A bit more poking & prodding revealed that slightly loosening the upper case screw (in the boss just left of the switch pads) made it work perfectly.

Ah-ha!

Come to find out that the rear case presses on the PCB to hold it in place, which moves it slightly toward the front of the case. The switch rotor, being firmly attached to the stem in the middle of the pads, doesn’t move, which suggested that the bifurcated spring contacts on the rotor had take a bit of a set.

Un-bending them very, very gently to add a millimeter of springiness solved the problem.

A piano solo topped out in the high 80s…

Update: Another meter owner shows how to cure the problem, rather than treat the symptom:

I found your older note about the switch problem on the digital R.S. SLM to be helpful, in that mine had a similar problem, but only on the 60 dB scale, not both the 60 and 70 dB scales. Your diagnosis about the back putting pressure on the board seems to be right on. However, for me, re-bending the switch contacts didn’t help.

What did fix it was filing ~2mm off the back case boss around the upper screw hole. That was the source of the pressure on the board. 1 mm didn’t quite fix it, but 2mm off did.

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Snowblower Muffler Replacement

What with all the snow this winter, I noticed that the muffler on the snowblower was rattling around something awful; eventually, the blue fire jetting directly from the engine block got to be distracting. Come to find out the bracket attached to the top of the block had ripped free from the muffler:

The two long bolts on the right explain why this particular anomaly didn’t get an immediate repair: they were firmly jammed, deep in the block, and resisted my gentle attempts to free them. For obvious reasons, you (well, I) don’t want to break off the end of a bolt in its tapped hole…

Snowblower muffler - failed bracket

Snowblower muffler – failed bracket

So, over the course of a few weeks, I applied a dose of PB B’laster to the bolts, down deep behind the muffler where they entered the block, and gingerly wiggled the bolts back-and-forth to their ever-increasing limits of travel. Doing that every time I went into the garage guaranteed plenty of excess oil to smoke off the engine during the first few minutes, but ya gotta do what ya gotta do. Two days before the next big storm, the block finally released the bolts. Whew!

Evidently, having the bracket tear loose wasn’t a rare failure and, perhaps, the situation attracted the attention of someone in accounting who pointed out the warranty repair costs (no, our blower wasn’t in warranty), because the new muffler has a different bracket:

Snowblower muffler - new bracket design

Snowblower muffler – new bracket design

Look at all those spot welds across that huge contact patch!

Yes, I used new bolts with a generous dollop of Never-Seez on each one…

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Reversible Belt Buckle: Post Restaking

A reversible belt lets me look perfectly natty, regardless of whether I’m wearing my brown pants or my khaki pants. The post joining the buckle and the base worked loose, so the spring wasn’t holding the two parts together; obviously, something must be done.

Loosen the four screws that hold the leather belt in place to reveal what’s inside:

Reversible belt buckle - spring post

Reversible belt buckle – spring post

Then push the two parts together and give the post a few shots with a sharp punch:

Reversible belt buckle - staked post

Reversible belt buckle – staked post

Reassemble in reverse order: done!

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