Using a Zener to regulate LED supply voltage?

[MAO]

Will this circuit keep an LED's brightest constant when switching between a 9v supply and a 18v supply?

 

[BuGG]

It should work, but when running at 18V you'll be shunting 9mA of current to ground through the 1K / Zener network.  

Instead of a Zener diode I would use a voltage regulator, 78L05 is the same size as a standard signal transistor and would work great.

[MAO]

Much better idea, thank you!