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: Machine Shop

Mechanical widgetry

SJCAM M20 Camera: NP-BX1 Battery and Charger Holder

A little tweakage to the NP-BX1 battery holder for the astable multivibrator blinkies produced a simple version with the wire exit holes on the bottom:

NP-BX1 Simple Holder – solid model

The four corner holes hold locating pins in the layered acrylic base:

SJCAM M20 Battery Replacement – case layers

Those pins got cut slightly shorter to fit in the battery holder; in this photo they’re serving to align the layers and adhesive sheets while I stacked them up.

The geometry is straightforward, with the outer perimeter matching the 3D printed battery holder:

SJCAM M20 Car-Mode Battery Hack – battery case

Cut one base and two wall layers from 3 mm (or a bit less) transparent acrylic, plus three adhesive sheets. I stuck adhesive on both sides of one wall layer, using the pins to align the adhesive, stuck the layer to the base, then topped it with the second wall layer, again using the alignment pins.

The motivation for transparent layered acrylic is being able to see the charge controller’s red and green status LEDs glowing inside the box. This probably isn’t required, but seemed like a Good Idea™ for the initial version.

With all that in hand, wire it up:

SJCAM M20 Battery Replacement – charger wiring

The USB charger PCB sits atop a layer of double-sided foam tape. After verifying that the circuitry worked, I globbed the wires in place with hot-melt glue to make it less rickety than the picture suggests.

The alert reader will have noticed the holes in the 3D printed NP-BX1 holder were drilled, not printed. In the unlikely event I need another case, the holes will automagically appear in the right place.

I haven’t yet peeled the protective paper off that top adhesive sheet to make a permanent assembly:

SJCAM M20 Battery Replacement – trial install

We use the car so infrequently that it’ll take a while to build up enough confidence to stick it together and stick it to the dashboard.

On the whole, it’s ugly but sufficient to the task.

A doodle with key dimensions, plus some ideas not surviving contact with reality:

SJCAM M20 Car-Mode Battery Hack – case doodle

I truly hope this entire effort is a waste of time.

2023-08-02
SJCAM M20 Camera: Battery Case Salvage

Remove the spicy pillow from an M20 battery case and carve a notch in one side to see if this might work:

SJCAM M20 Battery Replacement – battery interior

The circuit board is the charge controller for the evicted high-voltage lithium pouch cell, but I started by connecting an ordinary lithium cell with a Schottky diode to the PCB’s battery terminals.

This worked about as poorly as you’d expect, because the lower battery voltage minus the forward drop of the diode minus whatever happens in the PCB put the final voltage below the camera’s instant low-battery shutdown.

The terminals connecting to the camera in the rectangular bump are soldered to the back of the PCB, but the whole affair snaps out of the battery case. Unsoldering the PCB from the terminals, gingerly soldering directly to them, and adding a bulk storage capacitor produced a better result:

SJCAM M20 Battery Replacement – circuitry

The cap stores just enough energy to keep the camera happy while writing to the Micro-SD card, although the LCD screen dims slightly during each pulse.

Cut a pad from a sheet of closed-cell foam that happened to be exactly the right thickness:

SJCAM M20 Battery Replacement – wrapper layout

The elaborate thing below the case is a cardboard pad atop the sticky side of a PSA non-PVC vinyl sheet, laser-cut to fit:

SJCAM M20 Battery Replacement – case wrapper top

The bottom view, showing the latch retaining the contact block:

SJCAM M20 Battery Replacement – case wrapper bottom

Admittedly, that’s the last iteration of the wrapper, starting with a hand-trimmed Kapton tape version and three paper versions to get the dimensions right before trying vinyl. Looks good to me!

The final geometry has a 0.5 mm radius on all the corners:

SJCAM M20 Car-Mode Battery Hack – battery wrapper

The fillets reduced (but did not eliminate) mechanical oscillations while slinging the laser gantry around those corners. If I don’t point them out, maybe nobody will notice.

The PSA vinyl is marginally thicker than the original plastic wrapper, so the battery fits very snugly into the camera. On the other paw, getting the swollen battery out required a major effort; this one should not get tighter.

2023-08-01
SJCAM M20 Camera: Car Mode Battery Hack

The last lithium cell (a.k.a. battery) for the longsuffering SJCAM M20 transformed itself into a spicy pillow:

SJCAM M20 – spicy pillow lithium battery

SJCAM no longer sells those batteries and nobody else does, either, surely because the +4.35V marking shows they’re a special-formula high-voltage lithium mix that doesn’t work with ordinary chargers. Worse, you can’t substitute an ordinary (i.e. cheap) battery, because applying a high-voltage charger to a 4.2 V cell makes Bad Things™ happen.

Putting the M20 camera in Car Mode makes it begin recording when it sees 5 V on its USB input and shut down a few seconds after the USB input drops to 0 V. Without the internal battery, the camera’s clock doesn’t survive when the external power vanishes, which seems critical for a camera sitting on a dashboard.

