[millebe]

I think, to put it more simply, those two diodes exist to cause a 1.4 volt drop to the transistor.

The transistor is a PNP transistor, so the base (middle terminal) must be about .6V lower than the emitter (top terminal) for the transistor to turn on.

I'm guessing one diode wouldn't turn on the transistor reliably, so the author used two in series. This also probably allows for a more convenient value of R1.

[cnvogel]

Exactly. One diode will not turn on the transistor. Replace the two diodes with one LED, and the circuit will run the same.

One often finds LEDs used for the prose of simple voltage stabilization (often green ones @2.1V or so..)

[scoot]

So why not just use an appropriate resistor?

[darkarmani]

A resister has a voltage drop related to the current. A diode will always drop the same voltage (within operating parameters).

[nitrogen]

The voltage drop of a resistor is proportional to current.

[iiiears]

So in many appliances the LED serves two functions, Light and part of the active circuit to lower voltage? - This is why HN has for me replaced slashdot and digg. Thank You.