Discrete LM3909 Blue LED: Off at 1.0 V

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:

LM3909 Blue - 2.4 V alkaline
LM3909 Blue – 2.4 V alkaline

The LED waveform shows it needs about 2 V:

LM3909 Blue - 2.4 V alkaline
LM3909 Blue – 2.4 V alkaline

It’s barely visible in normal room light and strikingly bright at night.

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