Mashing all that together, I wondered if I could use one of the many leftover low-voltage NP-BX1 batteries from the Sony AS30V helmet camera without starting a dashboard fire, by preventing the camera from charging the battery, while still using it when the USB input is inactive (which, for our car, is pretty nearly all the time).

The circuitry, such as it is, uses a cheap 1S USB charge controller and a Schottky diode:

SJCAM M20 Car-Mode Battery Hack – circuit doodle

Power comes in on the left from a USB converter plugged into the Accessory Power Outlet in the center console and goes out to the camera’s USB jack, using a butchered cable soldered to the charge controller’s pads in the middle. The controller manages the NP-BX1 battery as usual, but a diode prevents the camera from trying to send charge current into the controller.

This should just barely work, as the diode reduces the battery voltage by a few hundred millivolts, so the camera will see the fully charged low-voltage battery as a mostly discharged high-voltage battery.

Suiting action to words:

SJCAM M20 Battery Replacement – circuitry

It’s built inside the gutted remains of an M20 battery case. The 100µF tantalum cap provides local buffering to prevent the camera from browning out during bursts of file activity while recording. The wire emerges through holes gnawed in the battery case and the camera housing:

SJCAM M20 Battery Replacement – camera cable exit

The charge controller on the other end of the wire lives in a layered laser-cut acrylic case attached to a modified version of the venerable 3D printed NP-BX1 battery holder:

SJCAM M20 Battery Replacement – charger wiring

More on the cases tomorrow.

Putting it all together, the lashup goes a little something like this:

SJCAM M20 Battery Replacement – trial install

The battery pack will eventually get stuck to the dashboard underneath the overhang, out of direct sunlight. Things get hot in there, but with a bit of luck the battery will survive.

The rakish tilt puts the hood along the bottom of the image, although raising the camera would reduce tilt and cut down on the skyline view:

SJCAM M20 Car-Mode Battery Hack – test ride

The battery icon instantly switches from “charging” to “desperately low” when the USB power drops, which is about what I expected, but the camera continues to record for about ten seconds before shutting down normally.

The NP-BX1 battery in the holder comes from the batch of craptastic BatMax batteries with a depressed starting voltage. An actual new cell with a slightly higher voltage would keep the camera slightly happier during those last ten seconds, but … so far, so good.

Another possibility would be a trio of 1.5 V bucked lithium AA cells, with the diode to prevent charging and minus the charger.

2023-07-31
Schwalbe Marathon Plus vs. Glass Chip

My pre-ride thumb check showed a flat rear tire on Mary’s Tour Easy:

Glass chip – end view in tread

So we fetched groceries with the car.

As usual, no tire armor can withstand a glass blade:

Glass Chip – side view

It’s a bit over 5 mm from the knife edge to the ground-flat end, just long enough to punch through a rather well-worn Schwalbe Marathon Plus tire and poke a slow leak in the tube.

The tire has covered enough miles to wear the tread down to maybe half a millimeter over the blue armor layer:

Glass chip – tire damage

Time for a new tire!

For the record, the odometer is just shy of 35 k miles and she rides about 1500 miles a year; somewhat less over the last year for reasons not relevant here. As best I can tell, the tire has been on there for about five years and 7000 miles.

2023-07-27
LightBurn: Nesting Shapes

A question on the LightBurn forum about packing shapes onto an irregular piece of leather let me work out the details of a LightBurn feature I thought I understood but had trouble explaining.

Start with an irregular shape:

Random fabric – overview

That’s made of rags from the box o’ wipes out of view on the right, laid out in no particular order, on a contrasting background to simplify the next step.

The camera tucked into the lid shows the view from above:

Random fabric – LB camera overlay

Tap the Trace button and fiddle with the sliders to get a nice solid outline, along with other junk off the edge of the cardboard:

Random fabric – LB trace

All of the traced vectors will be in a group:

Random fabric – LB shapes

Ungroup them, select the outline in the middle, invert the selection, and mass-delete the junk around the edges.

If you don’t move anything, the outline will be exactly over the shape on the platform. This will come in handy later.

Import all the shapes you want nested inside the outline, group them with the outline, and hand them to the Arrange → Nest Selected tool:

Random fabric – LB nesting setup

LightBurn saves the selected objects as an SVG file with the file name in the clipboard and fires up a browser tab at https://svgnest.com/. Upload the SVG and let the nesting algorithm chew away for a while:

Random fabric – LB nested

The weird triangles come from the Dot Mode perforations that ought not be there; inner shapes get subtracted from outer ones, which makes perfect sense. Your shapes will differ.

Download the nested shape SVG, load it back into LightBurn at the prompt generated after exporting the shapes, and LightBurn will apply the transforms to the original shapes. Put the outer shape on a tool layer and the inner shapes on whatever cutting layer you like, snap the outer shape (with the nested shapes inside) to the previously undisturbed outline of the stuff on the platform, and Fire The Laser!

