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.

Tag: Improvements

Making the world a better place, one piece at a time

Arduino Mega: Voltage Regulator Heatsink
The Arduino Mega has an SMD voltage regulator soldered to a copper pad connected with thermal vias to a similar pad on the bottom surface. The datasheet says the (roughly) 10×10 mm copper pad sets R_θJA=55 °C/W, more or less; probably a bit less with the double-sided setup.

It’s the chubby black slab snuggled up just to the right of coaxial power input jack. The four vias on each side go to an isolated copper pad under the solder mask on the other side.

Arduino Mega voltage regulator

The board draws about 75 mA with nothing fancy on the I/O pins, so the regulator dissipates half a watt with a 12 VDC input supply. Figuring an ambient of 30 °C, the junction temperature is ticking along at 50-60 °C.

That’s all well and good, but my rule of thumb for semiconductors is:
- If you can’t hold your thumb on it for any length of time, it’s too damn hot.
That regulator fails my rule of thumb even before I start adding LEDs and other doodads.

A bit of rummaging turned up an old Thermalloy sample box with a DIP heatsink. A dab of quick-setting epoxy and there it is:

Arduino Mega regulator with heatsink

Now, I’ll grant you there are a number of things wrong with that approach, but my thumb is much happier. If it gets unhappy, I’ll just crack that puppy off and stick something larger in its place.
2010-10-15
Arduino Mega: Mounting Standoff and Screw

The Arduino Mega has four, count them, four mounting holes in a more-or-less rectangular pattern around the edge of the board. Three of the four have enough room for standard pan-head 4-40 screws. The fourth hole is squeezed between two pin-header sockets, to the extent that no screw in my collection will fit.

Screw on Arduino Mega board

Having a lathe, however, makes this situation no big deal… and this time, I put the Sherline drill chuck in the 3-jaw because I wanted to, not because the 3-jaw was stuck. In truth, the lathe chuck simply won’t grab a 4-40 screw at all.

Turning down the screw

I had that pointy right-cutting tool in the holder, so that’s how I cleared off the burr on the backside of the head.

The chuck did grab the hex standoff just fine, though, and the final result looks like this…

Modified standoff and screw for Arduino Mega

For future reference, the turned-down sections are 0.180 inch in diameter; call it 4.5 mm. The snout on the standoff must clear the pins,so it’s 0.100 inch or 2.5 mm long.

2010-10-11
Pull Saw Blade Sheath

Trimming some poster frame edges brought this useful tool out of hiding. It’s an ordinary Stanley 20-331 flush-cut flexible-blade pull saw that I’d used while installing laminate flooring in the kitchen & laundry room some years ago; it cuts the bottom of moulding like nothing else.

Anyhow, what’s of interest here is the simple cardboard sheath I made to keep those delicate little teeth from getting bashed up in the tool drawer.

Saw and sheath

Here’s a closer end view. It’s nothing more than two large sheets of thin cardboard (think cereal box or maybe consumer electronics box) separated by two narrow sheets. The blade’s thinner than any practical cardboard you’ll find, so just do it. It’s held together with yellow wood glue, because that’s what I had nearby when I built it.

Saw sheath end view

The note written on the sheath reminds both my shop assistant and me which way the cutting happens. Sometimes, we need all the help we can get.

For what it’s worth, the new Stanley 20-331 listing shows a similar saw blade with a Fat Max handle. This is the older version, from back when thin was in.

2010-10-10
Homebrew Mini-ITX LPT Bracket
I’m putting together an Atom 510 box to replace the ancient Dell currently acting as the Sherline CNC controller, with the intent of seeing whether a rather anemic low-power CPU with two cores will work as well. The system board has room for one PCI card and I figured I’d install a second parallel printer while I had the hood up.

But then I realized that the only LPT cards in my stash had tall brackets that wouldn’t fit in the new mini-ITX case.

Well, it turns out that the LPT card itself would fit in the box, so all I had to do was reshape the bracket:
- A bit of filing on the bottom knocked off a millimeter and put a tidy taper on the tab
- A brief session with Mr Hammer bent the top flange over, so as to meet the case mounting flange
- A somewhat surprised tin snips removed the excess length
- A cylindrical file chewed out a somewhat generous screw clearance notch
Finished LPT bracket

And then it’s just a matter of screwing things together.

