How could this "Battery OK" indicator be improved, without adding too much complexity (or maybe even simplifying)?
The intended function is to have the LED illuminated at relatively-constant brightness when the supply is above a threshold voltage. As the supply voltage drops below that threshold, the LED should transition as rapidly as possible to completely off. Idle current should be as low as possible.
With the circuit as shown, this is a plot the current through the LED for supply voltage up to 10V:
The R2/R3 voltage divider is set so that when the supply is at the threshold voltage, the voltage across R3 is one VBE drop.
R1 sets the LED current above the threshold. (Q3 serves as a current regulator.) This way there's not a dimming of the LED as voltage decreases from maximum until the threshold is reached.
The Sziklai pair is used so that the transition occurs over a narrower voltage range, and so that Q1 base current doesn't load the divider. A Darlington pair could be used here, but the divider would have to be adjusted so that the lower voltage is 2 × VBE.
Edited to clarify the specifics of the sample circuit:
- The 9V source was just chosen because a 9V battery is readily available for testing. I would like something readily adaptable to other voltages.
- The 2.5 mA current through the LED is also somewhat arbitrary. Having the brightness be relatively constant is more important than the actual value.
- I could replace the JFET and its source resistor with a packaged current regulator diode and it would be essentially equivalent. (I didn't have a model for one of those.)