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.
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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