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.
Electrical & Electronic gadgets
My stock of single-row header pins seems to be running short, so it’s time for another slitting session:
Manual CNC, typing bare G-Code directly into LinuxCNC Axis: no reason to turn the cranks by hand.
This makes absolutely no economic sense, but it’s a sticky-hot day and the Basement Laboratory has the dehumidifier. Some day I’ll run into a killer surplus sale of single-row headers and that’ll solve the problem forever…
The Basement Warehouse Wing has an essentially unlimited supply of pristine CD cases (remember CDs?) that, with a bit of deft bandsaw work, will each emit a pair of 4×4 inch sheets of perfectly transparent acrylic plastic. The sheets are about 1.3 mm = 50 mils thick, which is just about exactly what you want for a Nixie-style display that doesn’t require high voltages, because you can edge-light a sheet with 0603 amber SMD LEDs. Obviously, this is not a Shining New Idea, but this post collects my doodles so they don’t get lost along the way.
The Squidwrench StickerLab session prodded me into lashing a prototype together to see how this would work; they have a Silhouette Cameo vinyl cutter driven with Robotcut that works well. I’d hoped to do some laser cutting at the subsequent session, but our schedules didn’t mesh.
The compelling advantage of laser cutting is that you could crack the CD cases apart, throw out the CD holder gimcrackery, lay the sheets flat on the cutter table with the latches & other junk upward, and burn the digits out of the middle without any further preparation. I think I could get much the same effect, at least for a crude prototype, by milling & engraving with the Sherline.
The sheets are about 4 threads of 3D printed plastic extruded at the M2’s default 0.4 mm width. You could print a black baseplate with slots to hold the sheets, put two threads between each sheet, and have the sheet 6 threads apart on center = 2.4 mm spacing:
Ten such sheets would produce a standard 0-to-9 display about an inch deep, plus protective sheets front and back, so the whole affair would be maybe 1.25 inch deep. You’d probably want to offset the tabs on adjacent sheets to reduce light leakage between LEDs. The baseplate fits atop a PCB with LEDs at the right locations, so you get an opaque holder for the sheets that’s easy to produce and easy to assemble:
If you were clever, you could have different tab locations on each sheet so they’d fit only in the proper orientation; that might be important for cough mass production.
The M2 has a platform big enough to build an entire clock base in one pass, plus a matching piece to capture the tops of the digits. I think edge-lit acrylic needs a complete opaque surround for each digit sheet to block light leaking from the edges; it might be easier to build the mount in the other direction, lying flat on the platform, stack the mounts together with the digit sheets, then bolt the whole assembly from the front; that would ensure perfect alignment of everything.
In that case, the 3D printed layers are 0.25 mm (or smaller), but the resolution for the tabs would be 0.4 mm. If you were exceedingly brave & daring, you could lay the digit sheets in place during the build and come out with a monolithic unit; that might require a bit of clearance atop each sheet, as a grazing touch from a hot nozzle would be painfully obvious.
There’s no reason you couldn’t have 16 sheets for a hexadecimal display; this would work out nicely with 8-bit shift registers using SPI from the usual Arduino-love controller. One might prefer current-limiting LED drivers.
There’s also no reason you couldn’t use a wider “digit” sheet and engrave, say, the days of the week or the units of measurement or something like that on each panel.
If the display will be 30 mm deep, then the digits must be large enough that the depth doesn’t turn each digit into a tunnel. Large Nixe tubes had digits about 40 mm tall, so I went with a 30 x 45 panel, plus 1 mm tabs on the top and bottom:
The “engraved” digit on the left came from a vinyl mask similar to the one on the right, using fine sandpaper to rough up the acrylic surface. I deliberately started with a battered old CD case in order to prevent myself from getting too compulsive with neatness; as you’ll see, edge-lit acrylic reveals any surface imperfections, so cleanliness is important.
The black border could be a light-shield gasket around the outer edge of the display panel to reduce glare from the edges. This might be more important for laser-cut pieces with highly reflective edges or for milled pieces with diffuse edges; there’s no way to tell without actually building one to see. I simply bandsawed the sheet around the edges of the mask, then filed off the larger chunks: the edges are very, very rough, indeed.
