Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
The SJCAM M50 camera gasket seems unable to cope with The New Normal weather conditions around here:
SJCAM M50 – screen condensation
I think this was probably another case of diurnal pumping, given the exceedingly hot days and cool nights in late July.
Plenty of water condensed on the bottom of the battery compartment cover:
SJCAM M50 – battery lid condensation
And inside the compartment around the AA cells:
SJCAM M50 – battery compartment condensation
Unlike the previous leak, the camera lens wasn’t involved, so I did not disassemble the case. I let the opened camera (without batteries) dry out in the hot hot sun for the rest of the day and it seemed fine by evening.
Keeping it out of full sunlight during the day definitely limits the locations I can use.
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.
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.
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.
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.
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.
Although the oven igniter I just installed worked, its 3.0 A current fell below the gas valve’s minimum 3.3 A, which, based on past experience, suggested it would fail in short order. Just to see what happened, I sent a note to the seller, who offered a warranty swap and, after a bit of fiddling, the replacement arrived:
Oven Igniter B – 3.3 A initial current
This one draws exactly 3.3 A, so it just barely meets both its product description and the gas valve’s minimum current.
The Smell of Molten Projects in the Morning 