LED on 1.5v Supply

A red LED requires about 1.7v before it will start to illuminate - below this voltage - NOTHING! This circuit takes about 12mA to illuminate a red LED using a single cell, but the interesting feature is the way the
LED is illuminated. The 1u electrolytic can be considered to be a 1v cell.

(If you want to be technical: it charges to about 1.5v - 0.2v loss due to collector-emitter = 1.3v and a lost of
about 0.2v via collector-emitter in diagram B.)

It is firstly charged by the 100R resistor and the 3rd  transistor (when it is fully turned ON via the 1k base resistor). This is shown in diagram "A." During this time the second transistor is not turned on and that's why we have omitted it from the diagram. When the second transistor is turned ON, the 1v cell is pulled to the 0v rail and the negative of the cell is actually 1v below the 0v rail as shown in diagram "B." The LED sees 1.5v from the battery and about 1v from the electrolytic and this is sufficient to illuminate it.

Follow the two voltages to see how they add to 2.5v.

 

Source : From Ebook On my Computer