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Archive for May 8th, 2017

Wearable LED vs. Astable Multivibrator vs. Dead Lithium Cells

Mashing the 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 desktop blinky:

CR123A Astable - front

CR123A Astable – front

Of course, I managed to swap the base resistors, which meant the LED stayed on most of the time, which accounts for the slightly off-kilter brown resistor just under the LED.

It doesn’t look like much with the LED off:

CR123A Astable - top - off

CR123A Astable – top – off

Running from a 2.8 V (= dead) lithium cell, the LED lights a dark room at 3 mA:

CR123A Astable - top - on

CR123A Astable – top – on

The LTSpice schematic gives the details:

Astable Multivibrator - CR2032 - schematic

Astable Multivibrator – CR2032 – schematic

The LED definitely didn’t come from Nichia and the 2N3704 transistors aren’t the 2N3904s found in the LTSpice library, but, by and large, this is the kind of circuit where nearly anything will work.

The actual LED current obviously depends critically on the particular LED and the cell voltage, so this represents more of a serving suggestion than an actual prediction:

Astable Multivibrator - CR2032 - waveform

Astable Multivibrator – CR2032 – waveform

Indeed, a Tek current probe clamped around one of those 10 AWG copper wires shows a much more enthusiastic LED current (1 mA/div):

Astable - CR123A 2.8 V - 1 mA -green

Astable – CR123A 2.8 V – 1 mA -green

I don’t trust the baseline very much. The simulation & back of the envelope agree: the LED-off current should be around 400 µA (which doesn’t depend on the LED at all), so it’s in the right ballpark.

Your mileage will definitely differ.

It runs without a trace of software, which everybody at Squidwrench thought was wonderful …

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