Fixed voltage regulator for simple circuit
Are you just driving an LED?  If so you don't need a regulator.  Also you shouldn't be driving an LED with a constant voltage, you need to drive it with a reasonably well defined current.  You can do that with just a resistor.







simulate this circuit &ndash; Schematic created using CircuitLab

EDIT:

If the input voltage only changes over a moderate range (2:1) then you don't need a regulator.  Just choose an appropriate resistor value to give the LED current you need. 

If for example if you have a 5V supply and you want 20mA through the LED which has a 3V drop (common for a white LED). Then the resistor you will need is:

R = (Vinput - Vled) * 1000/LED_Current.

For the example values this gives (5 - 3) * 1000/20 = 100 Ohm.

The LED voltage will vary with the color.  A Green LED is probably about 2.1V and a Red LED about 1.7V, Infra red ~1.2V.

If the input voltage varies the current will as well but you probably won't even be able to see a 30% change in brightness.  This would occur if the voltage changes between 4.4V and 5.6V.