Posts Tagged Improvements

Tensilizing Copper Wire

The “bus bars” on the battery holders are 14 AWG copper wire:

Astable - NP-BX1 base - detail
Astable – NP-BX1 base – detail

Slightly stretching the wire straightens and work-hardens it, which I’d been doing by clamping one end in the bench vise, grabbing the other in a Vise-Grip, and whacking the Vise-Grip with a hammer. The results tended to be, mmm, hit-or-miss, with the wires often acquiring a slight bend due to an errant whack.

I finally fished out the slide hammer Mary made when we took a BOCES adult-ed machine shop class many many years ago:

Slide Hammer
Slide Hammer

The snout captured the head of a sheet metal screw you’d previously driven into a dented automobile fender. For my simple purposes, jamming the wire into the snout and tightening it firmly provides a Good Enough™ grip:

Slide Hammer Snout
Slide Hammer Snout

Clamp the other end of the wire into the bench vise, pull gently on the hammer to take the slack out of the wire, and slap the weight until one end of the wire breaks.

With a bit of attention to detail, the wires come out perfectly straight and ready to become Art:

Straightened 14 AWG Copper Wires
Straightened 14 AWG Copper Wires

The wires start out at 1.60 mm diameter (14 AWG should be 1.628, but you know how this stuff goes) and break around 1.55 mm. In principle, when the diameter drops 3%, the area will decrease by 6% and the length should increase by 6%, but in reality the 150 mm length stretches by only 1 mm = 1%, not 3 mm. My measurement-fu seems weak.

Highly recommended, particularly when your Favorite Wife made the tool.

The Harbor Freight version comes with a bunch of snouts suitable for car repair and is utterly unromantic.



Astable Multivibrator: Monochrome Pirhana LED

The LED parts box disgorged some single-color Pirhana-style LEDs:

Astable - 2N7000 - Mono Pirhana LED
Astable – 2N7000 – Mono Pirhana LED

Didn’t quite catch the blink, but the Ping-Pong ball radome lights up just as you’d expect.

The radome sits on a stripped-down RGB LED spider:

Astable Multivibrator Battery Holder - mono LED Spider - fit view
Astable Multivibrator Battery Holder – mono LED Spider – fit view

The circuitry is the same as the First Light version, with a 1 MΩ resistor stabilizing the LED ballast resistor:

Astable - 2N7000 - Mono Pirhana LED - detail
Astable – 2N7000 – Mono Pirhana LED – detail

Those are 1 µF ceramic caps in the astable section, so I’m no longer abusing electrolytics, and a stylin’ 100 nF film cap metering out the LED pulse up above.

Just for pretty, I’ve been using yellow / black wires for the battery connections and matching the LED color with its cathode lead.

The OpenSCAD source code as a GitHub Gist:


Leave a comment

Minilathe MT3 Spindle Collet Fitting

I’ve used the LMS set of inch-size MT3 spindle collets on occasion, but releasing them required an unseemly amount of drawbar battering. It recently occurred to me to check their fit in the spindle taper:

Minilathe - MT3 collet - taper test

Minilathe – MT3 collet – taper test


The only place they touch the spindle is right around the base, so it’s no wonder they clamp poorly and release grudgingly. I tried several others with the same result.

Cross-checking shows a much closer fit along the entire length of the dead center, so it’s not the spindle’s fault:

Minilathe - Dead Center - MT3 taper check

Minilathe – Dead Center – MT3 taper check

Stipulated: we’re not talking toolroom precision here

I set the collets on centers:

Minilathe - MT3 collet - drive setup

Minilathe – MT3 collet – drive setup

And proceeded to file away the offending section to move the clamping force closer to the business end of the collet:

Minilathe - MT3 collet - filed result

Minilathe – MT3 collet – filed result

I did the small collets, the ones I’m most likely to need, and left the big ones for another rainy day.

They don’t have much clamping range and seem good only for exact-inch-size rods.

I should lay in a stock of ER16 and maybe ER32 collets for small stuff.




Debossed Printed Legends

[Update: It seems I interchanged “em” and “de” throughout this post.  ]

Up to this point, I’ve been labeling printed parts with emdebossed legends that look OK on the solid model:

Astable Multivibrator Battery Holder

Astable Multivibrator Battery Holder

Alas, the recessed letters become lost in their perimeter threads:

3D Printed Legend - Embossed

3D Printed Legend – Embossed

Raising the legend above the surface (“deembossing”) works reasonably well, but raised letters would interfere with sliding the battery into the holder and tend to get lost amid the surface infill pattern.

The blindingly obvious solution, after far too long, raises the letters above a frame embossed into the surface:

Astable Multivibrator Battery Holder - Legend Debossed

Astable Multivibrator Battery Holder – Legend Debossed

Which looks OK in the real world, too:

3D Printed Legend - Debossed

3D Printed Legend – Debossed

The frame is one thread deep and the legend is one thread tall, putting the letters flush with the surrounding surface and allowing the battery to slide smoothly.

The legend on the bottom surface shows even more improvement:

NP-BX1 battery holder - Raised vs Recessed Legend

NP-BX1 battery holder – Raised vs Recessed Legend

An OpenSCAD program can’t get the size of a rendered text string, so the fixed-size frame must surround the largest possible text, which isn’t much of a problem for my simple needs.

