Those wearable LEDs spent the last five months sitting on the kitchen window sash, quietly discharging their CR2032 lithium cells:
Occasional voltage measurements produced an interesting graph:
CR2023 primary lithium cells start out around 3.3 V, so these were pretty much dead (from their previous lives in dataloggers) when I slipped them into their holders. The LEDs seem to be blue LEDs, with threshold voltages around 3.6 V, with colored phosphors / filters, so they started out dim and got dimmer. The green(-ish) LED obviously fell over a cliff and went dark in late January; I have no way to measure long-term microamp currents, alas.
The reddish LED is still going, mmm, strong.
If you need a rather dim light for a surprisingly long time, these things will do the trick.
I should gimmick up another astable multivibrator to blink one LED.
The original data:
The Smell of Molten Projects in the Morning 
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6 responses to “Monthly Science: CR2023 Lithium Cells vs. Wearable LEDs”
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It used to be, you could flash a neon bulb with a “dead” photoflash or radio B battery for many months, even years. Then technology marched on, and the LM3909 chip came out, which let you flash an LED with a “dead” 1.5V cell for months. Then the LM3909 was discontinued (it was a somewhat clever chip, probably doing a charge pump style boost operation, like a “joule thief” without an inductor). Now I’m tempted to breadboard an LM3909 type circuit…
Somebody did a discrete version; http://home.cogeco.ca/~rpaisley4/LM3909.html. Cute little circuit, done by one of the nicer designers at National’s linear group, Pete Lefferts.
I am definitely soldering a discrete LM3909 atop a lithium cell holder!
[…] wearable LED from the completely dead CR2032 cell with a classic astable multivibrator circuit and a not-dead-yet CR123 cell produced a pure-analog […]