Low V led?

Alec_t

Joined Sep 17, 2013
14,280
Here's one way you could do it.
The LED here serves two purposes; as a voltage reference and as an indicator. The trip point is adjustable with the trimmer. When Vb1 drops below Vb2 the long-tail pair formed by Q1,Q2 see-saws and turns on Q3. This increases the LED current from an insignificant ~50uA to ~10mA. This in turn increases the LED forward voltage drop and provides positive feedback to enhance the LED turn-on speed.
 

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MikeML

Joined Oct 2, 2009
5,444
It occured to me later, while I was away from the computer that I originally misinterpreted the OP's question and misdirected the thread. I'm glad that Alec finally showed a circuit that turns on the LED as the voltage drops...

Another question for the OP. Do you really want to turn on a LED, and add load to a battery that is now depleted, and deplete the battery further? Why not just automatically turn off the load that is depleting the battery rather than adding to the load???
 
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