, ,


Astable Multivibrator: 2N7000 MOSFET First Light

Taping a cardboard support under the soldering fixture helped hold all the parts in place:

Astable - 2N7000 soldering

Astable – 2N7000 soldering

The struts fit neatly into an NP-BX1 battery holder and the circuit began blinking merrily:

Astable - 2N7000 assembled

Astable – 2N7000 assembled

My photography hand is weak …

The circuit schematic / layout resembles this:

LED Schematic - MOSFET transistors

LED Schematic – MOSFET transistors

The missing 1 MΩ resistor at the LED would serve as a physical support to tether the loose end of the 100 (-ish) Ω resistor, which desperately needed some stabilization under the LED spider.

The simulation says it should blink about every 4s:

Astable - 2N7000 buffered - true model

Astable – 2N7000 buffered – true model

The 2N7000 MOSFETs use a SPICE model from the Motorola ON Semi downloads, although they behaved about the same way using the LTSpice 2N7002 model.

It really does blink every 4s:

Astable - 2N7000 overview

Astable – 2N7000 overview

The LED pulse width should be about 50 ms:

Astable - 2N7000 buffered - LED current - true model

Astable – 2N7000 buffered – LED current – true model

The voltage at the bottom of the ballast resistor is directly proportional to the LED current:

Astable - 2N7000 pulse detail

Astable – 2N7000 pulse detail

So the pulse is actually 80-ish ms, which is Close Enough™ for my purposes.

The key advantage here is making both the astable’s period and the blink’s duration (roughly) proportional to the component values, so I can tweak them with some confidence the results will come out more-or-less right.

I love it when a plan comes together!




Astable Multivibrator: 2N7000 MOSFETs

Some poking around revealed an astable multivibrator using now-obsolescent ZVNL110A MOSFET transistors. The key idea seems to be large gate resistors putting the DC operating point exactly at the voltage required to hold each transistor in the linear region, pretty much guaranteeing the astable will eventually start up.

A bit of simulation suggests this variation ought to work:

Astable - 2N7000 buffered

Astable – 2N7000 buffered

Well, after the kickstarter in the lower left shorts the transistor for millisecond to enforce some asymmetry, whereoupon the simulation ticks along just fine.

The yellow trace shows the voltage across C2 ramping back and forth between ±1.3 V, with a period just over 4 s and almost exactly a 50% duty cycle: much better than the bipolar version, with sensible component values. As before, the cap sees both polarities, so an electrolytic cap isn’t appropriate.

The red trace is the drain voltage at M2 (presumably, “M for MOSFET”, rather than a plebeian “Q” or “T”), which is firmly at 0 V when it’s ON and ramps upward as R4 pulls C1 higher to turn it even more firmly OFF.

The green trace shows the LED current pulse when M2 turns ON at the end of each cycle. Rather than contort the astable into a very low duty cycle, I generate the pulse by dumping current through a smallish cap into the gate of M4. A few tens of milliseconds makes a perfectly serviceable blink and keeps the average current drain down around a milliamp or so.

In between, M3 buffers the astable’s output to deliver enough current to C4. Without the buffer, the cap draws enough current to mess with the oscillations; that’s how I got backed into this corner.

Figuring the LED at 20 mA for 50 ms, the astable at 10 µA, and the buffer at half of 40 µA, the average current of 1 mA comes entirely from the LED, so even a weak lithium camera battery should last a good long while.

If the low average drain ekes 1000 mA·h from the battery, the LED should blink for a month or two before the battery shuts down.

Leave a comment

Multimeter Probe Cable: FAIL

A reasonably good silicone-wire multimeter probe set arrived last spring and has worked well enough (I thought, anyhow) for the usual voltage measurements, but recently failed while measuring a small current. We all know how this will turn out, but the details may be of some interest.

Measuring the resistance from tip to plug located the fault to the black probe, after which I poked a pin through the insulation near the plug:

Multimeter probe - diagnosis

Multimeter probe – diagnosis

The two leads near the bottom go to my shiny Siglent bench multimeter. Despite their similarity to the failed probes, I’m pretty sure Siglent has better QC (well, mostly).

The probe’s resistance was near zero from the tip (offscreen to the left) to the pin and megohms from pin to plug (on the right). Figuring the wire worked loose, I pulled it away from the plug:

Multimeter probe - disassembly 1

Multimeter probe – disassembly 1


Although I wouldn’t have trusted those probes anywhere near their alleged 1 kV rating, seeing that exposed copper-like substance was disconcerting.

Hacking off the strain relief bushing around the wire got closer to the fault:

Multimeter probe - disassembly 2

Multimeter probe – disassembly 2

And, finally, the problem becomes obvious:

Multimeter probe - disassembly 3

Multimeter probe – disassembly 3

Yet Another Cold Solder Joint:

Multimeter probe - cold solder joint

Multimeter probe – cold solder joint

Pulling a black banana plug from the heap, I decided to drill a proper hole to anchor the wire:

Multimeter probe - drilling plug

Multimeter probe – drilling plug

Which looked like this afterward:

Multimeter probe - soldered plug

Multimeter probe – soldered plug

And produced a strongly mismatched pair:

Multimeter probe - repaired

Multimeter probe – repaired

Ain’t it amazing how much fun you can have for a few bucks, all delivered by eBay? [sigh]

, ,

Leave a comment