There doesn’t seem to be an easy way to stash the Inkscape SVG file on WordPress.
I solder-blobbed some wire-wrap wire, a 1206 SMD resistor, and a 0603 LED together:
The 0603 SMD LED fits neatly along the edge of the sheet:
A 3rd hand holds it upright on the bench over the LED lashup:
It looks marginally better with the lights out, but you can see all the scratches:
The hot spot at the bottom of the digit isn’t nearly that awful in person.
A top view shows the glowing edges, plus the nuclear glow from the LED:
A touch of soft focus, plus moving the LED under a tab location, helps a bit:
You’d want two LEDs per digit and maybe one at the top, but that’s in the nature of fine tuning.
All in all, I like how it looks. Getting from this crud to a workable display will require far more effort than I can devote to it right now…
The first two charges for those Baofeng BL-5 batteries show that the actual capacity isn’t quite up to the 1800 mA·h spec:
The (meager) instructions say that the batteries will reach “full capacity” after three charges. Unless there’s a miracle waiting in the wings for that third charge, I very much doubt that they’ll get any better than the 1400 to 1500 mA·h you see in that graph. Note that the two batteries have quite different capacities and that the capacity for Pack B decreased on the second charge (purple vs. green trace).
Compare that with the Wouxun batteries (plotted with Gnuplot, rather than a screen grab):
Those are all at 250 mA, which is certainly less than the peak current and probably more than the average current. It’s close enough for now, anyway, and shows that the Wouxun batteries actually live up to their spec.
Huh. Who’d’a thunk it?
It looks like the blinky lights should go into power-save mode under 7 V, because there just isn’t that much capacity left when the cells start rolling over the edge of the cliff.
Although the M2’s heated build platform works well enough, somebody who knows what he’s doing (you know who you are: thanks!) sent me an improved version. It’s a PCB heater, laid out to compensate for the usual edge cooling, firmly attached to a tempered glass plate with genuine 3M thermally conductive tape:
They designed the heater around the 30 VDC power supply used in their other equipment. Although I had high moderate hopes that a boost power supply would convert the 24 V supply I already had for the stepper driver bricks into the 30 V for the heater, it was not to be. So there’s a 36 V 9.7 A 350 W supply arcing around the planet that (I think) should work better: adjust the voltage down as far as it’ll go, soak up another few volts in the solid-state relay, and Things Should Be Close Enough to 30 V. One can buy a genuine 30 V supply, but it costs surprisingly more than either 24 V or 36 V supplies on the surplus / eBay market and won’t really provide the proper voltage without upward tweaking anyway.
I replaced their standard 0.156 inch square terminals with Anderson Powerpoles, soldered a length of shielded cable to the 100 kΩ thermistor pads, and gimmicked up a connection to the 24 V supply; it delivered 23.7 V at the PCB terminals. The thermistor is 100 kΩ at 25 °C and 11.4 kΩ at 77 °C. The PCB heater is 5.9 Ω at 25 °C and 7.3 Ω at 77 °C; it dissipates 77 W at 77 °C (no, that’s not a typo).
The ultimate temperature looks to be about 90 °C with a 24 V supply, which isn’t quite enough for ABS (which I’m not using in the M2 right now, but probably will eventually). The time constant, assuming the 1-e-1 point is 66 °C, works out to about 9 minutes; it’ll be up to final temperature in half an hour. Those numbers aren’t quite as accurate as one might wish, because the heater power drops as the temperature rises and the copper resistance increases.
A 30 V supply would dissipate 120 W at 77 °C and rumor has it that the ultimate temperature is around 125 °C, which would be fine for ABS. Goosing the power a bit would produce more heat, but I’v been running the Thing-O-Matic at 110 °C and that’s good enough. More power, of course, gets it to the temperature setpoint faster, which is probably a Very Good Thing.
Obviously, you need PWM to control the temperature; given a 9 minute time constant, a bang-bang controller will work perfectly well.
