The output from the sender follows a specific range of resistance and with a little clever electronics is all you need to trigger the LED's.
Standard senders run 33 Ohms full tank to 240 empty.
You could have all LED's on when full and loose one every time the fuel level drops past a certain point.
With 5 LED's you could have 3 green 1 yellow and one red.
Quote:
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United States Patent 4102191 Link to this page:http://www.freepatentsonline.com/4102191.html Abstract:A digital readout fuel gauge for monitoring the liquid level in a fuel storage tank. The device utilizes a pre-amplifier circuit which comprises a voltage divider network utilizing the fuel gauge tank variable resistor on one side of the divider and a variable resistance on the other side which is connected to a signal processor. The output voltage of the signal processor is connected to the analog input of an analog to digital converter. The analog to digital converter changes the DC voltage levels to a four bit BCD code. The output carrying the code is connected to a decimal digit to a seven segment LED decoder. The digits are generated and displayed on LED indicator units. In addition, a voltage comparator circuit compares the output of the signal processor with a reference voltage and when said output is below a predetermined level, the comparator circuit initiates a warning signal.
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Down to Maplin it is then