The Smell of Molten Projects in the Morning
Ed Nisley's Blog: shop notes, electronics, firmware, machinery, 3D printing, and curiosities
Blinking & glowing LEDs
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.
A red LED has a sufficiently low forward voltage to run with a MOSFET astable multivibrator and a pair of run-down AA alkaline cells:
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.
Adding an amber LED to the collection:
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.
Next step: wire up an astable with a yellow LED to go with the green and blue boosted LEDs.
After eight months from a full charge, an old NP-BX1 lithium battery has come to this:
The astable still ticks along at 1.4 seconds per blink, but the green LED barely lights up from a 2.1 V battery:
A pulse of 12 mV across the 100 Ω resistor puts the LED current at a mere 120 µA: no wonder the poor thing wasn’t visible in ordinary room light.
Another full charge restored its vigor for another couple of seasons.
The green-LED discrete LM3909 is still flashing, even with its AA NiMH cells burned down to 1.15 V:
If the truth be known, one of the cells is now reverse-charged to 200 mV, so that’s a bit beyond as low as it can go.
The flash period has stretched to 8.7 s:
The circuit boosts the battery by 800 mV to put 1.94 V across the green LED at the start of each flash:
Admittedly, the LED isn’t particularly bright at 2.8 mA:
But it’s still flashing!
Swapping the cells into the LM3909 with a blue LED doesn’t produce any blinking, which is about what the earlier tests showed.
Having a handful of not-dead-yet AAA alkalines and a bunch of LED blinkies built for AA alkalines, a pair of adapters seemed in order:
The blinkies need a somewhat wider base than they’d get from a pair of AAA alkalines, so it’s not quite as dumb as it may seem.
In any event, the positive terminal comes from a brass rod:
Nobody will ever see the fancy Hilbert Curve infill around the brass:
In this application, they’ll go from not-dead-yet to oh-it’s-dead faster than AA cells, so I can watch how the blinkies work with lower voltages.
The Smell of Molten Projects in the Morning 