Once again, the discrete LM3909 circuitry can blink a blue LED while running a pair of alkaline cells all the way down to about 1 V, with one cell ending at 0.2 V and the other at 0.8 V. They started out discharged to 1.2 V each during their useful life, then blinked for a month; it’s as good a use for dead cells as I can think of.
With another pair of not-dead-yet cells providing 2.4 V, it started up again:
That’s a frame from a short video taken in subdued light, just to show it really does work.
The blue LED inside the radome got fainter as the alkaline AA cells faded away, but remained visible in a dark room until the discrete LM3909 circuitry stopped oscillating with the battery at 1.0 V. One of the cells had flatlined, with the other supplying what little current was needed.
The circuitry restarted with a pair of weak alkalines applying 2.4 V across the bus bars:
The LED waveform shows it needs about 2 V:
It’s barely visible in normal room light and strikingly bright at night.
The red LED is actually part of an RGB Piranha, just to see how it compares to an as-yet-unbuilt version with a single red LED in the same package.
The LED drops 1.9 V of the 2.75 V from the mostly used-up AA cells:
The original 33 Ω ballast resistor showed a peak current of 11 mA in a 30 ms pulse:
Replacing it with a 12 Ω resistor boosts the current all the way to 12 mA:
The 2N7000 gate sees a just bit more than 2 V, barely enough to get the poor thing conducting, which makes the ballast resistor mostly decorative. The MOSFET datasheet puts its 1 mA threshold somewhere between 0.8 and 3 V, so it could be worse.
Keep in mind the DSO150’s 1 MΩ input impedance sat in parallel with the 1 MΩ gate pulldown resistor forming the RC differentiator when I measured the gate voltage; I’ll leave the simulation as an exercise for the interested reader. The blinks were noticeably dimmer and perhaps a bit shorter, although eyeballometric calibration is notoriously hard.
The slightly revised schematic-layout doodle stacks the transistors along the negative bus bar:
Flipping the bottom transistor over to snuggle the two timing caps next to each other would eliminate the long jumper wire and probably look better.
Because a yellow / amber LED runs at a lower voltage than blue and green LEDs, it sits atop an astable multivibrator, rather than a discrete LM3909. The battery holder has a pair of carbon-zinc “Extra-Heavy Duty” AAA cells, so corrosion and leakage pose a foreseeable hazard.
The voltage across the 100 Ω LED ballast indicates a 9 mA peak LED current, which is somewhat dim in ordinary room light:
The corresponding LED voltage says the LED runs at 2.1 V for that much current:
Something around 39 Ω should make it more visible.
Adding a bit of trim to the bottom of the LED spider makes it look better and helps keep the strut wires in place:
It’s obviously impossible to build like that, so it’s split across the middle of the strut:
Glue it together with black adhesive and a couple of clamps:
The aluminum fixtures (jigs?) are epoxied around snippets of strut wire aligning the spider parts:
Those grossly oversized holes came pre-drilled in an otherwise suitable aluminum rod from the Little Tray o’ Cutoffs. I faced off the ends, chopped the rod in two, recessed the new ends, and declared victory. Might need better ones at some point, but they’ll do for now.