Alkaline AA Astable vs. RGB and Yellow LEDs

A fresh pair of alkaline AA cells at 3.2(-ish) V can’t light a Vf = 3 V blue LED with any authority, but I laid out an astable multivibrator circuit with a Piranha RGB LED to see how the colors looked:

Astable AA Alkaline - build test
Astable AA Alkaline – build test

Lighting all three LEDs at once doesn’t make much sense, although I did try it just for the amusement. Spoiler: red wins, even with more-or-less equal currents.

Mounting the hairball on an AA alkaline holder looks better:

Astable AA Alkaline - RGB LED test
Astable AA Alkaline – RGB LED test

Red being the only LED color making any kind of sense meant the Piranha was overqualified for the job, so I replaced all that clutter with a simple 5 mm yellow LED:

Astable AA Alkaline - yellow
Astable AA Alkaline – yellow

It’s shatteringly bright at 20 mA from fresh alkalines at 3.2 V and remains visible down to 1.8 V.

The original circuit schematic / layout doodle:

Astable wiring layout - Piranha RGB test
Astable wiring layout – Piranha RGB test

No surprise in any of this, as it’s why the discrete LM3909 circuitry happened, but it’s nice to have a simple LED atop some alkalines for show-n-tell. If, of course, show-n-tell events ever happen again …

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