Category: Software

General-purpose computers doing something specific

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

Mini-Lathe Chuck Stops: CNC Pocketing

With the fixture aligned and the chuck stop blank clamped down, all that’s left is to make three little pockets:

Lathe Chuck Stop - Pocketing - LinuxCNC backplot — Lathe Chuck Stop – Pocketing – LinuxCNC backplot

Although Javascript may be the gom jabbar of programming, the blinding syntactic noise of raw G-Code puts you in a similar world of hurt:

#<chuckrad>=20.000                  (radius to center of magnet)
#<chuckjaws>=3                      (number of jaws)
#<chuckang>=[360.0/#<chuckjaws>]    (angle between jaws)

#<bitrad>=[2.900/2]                 (cutter radius)

#<pocketrad>=[4.100/2]              (magnet pocket radius)
#<pocketdeep>=2.200                 ( … depth)
#<xoffs>=[#<pocketrad>-#<bitrad>]   (pocket center to cutter center)

#<safez>=20.0                   (above all the clamps & gadgets)

G21 G54 G80 G90 G94             (metric!)

F600                            (full speed for the Sherline)

G0 Z#<safez>

Obviously, those magic numbers must match the laser-cut blanks, the magnets, the cutting bit in the spindle, the clamps on the table, the speed of the machine, and everything else you overlooked.

So. Much. Pain.

Knowing the angle to the current pocket, polar coordinate notation gets to the center point, with a jaunt in relative motion to the starting point for the helix into the pocket:

#<ang>=[#<chuckang>/2]          (set starting angle)
O100 REPEAT [#<chuckjaws>]

G0 @#<chuckrad> ^#<ang>         (to hole center)
G91                             (relative motion …)
G0 X#<xoffs>                    ( … to helix start …)
G90                             ( … and done)

G0 Z0                           ( to surface)

Each pocket consists of a helix cut to the bottom, two clearing passes, and another helix back to the surface:

G2 I[-#<xoffs>] Z[-#<pocketdeep>] P[1+FUP[#<pocketdeep>]]   (into hole)
G2 I[-#<xoffs>] P2                                          (clean bottom)
G3 I[-#<xoffs>] Z0 P[1+FUP[#<pocketdeep>]]                  (shave sides)

That dance produced rounder pockets with cleaner bottoms than just a single helix down and a straight pull upward.

Then set up for the next hole and clean up after the last one:

G0 @#<chuckrad> ^#<ang>         (back to center)
G0 Z#<safez>

#<ang>=[#<ang>+#<chuckang>]     (set up next hole)
O100 ENDREPEAT

G0 Z[2*#<safez>]
G0 X0 Y0

M2

I ran the Sherline XY axes at their 600 mm/min top speed, the spindle at 10 kRPM with a shiny new 3 mm (nominal!) cutter, ramped into the helix at ≅10° (on a 1 mm circle!), and it sliced the acrylic into nice chips without getting all melty.

Unlike with Javascript, when you get something wrong in G-Code, you can hear the crash.

The LinuxCNC pocketing code as a GitHub Gist:

	(Magnet pockets for laser-cut lathe chuck stops)
	(2023-07 Ed Nisley)

	#<chuckrad>=20.000 (radius to center of magnet)
	#<chuckjaws>=3 (number of jaws)
	#<chuckang>=[360.0/#<chuckjaws>] (angle between jaws)

	#<bitrad>=[2.900/2] (cutter radius)

	#<pocketrad>=[4.100/2] (magnet pocket radius)
	#<pocketdeep>=2.200 ( … depth)
	#<xoffs>=[#<pocketrad>-#<bitrad>] (pocket center to cutter center)

	#<safez>=20.0 (above all the clamps & gadgets)

	G21 G54 G80 G90 G94 (metric!)

	F600 (full speed for the Sherline)

	G0 Z#<safez>

	#<ang>=[#<chuckang>/2] (set starting angle)
	O100 REPEAT [#<chuckjaws>]

	G0 @#<chuckrad> ^#<ang> (to hole center)
	G91 (relative motion …)
	G0 X#<xoffs> ( … to helix start …)
	G90 ( … and done)

	G0 Z0 ( to surface)

	G2 I[-#<xoffs>] Z[-#<pocketdeep>] P[1+FUP[#<pocketdeep>]] (into hole)
	G2 I[-#<xoffs>] P2 (clean bottom)
	G3 I[-#<xoffs>] Z0 P[1+FUP[#<pocketdeep>]] (shave sides)

	G0 @#<chuckrad> ^#<ang> (back to center)
	G0 Z#<safez>

	#<ang>=[#<ang>+#<chuckang>] (set up next hole)
	O100 ENDREPEAT

	G0 Z[2*#<safez>]
	G0 X0 Y0

	M2

view raw Lathe Chuck Stop – Pocketing.ngc hosted with ❤ by GitHub

2023-07-14

Mini-Lathe Chuck Stops: Pocketing Fixture
Putting pockets in the legs of the mini-lathe chuck stop blanks requires a fixture to align them in the Sherline mill:

Lathe Chuck Stops – pocketing setup

Because it need not withstand much lateral force and will get used only a dozen-ish times, the base is MDF and the stop alignment happens in three matching chipboard layers:

Lathe Chuck Stops – Pocketing Fixture – LB layout

The three stops (over on the right) are copy-pasta from the originals. A 0.1 mm outset in the chipboard (center) lets the acrylic shapes drop into the chipboard sheets with Good Enough™ alignment accuracy. The MDF layer (left) provides some overshoot comfort below the chipboard.

