# LM393 input question

Discussion in 'The Projects Forum' started by ddaytona1, Feb 18, 2011.

1. ### ddaytona1 Thread Starter Member

Nov 3, 2008
13
0
I'm trying to measure the voltage at a fuel guage 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 gage voltage at the + input of IC 1A (not requiring R1 and R2 as shown) and then adjust RV to 3V? So in therory 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" ?

thanks!

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2. ### John P AAC Fanatic!

Oct 14, 2008
1,638
225
There are several points here.

The LM393's outputs never go "high". It has open-collector outputs, which means the electronic equivalent of a switch to ground, but never a switch to the high level. The diagram shows a resistor to V+, and that's what makes the output "high". "Absence of low" would be a better way to say it.

That open collector isn't really a switch. It's a transistor with a small current-carrying ability. You could run an LED off it (put 1kilohm in series) but most likely not an automotive light bulb.

The output goes "high" when the + input is higher than the - input. If you wired it with an LED connected to the output, you'd want a low output when the reference (fixed voltage) input is higher than the fuel tank sensor's output. In other words, it would be backward relative to the diagram. So if the fuel gauge output is highest for a full tank, you need to switch the LM393 + and - inputs.

The LM393 doesn't have 4 comparators. You're confusing the LM393 (dual) with the LM339 (quad). But whichever you use, tie the inputs to all unused comparators to Gnd.

So much to say about such a simple component.