Constant current LED driver
Delivers 19.5mA at diode foward voltages from the 0.1V of a germanium signal diode up to a ridiculous 3.8V. (Ridiculous for a simple PN-junction diode, whether LED or anything else.)
Delivers 19.5mA into a 2.2V LED at power supply voltages from 3.4V to (I presume) transistor breakdown voltage. (Simulation fine at 10kV!)
Switched by a lower current, taking 3.2mA there. Could take less with some adjustment.
The dark LED is merely a voltage reference, it could be replaced by a Zener diode. Current depends on this voltage and the voltage developed across the emitter resistor. The transistor will turn on until the voltage at its emitter is about 0.8V less than at its base.
