Low fuel warning light circuit

G'day all ... I am in need of some advice. I am not learnered in the ways of electronics, and was hoping someone may be able to point me in the right direction. I want to construct a low fuel warning light using a led, i am not sure of the circuit i require or components.. But i do know that when i have a full tank of fuel the resistance from the internal fuel sender unit is low, and when the tank is empty, the resistance is high. I want the low fuel light to come on when i have around 3 litres left in my tank. (which holds 18 litres) Any help would be appreciated.. I have surfed the net for any form of diagram but to know avail.

again any help would be great.
Johno

You just need a comparator, a resistor and LED, and a potentiometer to make the set point. Maybe a regulator to make the set point independent of the system voltage. Dat's it.

try this site i built it and it works **broken link removed**