The chipboard layers each have four alignment targets at (±30,±20):

Lathe Chuck Stops – pocketing fixture touchoff

Touch off the lower-left target at (-30,-20) and G0 X30 Y30 should drop the laser dot in the middle of the upper-right target. With the (0,0) origin at the geometric center of the stop, LinuxCNC’s polar notation picks out the three pockets:
```
G0 @20 ^-60
G0 @20 ^180
G0 @20 ^60
```
The plywood disk under the Sherline’s clamp has a glued ring to put the clamping force out near the ends of the legs. I started with just the aluminum clamp, but the legs needed a bit more stability; a laser cutter makes impromptu widgets like that trivially easy.

Next: write the G-Code to make the pockets.

The LightBurn SVG layout as a GitHub Gist:

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view raw Lathe Chuck Stops – Pocketing Fixture – LB layout.svg hosted with ❤ by GitHub
2023-07-13
Mini-lathe Chuck Stops

Having occasionally been in need of a lathe chuck stop, I finally cleared that project off the heap:

Lathe Chuck Stops – demo setup

These are definitely not up to commercial standards, but also don’t cost fifty bucks each. A trio of 4×2 mm neodymium disk magnets stick the stop to the chuck (and to each other) with enough force to hold it there, but not enough to make removing it a hassle.

I imported the Z axis orthogonal view of the chuck jaws from the ball fixture for the running lights:

Lathe Chuck Jaws – solid model axial

Trace the right-side jaw, clean it up, put the tip a known distance from the origin, make a circular array, and draw a comfort circle the size of the chuck OD.

The stop geometry comes from a hull wrapped around a circle a few millimeters larger than the 4 mm magnet (out 20 mm from the center) and a circle at the center sized so the hull clears the jaws:

Lathe Chuck Stops – LB layout

Then a small circle at the center allows me to drop the stop atop a known coordinate and rotate it around the circle, because the XY coordinate center is not at the geometric center.

I cut out a few chipboard samples to verify the sizes, a few more from scrap acrylic to set up the pocketing operation, then half a dozen of each in cheerful kindergarten colors:

Lathe Chuck Stops – on-lathe storage

The 5 mm stop is obviously too fragile for commercial success, but I figured it’ll survive long enough around here. Worst case, I can make another handful as needed.

Although I have laser-engraved pockets in plywood, a few experiments in acrylic confirmed the surface finish is terrible and the depth control is iffy, at best. Given that I need a 2.2 mm deep pocket in 3 mm acrylic, a CNC mill seems the right way to poke the pockets:

Lathe Chuck Stops – pocketing setup

More on that tomorrow.

The LightBurn SVG layout as a GitHub Gist:

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view raw Lathe Chuck Stops – LB layout.svg hosted with ❤ by GitHub

2023-07-12
Eyeglass Case Padding Redux
Confronted with a nice metal eyeglass case that had lost its original liner, I traced the outline on paper and scanned it:

Metal case outline

Unlike the plastic Zenni cases, this one has nice straight edges, so:
- Eyeball a LightBurn rectangle over the traced image
- Round the corners to suit
- Shrink it by a few millimeters to make it fit inside
Then:
- Add a perimeter line offset by the 6 mm required to cover the sides
- Draw a dart in each corner to allow for bending the foam
- Set the perimeter priority to 1 so it cuts last
- Put the original outline to a tool layer to remind me how to do this the next time around
Which looks like this:

Metal case pad – LightBurn layout

Then Fire The Laser into a sheet of EVA foam:

Metal eyeglass case – padding cut

Stuff it into the case, do another one in brown, and the result looks kinda like it should:

Metal eyeglass case – padding installed

That was easy …
2023-07-03

Patio Chair Foot Adapters

Some years ago we acquired a free quartet of aluminum-frame patio chairs in need of new straps and feet. Eventually enough straps broke to force me to re-strap the things and I finally got around to replacing the badly worn OEM feet:

Patio Chair Foot Adapter - OEM feet — Patio Chair Foot Adapter – OEM feet

The small drilled holes let me yoink most them out with sheet-metal screw attached to a slide hammer, then apply the Designated Prydriver to the most recalcitrant / broken ones.