Now there’s a pretty good chance I can do that again …

2023-07-26
Laser Perforations
A discussion on the LightBurn forum produced a hacky way to laser-cut pinholes at precise locations:

Laser-cut pinhole – aligned exit

That’s the 0.3 mm exit wound in 3 mm acrylic, one of the mini-lathe chuck stops, carefully hand-held to align the channel.

Squinting at similar holes through clear acrylic shows they’re smoothly melted (as you’d expect), but not exactly perpendicular to the surface. I’m sure the acrylic gas pushes the beam around and erodes the sides of the channel as it boils out of the progressively deepening hole.

The entry wound is about half a millimeter:

Laser-cut pinhole – entry

The heat-distorted strip around the perimeter is less obvious in real life without magnification. The protective plastic film over the surface melts easily and, although it does keep the fumes from condensing, causes a bit of damage.

Each pinhole comes from a single dot in LightBurn’s Dot Mode, so you must arrange the dot spacing to match the path:

Lathe Chuck Stop – Pinhole distance

The pockets are on a 40 mm BCD, so they’re out 20 mm from the center and the hole-to-hole distance is:
```
34.64 mm = 2 × 20 mm × cos(30°)
```
Set the dot distance to that exact number and It Just Works.

The laser turns on for a specific number of milliseconds at each dot. In this case, I used 50 ms with the layer set to 70% PWM. You could surely optimize the values.

The starting pinhole gets drilled twice, which happens because Dot Mode expects to make a line of perforations with one dot at each end. In this case, the end of the last line overlaps the start of the first line; two lines would work better than a triangle.

You could make a square array from a single line with (many) dots at the desired spacing, separating the lines by the same spacing.

A circular array might work, too, with a straight line joining successive holes.

Undo would definitely be my copilot while figuring those out.

This could make an easily clogged trash strainer or a filter for small chunks.
2023-07-20
Laser Power Measurement: Geometric Beam Absorber
CO₂ laser power meters seem to depend on a flat-black absorbing surface to soak up a (typically unfocused) beam pulse, backed by a known metal mass with a thermocouple to measure the temperature rise above ambient. Knowing the pulse width, the temperature rise, the absorber mass and specific heat capacity, you can compute the pulse energy and average power during the pulse.

Previous tinkering with an old Gentec ED-200 showed this works well, although the absorber surface took something of a beating because it was definitely not rated for the OMTech’s 60 W (claimed) beam power.

Rather than using a spendy absorber surface with a durable coating, perhaps a geometric absorber using reflective surfaces arranged to channel the energy into the material, rather than away from it, might suffice.

Consider a pack of ordinary utility knife blades:

Beam absorber – utility blades – overview

Seen kinda-sorta perpendicular to the sharpened side of the blade edge, they’re wonderfully reflective:

Beam absorber – utility blades – edge flat

Seen perpendicular to the edge itself, they’re dead black:

Beam absorber – utility blades – edge-on

Well, pretty close to dead black. It’s darker in real life, with glimmers along the edge and the rest of it a deep black. The edges are sharp, but utility knife blades will lead a rough life and they don’t start out Scary Sharp.

Xacto blades come closer to an ideal razor edge:

Beam absorber – Xacto 11 blades – edge-on

The only things you (well, I) see is dust on the edges. The rest is dead black, because light hitting any shiny surface is reflected deeper into the notch between two blades and eventually absorbed.

Double-edge razor blades are sharper and would likely be even blacker, particularly cheap ones without fancy lubricating coatings.

Bonus: the wavelength of CO₂ laser IR light is 10-20× that of visible light, which makes the surfaces that much more reflective. The geometry still channels the reflections into the block and nothing comes out.

There are some fairly obvious reasons why nobody uses a stack of razor blades as a beam absorber in real life:
- Lethally sharp cutting hazard
- Impossible to clean without wrecking the edge
But for personal use, why not?

Some doodles:

Steel has a specific heat around 0.47 J/g·K and a stack of utility blades weighing 140 g is 23 mm across. Soaking up a 60 W beam will raise the temperature of the stack by:
```
0.91 K/s = 60 J/s / (0.47 J/g·K × 140 g)
```
Which seems reasonable: fire a 10 s burst, measure the temperature rise, and multiply by 0.91.

Similarly, a stack of Xacto #11 weighing 15 g is 11 mm across and the temperature will rise 8.5 °C/s. You’d use that for lower power beams.

You could clamp the blades into a larger heatsink, perhaps with a thermocouple / thermistor in a hole drilled into the block.

Calibrate the stack / heatsink with an embedded cartridge heater: voltage × current × pulse width gives the power dumped into the block, so measuring the temperature rise gives you the temperature-power relation.

This feels like a great Arduino project, although it’s nowhere near getting started.

At least I got that scrap of paper off my desk …
2023-07-19