lm3914 fuel gauge (help)

[SteveStation]

hello
i am making a lm3914 fuel gauge using this project from Caslor, that is exactly what i need.
https://www.electro-tech-online.com/threads/lm3914-question.105134/page-3
My problem is that i cant get it calibrated to my needs.
my car uses this values.
7 ohm Full
51 ohm 3/4
100 ohm 1/2
159 ohm 1/4
300 ohm Resserve (empty)
can someone help on this.

 

[4pyros]

Did you look at the LM3914 tutorial on this site?

 

[SteveStation]

yes i did as i understood i have to adjust the resistor r6 to empty that gave me the value to 54k1, that will give me 77.01 mv wend is empty and 6.98v wend is full, but wend i make the test to 51 ohm (3/4) i have the same volt out 6.98v i know that i have to adjust the resistor r2 and r3 and maybe r7 and r1 but i down t know how calculate the resistor values.

 

[4pyros]

Post the circuit you are looking at.

 

[SteveStation]

https://www.electro-tech-online.com/attachments/caslor1-zip.41999/

 

[4pyros]

https://www.electro-tech-online.com/attachments/caslor1-zip.41999/

Dont get it, thats for a fuel gage not a tack!

 

[4pyros]

Are you using a 8 volt supply?

 

[SteveStation]

yes by using the lm7808 conected to 12v suply.
the real problem because i have to use this circuit is because i have to invert the voltage because the sender use the resister connected to ground to tell that the tank is full or empty.
this can be simulated by change the r5 to 300R to empty and 7R to full.
Take a lock to this schematic from Caslor
https://www.electro-tech-online.com/attachments/final-floter-jpg.41988/

 

[crutschow]

Below is a simulation of my take on the circuit. It uses an op amp to invert and scale the signal. D11 and R11 are to provide a rough compensation for the non-linear output versus sender resistance versus position. You may have to tweak some of the component values to get the exact readout scale desired.

 

[4pyros]

Below is a simulation of my take on the circuit. It uses an op amp to invert and scale the signal. D11 and R11 are to provide a rough compensation for the non-linear output versus sender resistance versus position. You may have to tweak some of the component values to get the exact readout scale desired.

View attachment 86565

Came out real small, cant read it.

 

[crutschow]

Came out real small, cant read it.

Don't understand. Comes out large on my computer when I click it. Have you tried right-click to a different tab/window?

 

[4pyros]

Don't understand. Comes out large on my computer when I click it. Have you tried right-click to a different tab/window?

Right click works.

 

[SteveStation]

tanks to all that is trying to help me
This is what i came out i don't know if is ok or not but here is a screenshot.
Take a look and see if is something wrong or some improvement can been done

 

[4pyros]

How are you guys coming up with all these exotic resister values for the feed back resistors?

Where are you going to get a 95.5K resister?

 

[crutschow]

How are you guys coming up with all these exotic resister values for the feed back resistors?

Where are you going to get a 95.5K resister?

A 95.3k would likely work instead.

 

[SteveStation]

the 95.3k was already there on caslor project, but beside that is the rest ok.

 

[crutschow]

the 95.3k was already there on caslor project, but beside that is the rest ok.

If you want a linear output with resistance change, that looks okay. But that will not translate into a proper readout for an LM3914 gauge. Note, for example, that 100 ohms (1/2 tank) gives over 3.5V at the output. For a 1/2 reading the voltage should be closer to 3V. That's why I added a diode and resistor in my circuit, to make the voltage more closely correspond to the tank level.

 

[SteveStation]

Yes i notice that but i have try to use your circuit but i never came out if the right resistor values,
I am new if this in electronic and i am still learning.

 

[crutschow]

Yes i notice that but i have try to use your circuit but i never came out if the right resistor values,..........

That's because you need a non-linear output, which requires a non-linear circuit element such as the diode I used.

 

[SteveStation]

tanks crutschcow for all the help that you give to my project but if all the test that i made even if your circuit, but the my last circuit was the best that i came if.