Some feet had worn enough to expose the aluminum tubes, but most had at least a thin layer of plastic:

Patio Chair Foot Adapter - OEM foot erosion — Patio Chair Foot Adapter – OEM foot erosion

Obviously, I should have stripped and repainted the frames (if that’s possible, as they’re probably powder-coated), but a man’s gotta know his limitations and this job needed to get done.

One might think patio furniture replacement feet are cheap & readily available, but no amount of keyword engineering produced search results with any degree of assured fit, so I conjured adapters for screw-in feet from the vasty digital deep:

Patio Chair Foot Adapters - solid models — Patio Chair Foot Adapters – solid models

This was a long-awaited opportunity to explore the BOSL2 library and it worked wonderfully well. Each adapter is whittled from a huge hex nut with threads that perfectly fit the M8×1.25 stud, which stands vertically through the middle of the (slightly oval) bottom surface parallel to the floor.

The front tubes have a 5° angle with respect to the vertical:

Patio Chair Foot Adapter - front — Patio Chair Foot Adapter – front

And the rear tubes are 15° off:

Patio Chair Foot Adapter - rear — Patio Chair Foot Adapter – rear

Each adapter has an orientation notch pointing toward the front of the front leg and the rear of the rear leg:

Patio Chair Foot Adapter - orientation notch — Patio Chair Foot Adapter – orientation notch

I expected to apply adhesive on the inside and outside of the adapters, but they tapped firmly into place inside the legs and the studs screwed firmly into them, so we’ll see how they survive in actual use. I expect the studs to rust after a while, but that might not be the most awful thing ever to happen.

The OpenSCAD source code as a GitHub Gist:

	// Patio chair foot adapter
	// Ed Nisley – KE4ZNU – 2023-06

	include <BOSL2/std.scad>
	include <BOSL2/threading.scad>

	LegAngles = [5,15];

	/* [Hidden] */

	ThreadThick = 0.25;
	ThreadWidth = 0.40;

	HoleWindage = 0.2;

	Protrusion = 0.1; // make holes end cleanly

	function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);

	ID = 0;
	OD = 1;
	LENGTH = 2;

	inch = 25.4;

	//———————-
	// Dimensions

	LegTube = [18.8 – HoleWindage,22.5,0];

	Stud = [8.0,1.25,10.0]; // M8x1.25 foot stud

	FlangeBase = 3.0;

	//———————–


	for (i=[0:len(LegAngles)-1]) {

	ang = LegAngles[i];

	FlangeIncr = LegTube[OD]*tan(ang);
	Flange = [Stud[0],LegTube[OD],FlangeBase + FlangeIncr];

	translate([i1.5Flange[OD],0,0])
	difference() {
	translate([0,0,0*-FlangeIncr/2])
	threaded_nut(2Flange[OD],Stud[0],1.5Stud[2],Stud[1],
	anchor=BOTTOM,bevel=false,$slop=0.2);
	rotate([0,ang,0]) {
	translate([0,0,FlangeBase + FlangeIncr/2])
	tube(4Stud[2],2Flange[OD],LegTube[ID]/2,
	anchor=BOTTOM);
	tube(4Stud[2],2Flange[OD],Flange[OD]/2,
	anchor=CENTER);
	}
	cube([Flange[OD],ThreadWidth,2*ThreadThick],anchor=BOTTOM+RIGHT);
	}
	}

view raw Patio Chair Foot Adapters.scad hosted with ❤ by GitHub

2023-06-29

Danger Zone Earrings: More SCP Warning Labels

More tinkering produced a full set of SCP warning labels in vector format suitable for laser cutting:

SCP warning signs – LB layout

The faint blue corresponds to the LightBurn tool layer, because you’ll want to assign your own cutting parameters.

The circumscribing circle provides a convenient way to snap the pattern into something else, because the symbols in the middle are not necessarily centered around their geometric midpoint.

Suiting action to drawings:

SCP Earrings – black on yellow – cutting

The acrylic fire shows they’re called Danger Zone earrings for well and good reason!

Anyhow, weeding the black vinyl produces crisp results:

SCP Earrings – black on yellow – overview

The fallout shelter symbol (top right) should have a circle around it, but that’s in the nature of fine tuning. It’s also not part of the SCP canon, but it kinda goes along with the radiation warning sign.

They’re cut from transparent amber non-edge-lit acrylic with black vinyl PSA patterns:

SCP Earrings – black on yellow – detail

Still not enough to get me to go full-frontal Mr Clean.

The LightBurn SVG layout as a GitHub Gist:

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view raw SCP Warning Labels – LB vector.svg hosted with ❤ by GitHub

I have no explanation for the different stroke widths, other than that SVG files seem to maintain a memory of every transformation applied to any object. LightBurn doesn’t use the stroke widths, so it should work out just fine.

2023-06-12