Archive for May, 2019

MPCNC Drag Knife: PETG Linear Bearing

Having reasonable success using a 12 mm hole bored in a 3D printed mount for the nice drag knife holder on the left, I thought I’d try the same trick for the raw aluminum holder on the right side:

Drag Knife holders - detail
Drag Knife holders – detail

The 11.5 mm body is long enough to justify making a longer holder with more bearing surface:

Drag Knife Holder - 11.5 mm body - Slic3r preview
Drag Knife Holder – 11.5 mm body – Slic3r preview

Slicing with four perimeter threads lays down enough reasonably solid plastic to bore the central hole to a nice sliding fit:

Drag Knife - 11.5 mm body - boring
Drag Knife – 11.5 mm body – boring

The top disk gets bored to a snug press fit around the flange and upper body:

Drag Knife - 11.5 mm body - flange boring
Drag Knife – 11.5 mm body – flange boring

Assemble with springs and it pretty much works:

Drag Knife - hexagon depth setting
Drag Knife – hexagon depth setting

Unfortunately, it doesn’t work particularly well, because the two screws tightening the MPCNC’s DW660 tool holder (the black band) can apply enough force to deform the PETG mount and lock the drag knife body in the bore, while not being quite tight enough to prevent the mount from moving.

I think the holder for the black knife (on the left) worked better, because:

  • The anodized surface is much smoother & slipperier
  • The body is shorter, so less friction

In any event, I reached a sufficiently happy compromise for some heavy paper / light cardboard test shapes, but a PETG bearing won’t suffice for dependable drag knife cuttery.

Back to the laboratory …


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M20 Camera Operation

A reader asked how the M20 camera mount on my bike works with respect to the camera’s clock; this description explains a few things missing from the original writeup.

SJCAM M20 Mount - Tour Easy side view
SJCAM M20 Mount – Tour Easy side view

Do you have to set the time & date at start of every ride?

The internal clock shuts down about ten seconds after you pull the battery. If-and-only-if you swap batteries fast enough, it’ll keep time forever. Screw up once and it snaps back to Epoch Zero.

“Car mode” automagically begins recording when USB power goes on, but the manual advises:

TIP: When using your camera as a dashcam, use a car charger cable and remove the internal battery to make sure it does not die out while you travel.

That’s because the M20 continues to run from its internal battery when USB power drops. After recording an hour of a parking lot or your garage wall, the battery dies and so does the clock.

Of course, without the internal battery, the clock dies ten seconds after you turn off the car.

The internal battery has many days of capacity with the camera turned off (whew!), so I conjured the case & PowerCore battery tray to handle our normal rides. The internal battery keeps the clock alive overnight and during the rain we’ve had for the last week, the PowerCore supplies juice during the ride, and I recharge the PowerCore every few weeks.

The M20 doesn’t draw charging current when I turn it on, but poking the PowerCore’s status button also turns on its outputs, whereupon the M20 decides it should begin charging and, bonus, draw power from the PowerCore during the entire ride. The M20 finishes charging while we ride, but the PowerCore continues supplying power and, when I turn the M20 off, the PowerCore sees no current draw and shuts itself off.

Only a geek could love a lashup like that, but it works around the M20’s broken clock and removes its battery maintenance hassle.


LED Nightlight Base Teardown & Simulation

I volunteered to take a look inside a small LED nightlight base to see how well it might work as a power supply for other circuitry:

Nightlight - overview
Nightlight – overview

Note: the AC plug is not polarized. Either blade can contact the hot side of the AC line.

The cadmium-selenide photocell in front turns the white LED on when it sees darkness and off when it sees lightness, with a more-or-less proportional response during dimness. The LED has an obvious 60 Hz flicker, particularly during its partially on phase, so I didn’t expect much inside.

