Voltage Drop

Who's good at drawing a circuit? I have a 24 V DC that I want a zener and a
yellow LED that comes when the voltage drops to a low of 15 Volts.

 

one way to do this is to use a opamp circuit

feed the negative input with the reference voltage you want as a threshold
(15 volt)and feed the positive input with the voltage you want to monitor
when the positive input will be lower then the negative input the output
will switch to ground,so place the led in series of a resistor connected
from the positive side of the power supply that feed the opamp (must be
higher then 15 volt by 3 volt at least and not be the monitored source...)
and the other side of the resistor to the output of the opamp,I didn't put
any histerisist in the circuit so it could flicker when near the 15 volt
level,and don't forget to have a common ground with the monitored
voltage....

if you have no idea what I am talking about let me know what will draw you a
circuit...

one other way would be to play with voltage drop and such thing when using a
zener,but that need experimentation on a lab bench to be sure of what will
be the reaction and it need to be tweaked for the zener and led used....