[Milsurplus] 1.5 volt LED screw base bulb ?

[Hubert Miller]

In the search for better dial illumination for the U.S. Army GRC-9 receiver, besides the 'Maglite' bulb adapter I built, I also bought what were sold
as "1.5 volt, screw base LED bulbs", replacements for a type 112 ( I think it is  ) flashlight bulb.
A 1.5 volt LED bulb?
This was something entirely new and greatly puzzling to me, just like the time a woman co-worker a few years back told me she had rewired a
floor lamp using "liquid solder". "Now, how they done that???" I wondered. Well, real miracles are very rare.
The bulb looks just like what it is, a screw base flashlight or dial lamp bulb. Sure enough, it lit bright at 1.5 volts. The draw current, 80 mA, was the
big clue. The actual incandescent 112 bulb draws more like somewhere between 100 and 200 mA. I reckoned at about 30% efficiency, a circuit could
generate the about 20 mA an LED needs at about 3 volts.
So I crushed the base of one and cut and peeled back the metal base, the sharp metal peel also causing a hydraulic fluid leak from one of my fingers.
Here's a photo of an intact bulb and the innards. It is the "joule thief" oscillator circuit. I don't know what frequency the oscillation is; the torroid looks
very low inductance and very HF-y.  The tiny circuit board has a few surface mount parts. I would really like to trace and draw the circuit but as you
can see, it looks like a bit of a challenge.

The repair job using "liquid solder" ?  It wasn't soldered using mercury or "Wood's metal", the only low melt temperature metals I know. She twisted
the wires together, then covered them with "Liquid Solder", which is a metallic looking glue. No miracle there either.

The GRC-9 receiver dial illumination: Richard Brunner and I have been exchanging emails on this. He finds that the receiver audio stage B+ draw will
nicely light a bright white LED in series. He says he will post his solution once he has the project finalized.
-Hubert Miller

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