The original data, including the thermistor resistance after I got my act together, plus a cute little temperature-vs-time graph:
The colored flyspecks are part of the paper; I salvaged a stack of fancy menu cards from a trash can and padded them up as geek scratch paper.
After I mentioned I was thinking of repurposing the nearly unused lithium-ion batteries from the Wouxun KG-UV3D radios for a blinky light, Dragorn of Kismet introduced me to his Baofeng UV-5 radio. The radio itself seems to be the worst amateur radio you’d be willing to use, but when seen as a standardized battery and drop-in charger with a free radio and antenna tossed into the deal, it’s not all that bad:
The Wouxun and Baofeng 7.4 V batteries allegedly have similar capacities: 1700 vs 1800 mA·h. The Baofeng also has a 3800 (or 3600) mA·h pack that extends well below the base of the radio (not all large packs seem to be compatible with the UV-5RE radios I got); that would be roughly equivalent to the larger packs that power the Wouxun / APRS / voice gadgetry on the bike.
The Baofeng battery pack is smaller and has features that seem less likely to misbehave on a bike.
It has a latching tab with a ramp and a positive notch, with ridges around the edge that engage the radio shell:
The radio body (which is what I must duplicate) has a movable latch tab above the battery contact pins, so the latch holds the battery into the compartment. The spring-loaded pin pairs are wired in parallel, presumably for redundant contact with each battery terminal:
The battery terminal pads are reasonably well protected by the tab:
The battery slides into the radio compartment and latches with a snap. Two holes on the battery base engage a pair of pegs on the radio case:
The holes are rounded rectangles and the pegs have one corner sliced off. The pegs seem entirely too fragile and not well suited for 3D printing, so some metalwork may be in order. The pegs must resist only pulling forces perpendicular to the case back, not sliding forces, and the case constrains side-to-side motion.
The two square posts (with two others not shown) form the “feet” that support the radio when it’s standing on the desk or in the charger.
Now, to doodle up the dimensions and measure the actual capacity.
Speaking of capacity, BL-5 batteries on eBay range from $23 for “genuine Baofeng” that may or may not actually have that name on the label, all the way down to $8 for the usual no-name equivalent.
For reasons that will become apparent in a while, I got a pair (*) of boost power supplies from the usual eBay source, allegedly capable of boosting a 10-to-32 VDC input to a 12-to-35 VDC output, up to 10 A and 150 W:
After establishing that it would not produce 30 V into a 5.9 Ω load (5.1 A, but 152 W), I got systematic.
A 100 Ω resistor drew 1/4 A while I set the output to 28 V. Doubling up the resistors showed that it worked OK at half an amp:
Four 6 Ω resistors in series draw 1.2 A, then (channeling the true spirit of DIY 3D printing) two in series drew 2.3 A:
That’s 32 W each and, yes, they did get toasty, but, no, I didn’t leave them turned on all that long.
But a 6 Ω resistance still didn’t work, so the supply can’t provide 4.7 A at 130 W. In case you were wondering, that’s two 6 Ω resistors in series and a pair of those strings in parallel, so each resistor still sees 32 W.
In terms of driving the actual load, these supplies aren’t going to light it up.
Ah, well, whaddaya want for five buck from halfway around the planet?
(*) Davy’s Aphorism: Never buy only one of any surplus item, because you’ll never find another. Get at least two, maybe three if it’s something you might actually use.
We’ve been kicking around ideas for introductory soldering / electronics projects at Squidwrench, which prompted me to come up with a classic astable multivibrator circuit:
Everything is totally non-critical: it should oscillate as long as the base resistors are small enough to lightly saturate the transistors. The LEDs let you know it’s actually working.
You’d probably want a few decades of caps and a decade’s worth of resistors on pin headers, so as to cover the range from visible blinkiness to nasty audio squeal.
The pulse generator and cap at the bottom apply a jolt at T=0 to knock the circuit off balance. Otherwise, the simulation will just sit there and do nothing; in the real world, of course, nothing stays balanced for very long.
The Smell of Molten Projects in the Morning 