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.
The toad population has apparently been spending more time near the Mighty Wappinger Creek, rather than around the house, during this very dry summer, so this small toad at the garage door came as a surprise:
A few days later, Mary spotted a larger toad tucked into the spice garden:
Small tree frogs sound off in the darkness around the house, but we’ve never seen any of them.
We wish them great success in their future bug hunts!
After un-wedging the astable’s synchronized periods and recharging the harvested 18650 lithium cell, I put a sock (not shown) over the radome and let it blink on a corner of the desk:
The periods are much too short and the NPN astable currents much too high, but the thing runs for about ten days before the over-discharge circuit shuts it down.
So a single NPN astable driving a single-color LED with a more reasonable period should get a month or so from an end-of-life 18650 cell and a MOSFET astable might run for two months.
I’ve been using not-dead-yet lithium batteries to power astable multivibrators blinking LEDs on the red-to-yellow end of the spectrum, because the over-discharge protection circuitry in the batteries shuts down at 2.5 V, while not eking much light from LEDs toward the blue end of the spectrum.
Back in the late 60s, when integrated circuits were new, National Semiconductor designed and, in the early 70s, introduced the LM3909: “a monolithic oscillator specifically designed to flash Light Emitting Diodes”. The IC used an electrolytic capacitor as both timing element and voltage booster by charging the cap, then switching it in reverse series with the LED, to produce a voltage drop larger than the 1.5 V battery supply. The original National Semiconductor LM3909 datasheet will get you started and Application Note 154 gives more details and insight.
Rob Paisley’s work from 2008 suggested a discrete-transistor version might look just as attractive, in a techie sort of way, as the astables, and perhaps boost the 2 V from a pair of not-dead-yet alkaline cells high enough to light a blue LED.
Some LTSpice twiddling produces a suitable circuit:
The labeled nodes correspond to pin numbers on the IC package, with a suffix indicating what they did for a living. R2 combines the two timing resistors in the IC into a single unit, so “P18-RC” combines the pins. The Q2 pair over on the right forms a current mirror driving Q3, which the doc calls the “power transistor”, to yank the positive end of the capacitor to ground to light the LED.
The LED is faked by a PT-121-B diode with a 2.34 V forward drop at 20 mA. It’s rated for 20 A average current, so it’s not a particularly good model for a piddly 5 mm LED, but I’ll define it to be Good Enough for now.
Running the simulation at 1.5 V is encouraging:
The green trace gives the voltage across the capacitor. Under these conditions, the voltage stays positive, although not by much.
Running it from a 3 V supply changes the results:
The cap charges to about the same voltage, but the pulse now lasts long enough to charge it nearly half a volt in the wrong direction. This is Bad Practice, even though my similarly offending astables have been doing it for years.
The data sheet points out that the forward drops of Q1 and Q2 determine the trigger level for the start of the LED pulse, so adding another forward-biased junction in series should let the cap charge to a higher voltage and, for the same pulse duration, pull the low end up above zero to increase overall happiness.
I formerly straightened the copper wire into “bus bars” for the astable multivibrators by whacking it with a slide hammer, but someone whose name is lost in the mists of time told me the right way to do it:
Yup, grab a piece of wire at both ends in Tiny Lathe and give it a few low-speed turns while pulling firmly on the tailstock.
No muss, no fuss, no drama, just bar-straight and slightly work-hardened copper wires:
I slide-hammered the top wire before remembering the clue. The bottom two wires have peppermint-stick swirls.
Thank you, whoever you were!
A Rudbeckia Black-eyed-susan coneflower from the garden carried a passenger to our patio table:
Even linearized, the inchworm was barely 20 mm long; it’s the thought that counts.
The stamens mature in concentric rings, each stamen topped by a pollen grain. Apparently, those grains are just about the most wonderful food ever, as the inchworm made its way around the ring eating each grain in succession:
Of course, what goes in must come out:
I had to brush off the table before washing it; the pellets are dry, but smear when you get them wet.
Another flower in the vase held a 10 mm inchworm with plenty of upside potential:
After nearly a week, the flowers were done and the inchworms had moved on. We wish them well, although we likely won’t recognize them in the future.
Lithium battery packs have overcurrent protection cutouts, but alkaline cells depend on their internal resistance and may overheat in response to a serious short circuit. So adding a PTC fuse to the circuitry over an alkaline battery case seemed appropriate:
That’s a test setup for a discrete-transistor version of an LM3909 LED blinker, about which more later. The PTC fuse looks a lot like a ceramic capacitor with one leg caught in an alligator clip.
Two bags of PTC fuses recently arrived from halfway around the planet, rated at 100 mA and 170 mA. One allegedly came from JinKe and the other probably didn’t pass through a Littelfuse factory despite its part number, but the only datasheet I can find is for the Littelfuse RXEF PTC PolySwitch series, which is surely close enough.
I set up a torture test involving a bench power supply and an ammeter, both offscreen and left to your imagination:
At 75 °F:
With a dead short simulated by 3 V from the supply, the current stabilized at:
Cranked to 5 V for that good old TTL vibe:
The datasheet says they’re good up to 60 V, but that’s just crazy talk.
The abuse put a shiny gloss on the epoxy coating, sort of like when you overcooked one of those wax-insulated capacitors back in the day.
Despite that, a PTC fuse is better than a dead short, if only because the plastic battery case won’t get all melty with the batteries supplying less than half a watt.
For whatever reason, a two-outlet junction box stands outside the Credit Union:
The slanted conduit certainly looks in need of an elbow to line it up, doesn’t it?
It seems whoever installed it, many years ago, simply forced the conduit to line up, no matter the consequences:
The threaded entries on the die-cast outlet box were never intended to cope with that much misalignment; half the bottom has vanished. I think the round box on the top originally held a floodlight to wash the (uninspired) building facade at night, but those days are long gone.
If the conduit has horizontal underground runs, both are certainly full of water by now. The white(-ish) “Romex” cable insulation looks like ordinary indoor wiring, not the grayish direct-burial sheath, but it may be sun-bleached after years of exposure.
Surely there’s a tripped GFI on that circuit …
The Smell of Molten Projects in the Morning 