Archive for January, 2019
Because the COB LEDs dissipate 6W, far more power than I’m comfortable dumping into a 3D printed structure, I redefined a length of aluminum shelf bracket extrusion to be a heatsink and epoxied the module’s aluminum back plate thereto:
Unlike the flexible LED strips, the COB LED modules have no internal ballast resistors and expect to run from a constant-current supply. Some preliminary testing showed we’d want less than the maximum possible light output, so a constant-voltage supply and a few ohms of ballast would suffice:
With all that in hand, the heatsink extrusion cried out for smooth endcaps to control the wires and prevent snagging:
The central hole in the left cap passes 24 AWG silicone wires from the power supply, with 28 AWG silicone wires snaking down through the L-shaped rectangular cutouts along the extrusion to the LED module’s solder pads.
The model includes built-in support:
Assuming the curved ends didn’t need support / anchors holding them down turned out to be completely incorrect:
Fortunately, those delicate potato chips lived to tell the tale and, after a few design iterations, everything came out right:
The “connector”, such as it is, serves to make the light bar testable / removable and the ballast resistor tweakable, without going nuts over the details. The left side is an ordinary pin header strip held in place with hot melt glue atop the obligatory Kapton tape, because the heatsink doesn’t get hot enough to bother the glue. The right side is a pair of two-pin header sockets, also intended for PCB use. The incoming power connects to one set and the ballast resistor to the other, thusly:
The diagram is flipped top-to-bottom from the picture, but you get the idea. Quick, easy, durable, and butt-ugly, I’d say.
The next step was to mount it on the sewing machine and steal some power, but that’s a story for another day.
The relevant dimensions for the aluminum extrusion:
The OpenSCAD source code as a GitHub Gist:
Continuing the process of silk-purse-izing the DSO150, a batch of USB 1S lithium battery charger modules arrived from halfway around the planet. I drilled & filed a suitable hole / slot / aperture in one of the few remaining spots in the case, then stuck the PCB to the bottom with good foam tape:
Because the charger includes cell protection circuitry, I replaced the original protected 18650 cell with a bare cell sporting solder tabs. The cell should go directly to the charger board, but the switch disconnects the + wire; I’m unwilling to believe the charger won’t slowly and inexorably discharge the cell if I don’t use the DSO150 for a few months. It could happen.
A label makes the hole look almost professional:
Well, makes it look Good Enough™, I suppose.
The power switch gets a label, too:
Flipping the switch ON lights up the scope from the battery.
The charger (sensibly) will not route power from the USB port to the scope without a battery, so you must plug in a USB source with the switch ON, then flip the switch OFF. I don’t know why you’d want to do that, but there you go.
Now it’s a real portable instrument, with all the inconvenience of managing a built-in lithium cell.
Definitely not a vacuum tube:
The LED holder is identical to the Pirhana holder, with a 10 mm diameter recess punched into it for the SK6812 PCB:
The 3.6 V (and declining) power supply may not produce as much light from the SK6812 LEDs, but it’s entirely adequate for anything other than a well-lit room. The 28 AWG silicone wires require a bit of careful dressing to emerge from the holes in the radome holder:
The firmware cycles through all the usual colors:
A pair of tensilized 22 AWG copper wires support the Pro Mini between the rear struts. The whole affair looks a bit heavier than I expected, though, so I should reduce the spider to a single pair of legs with a third hole in the bottom of the LED recess for the data wire.
The OpenSCAD source code needs some refactoring and tweaking, but the Pirhana LED solid model version of the battery holder should give you the general idea.
A year or so ago, a certain Young Engineer suggested my Vacuum Tube Lights really needed battery power and rebuffed my feeble objections concerning low LED intensity (3.6-ish V, not plug-in 5 V USB) and short run time (because three constantly lit LEDs draw too much current). Having a spare NP-BX1 holder lying about, here’s a feasibility study:
Not much to it, eh?
Hitching the DSO150 to a Tek current probe (which needs a 50 Ω load, thus the terminator on the BNC tee) seems a clear-cut case of a sow’s ear joining forces with a silk purse:
It was just sitting there, so why not?
Seen with a bit more detail on a better scope:
Each vertical increment represents the current into a single LED (at 10 mA/div), with the PWM cycles ticking along at 1.3 kHz.
The current steps aren’t the same height, because the LEDs have different forward voltages. The taller step (at the top) probably comes from the red LED, with the other two being blue and green. The maximum current is only 40 mA, not the 60 mA you’d expect with a 5 V supply.
The PWM width, of course, determines the brightness of each LED. Eyeballometrically, the average current will be half of 40 mA for (just less than) half of each PWM cycle, so figuring each SK6812 module (there’s only one here) will draw 10 mA seems reasonable.
The “base load” from the Arduino looks like 2 mA, so there’s not much point in removing its power and status LEDs.
The NP-BX1 lithium cell has lost enough capacity to no longer power my Sony HDR-AS30V helmet camera for at least half of a typical ride. The camera draws around 1 A, so you can clearly see the defunct batteries:
If the average voltage during discharge is 3.3. V, then a 10 mA load would be 33 mW and a defunct NP-BX1 battery with 2 W·h capacity (at 1 A) might provide 60 hours of continuous use. I’d expect more capacity at lower current, although it’s not clear the cells actually behave that way.
So a battery-powered Vacuum Tube Light might make sense, perhaps as romantic illumination for techie snuggling:
Ya never know …
We spotted this anomalous situation halfway up Cochran Hill Road:
It looks like a Verizon FiOS cable box filled with jumpers for all the houses along the way:
You’ll note the missing lock and misplaced latch. The box face isn’t scarred, so getting in must not have required much effort.
The box carries no company identification or emergency numbers, but it does have two theft deterrents:
Perhaps the deterrents worked better in warmer months.
Given how little Verizon wants to hear from its FiOS customers, I have sub-zero motivation for devoting the hours required to find out if it’s their problem. Somebody along Cochran should have enough standing for the case.
We spotted this assortment of jewelry gleaming along Clove Creek:
A closer look at a necklace:
And the brooch:
The water level has been dropping for several days as the air temperature went from tolerably cold to well below freezing.
It’s better in person; I couldn’t get close enough to avoid using the Pixel’s digital zoom, so the images have more gritty texture than you’d expect.
After the Great DMM Probe Debacle, I picked up similar-but-different set of cheap probes and clip leads.
The needle-tip probes carry a 20 A current rating:
If you look out along the wire, though, you’ll find a 10 A rating:
Now, even though 20 AWG wire in silicone may carry a 17 A spec, the corresponding 200 °C temperature seems excessive for a test probe. Limiting the current to 10 A would reduce the power dissipation by two thirds, which should limit the temperature rise. Whether the wire actually contains 20 AWG of actual copper strands remains an open question.
The kit also had banana plug / test hooks with no particular rating, although the wire allegedly has 16 AWG conductors:
The banana plug / alligator clip combo claims 30 A, also with 16 AWG conductors. Who knows? It could be true.
For comparison, the Siglent SDM3045 DMM came with these probes:
The probes carry a 10 A rating and, although the wires aren’t branded, I’ll assume they have good-enough QC to ensure the copper matches the claims. The production values seem a bit higher, too, even if they bear a striking resemblance to the cheap probes.
And, for reference, the probes with the cold solder joint also claim 20 A:
Wouldn’t trust any of ’em for more than a few amps, tops …