The component side of the PCB faces toward the blades, which you’re looking along the lengths of:

Nightlight - PCB component side
Nightlight – PCB component side

The solder side faces away from the outlet:

Nightlight - PCB solder side
Nightlight – PCB solder side

Flipping the solder side left-to-right and overlaying the two images produces an X-ray-ish view useful for tracing the circuitry:

Nightlight - PCB trace overlay
Nightlight – PCB trace overlay

Some doodling extracts an LTSpice schematic:

Nightlight schematic
Nightlight schematic

None of the component values seem particularly critical; the diodes and transistor are close approximations to what’s really inside. I think the 100 Ω resistor also serves as a fuse, should anything else go wrong.

Setting the CdS cell to 1 MΩ = “dark” turns the LED on:

Nightlight - ON waveforms
Nightlight – ON waveforms

Although I don’t trust the numbers very far, the LED current waveform definitely suggests the flicker isn’t all in my head.

Setting the cell to 10 Ω = “light” turns the LED off, by the simple expedient of clamping the filter capacitor voltage well below the LED’s forward drop:

Nightlight - OFF waveforms
Nightlight – OFF waveforms

When the LED is off, the transistor current is slightly higher than the LED’s on-state current, because saturation voltage:

Nightlight - OFF - transistor current
Nightlight – OFF – transistor current

The current runs right through the 820 nF capacitor, which serves as a more-or-less 3.2 kΩ ballast resistor:

Nightlight - OFF - 820 nF cap current
Nightlight – OFF – 820 nF cap current

It’s a nice film cap and should have a low ESR, but this seems a bit sketchy to me.

So, basically, the nightlight doesn’t really have a power supply in the usual meaning of the term and isn’t suited for driving anything other than the white LED inside the case. Relocating the LED outside the case is an Extremely Bad Idea™, because the anode is one diode away from what might well be the hot AC line; one little oopsie and you’ve got a lethal shock hazard.


Reversible Belt Buckle Re-staking

It’s a shorter belt with the same failure mode:

Reversible belt buckle - parts
Reversible belt buckle – parts

I clamped the whole affair on a block to align all the parts:

Reversible belt buckle - alignment clamping
Reversible belt buckle – alignment clamping

Drop the pin and spring in place, whack it with a punch, and it’s all good:

Reversible belt buckle - restaked
Reversible belt buckle – restaked

That was easy …

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Pedal Spindle Wrench Flat Tweakage

A new-old-stock pair of pedals for Mary’s bike had wrench flats just slightly too narrow for my 15 mm wrench:

Titanium pedal spindle - as built
Titanium pedal spindle – as built

Well, that’s easy to fix:

Titanium pedal spindle - filed to flats
Titanium pedal spindle – filed to flats

For reasons lost in the mists of time, those are titanium spindles. They file just like steel; I’m not fussy.

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Epson R380 Printer: Waste Ink Counter Reset

Following the same drill as before, the Epson R380 printer once again thinks I’ve changed its diaper before resetting its waste ink counter. Instead, I’ve poured what would be a moderate fortune of waste ink down the drain from the external tank, had I not grafted a continuous flow ink supply onto the thing.

To judge from how often I must reset the counters, I’m expected to buy a new printer every three years. For sure, it’s uneconomical to have anybody else (the nearest Epson Authorized Customer Care Centers is 68 miles away on Long Island) do the deed. As Epson delicately puts it “replacement of ink pads may not be a good investment for lower-cost printers”.

Epson now provides a utility allowing you to reset the counters exactly one time. Having a scrap Windows PC ready to go, I didn’t bother capturing the partition before firing off the previous Sketchy Utility™, nor did I restore it, so the whole process took about half an hour.

The hard drive platters will eventually become nightlights.



Equipping an Electronics Lab on the Cheap

I described how to take advantage of Living in the Golden Age of Electronic Making for HVOpen:

Although you don’t get my patter, perhaps the linkies will make up for the silence:

I filled a table with Show-n-Tell widgets and a good time was had by all: hardly anybody fell asleep.

[Update: The talk addressed folks interested in starting out with electronic projects who have no test equipment at all. The choices would be different for other audiences, but … boat anchors aren’t appropriate here.]