Low fuel light with LM311N

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mentholboy

Joined Jan 26, 2018
6
Hey guys, I'm in the process of making a low fuel light for my car and I'm having a bit of trouble with hysteresis. When the state switches and the led should be off it is still on, but dim. Removing R4 removes the problem, but obviously the hysteresis as well. Changing the value of R4 seems to change the brightness as well as the amount of hysteresis too, can you have too much hysteresis? I would like a bigger threshold so it doesn't blink too much under acceleration/cornering ect. I'm very new to this so and I'm just trying what I've been reading, so please be gentle. Is there a way to remedy this? Thanks!
 

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AlbertHall

Joined Jun 4, 2014
12,026
When the output is off there is still a path for current to flow through the LED and R4 back to the 10k/6k8 potential divider.
I see two possible easy fixes:
1. Increase the values of R4 and the potential divider by a factor of ten or more. This might reduce the current to where you can't see the glow any more.
2. More certain method is to connect a resistor between the LM311 output and the supply. The value of the resistor should be such as drop less voltage than the LED needs to light, say 0.8V, when the output is off. I can't suggest a value as I don't know the value of R4.
 
try buffering the signal first, then add an RC filter with say a -3db point of 1 Hz.

It's probably sloshing in the tank. That's kinda handled by the guage. e.g. fluid filled.

Make sure the gas gage system doesn't use a pulsed voltage source.
 
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