•  Velly
  • 51.2% (Neutral)
  • Newbie Topic Starter
I'm trying to measure the voltage at a fuel gauge and determine that when the voltage is 3V (representing 1/4 tank) I need a 12V light to go on representing low fuel. Looking at the LM393  on the attached diagram, V+ would be the 12V input from the car battery.

I'm unsure how to bring in the input from the fuel guage so when the voltage at the fuel guage drops to the 3V the output of IC 1A goes high.

On the attached diagram would I need to connect the fuel gauge voltage at the + input of IC 1A (not requiring R1 and R2 as shown) and then adjust RV to 3V? So in theory when the + input of IC 1A drops to 3V, it matches the input of 3V controlled by RV at the - input of IC 1A which makes the output go high?

As far as the 12V light, can I just connect it directly to the output or would I need to consider a transistor & relay configuration?

Since LM393 has 4 comparators and I will be using 1 of them, do any of the other comparators need to be tied to voltage as well to prevent "floating" ?


Mark Bischetsrieder
Based on a few of the schematics i looked at they use regular leds in this instance and resistor values are chosen based on the requirements of the led. if you are looking to toggle a 12v light you already have you may need some additional components such as a mosfet to allow the light to be switched on or off. a few of the resources i used were for making charging circuits (ex. light goes on when charging off when charged) take a look at these and see if they might apply to your project.


they have much to do with the voltage falling and causing the light to function.

from what i understand you may need a constant +12v and the adjusted voltage from gas sending unit on the inputs of the comparator to give you the needed output

i also looked here to get a better idea of what this IC does.


worse comes to worse you can always get a arduino nano to do what you want!