LPT Bracket – outside

LPT Bracket – top

I’ll admit the clearance from the top mounting screw to the flange is terrifyingly cozy, but I’m not averse to applying force majeure to either an unsuspecting LPT connector or the case itself…

The top view omits the screwdown clamp that secures the card to the case so you can see where the screw notch goes.
2010-10-06
Door Stop Bumper Fix

After slightly over half a century, the rubber bumpers on the doorstops around the house have stiffened up and, occasionally, one falls off.

Although I suppose I should just buy a new doorstop, molding a dab of silicone snot around the end of the nice brass post takes only a few minutes (plus an overnight cure). If what they tell us about silicone adhesives is true, this one is good until the sun goes dark…

Re-bumpered door stop

Another no-CNC repair!

2010-10-02
Dual Displays vs Wacom Tablet vs xorg.conf
So, as I wondered there, it turns out that the tablet cursor can fall into the crack between my landscape left monitor and portrait right monitor, with the distressing result that the X server jams with the pointer jittering between the two displays. Worse, the keyboard isn’t active, so I can’t switch to a console screen and shoot X in the head.

I’m guessing this is a picket-fence error, where Something Bad Happens when the cursor hits the maximum possible value: X=16704, in this case.

The solution seems to be telling the Wacom driver that the tablet is just slightly wider than it really is, so that the X value can’t ever reach the maximum. Having done this before, the line is already in my xorg.conf, just waiting to be un-commented:
```
Section "InputDevice"
    Identifier      "Wacom - stylus"
    Driver          "wacom"
    Option          "Device" "/dev/input/wacom"
    Option          "USB" "on"
    Option          "Type" "stylus"
    Option          "Button2" "3"
    Option          "Button3" "2"
    Option          "MMonitor" "off"
    Option          "ScreenNo" "0"
    Option          "BottomX" "16710"
#    Option          "BottomY" "11893"
EndSection

Section "InputDevice"
    Identifier      "Wacom - eraser"
    Option          "Device" "/dev/input/wacom"
    Driver          "wacom"
    Option          "USB" "on"
    Option          "Type" "eraser"
    Option          "MMonitor" "off"
    Option          "ScreenNo" "0"
    Option          "BottomX" "16710"
#    Option          "BottomY" "11893"
EndSection
```
That seems to work, but the failure was intermittent. We shall see…
2010-10-01
CPU Heatsink: Flattening Thereof

I suppose I should have known better: the bottom of that heatsink wasn’t anywhere near flat. I think it mated directly with the top of the CPU through thermal grease, not a compliant pad.

Curved copper heatsink surface

The obvious solution is to flycut the thing, which is where the Sherline’s limited Y-axis travel and teeny table put a cramp on your style. Normally, you’d put the length of the heatsink parallel to the X axis so the flycutter would clear on both ends, but there’s no obvious (read: quick and easy) way to clamp the thing that way.

So I mounted it parallel to the Y axis, which meant I couldn’t get the flycutter completely off the near end. The first pass at Z=-0.1 mm, however, showed that not only was the surface curved, but it wasn’t parallel to the top of the fins (which were flat on the tooling plate). I suppose I should have expected that.

This cut is has Z=-0.1 mm referred to the front end. It completely missed the other end:

First flycut pass

I flipped the heatsink around, measured the front-to-back tilt (about 0.16 mm), stuck a couple of brass shims under the front, and the second pass at Z=-0.05 mm from the new low point did the trick. Copper is nasty stuff and I did these cuts dry: the chips visible near the front are stuck firmly to the surface.

Final flycut pass

I scrubbed both the heatsink and the spreader plate on some fine sandpaper atop the sacrificial side of my surface plate until they were all good. I can see the remaining flycutter marks, but I can’t feel them, and the plates slap solidly together with a pffff of escaping air:

Flattened heatsink and spreader

A dab of heatsink compound should work wonders; the maximum dissipation will be under 20 W, roughly comparable to that old K6 CPU, but now the heatsink will be contacting the entire hot surface.

2010-09-22