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Fuel Gauge Op-amp Trouble

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slowted

New Member
Hello all. This is my first post on this forum and I'm looking for some help with a project. I'm a real novice so I'm sure this will be an easy one for the assembled minds.

I'm trying to build a replacement fuel gauge for a 1974 VW Camper. Originals are very fragile hot wire gauges so difficult to get hold of, and aftermarket replacements require me to cut holes in my dash. I can fit a 10-seg LED lightbar in the space utilising the old gauge casing to create a replacement in place of the original gauge.

The tank sender is a variable resistor ranging from 10Ω (full) to 75Ω (empty). I'm looking to display the output on a single 10-seg LED lightbar controlled by an LM3914 display driver. I'm using a fixed voltage regulator (TS78M06) and a voltage divider to provide the signal, but am limited by the current I'm prepared to put through the tank sender. This means I need to amplify and offset the signal before sending the length of the van, for which I planned to use an LM741 op-amp in negative feedback configuration. The circuit diagram is attached.

The problem, then, is with the op-amp. I've built the display part of the circuit, faking the signal voltage, and it works fine. I've built the transmit side of the circuit and it doesn't work. The voltages shown on the circuit are correct up until the Vsig output from the op-amp, which always saturates.

I'm sure this will be something simple, as I don't really understand electronics. I'm driven by necessity here rather than as a hobby, but I've got my teeth into it and don't want to be beaten now! Any ideas much appreciated.
 

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MikeMl

Well-Known Member
Most Helpful Member
Here is my suggestion:

A 741 is a piss-poor excuse for an opamp. It was obsolete 30 years ago.

It cannot difference two voltages close to it own negative supply pin. It can only pull its output within about 2V of either its positive supply pin or it negative supply pin, meaning that when running on 6V, the output can only swing from 2V to 4V.

In my version, I'm using a LM358 which is cheap, plentiful (and I have lots of them in my parts bin, and I have a trusted Spice model)

Note how I split the Sender pull-up resistor into two resistors, and used that as the signal to be amplified.

Note also that you will likely have to trim the Offset. See the variable resistor in the simulation.

The Plot is versus the Sender Resistance at various values of the TRIM resistor. If the gain is not suitable, then vary the feedback resistor.
 

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crutschow

Well-Known Member
Most Helpful Member
MikeMl, the circuit you show gives a increase in output for an increase in resistance, for a maximum output voltage when the tank is empty (75Ω). I believe the opposite would be preferable with the LM3914 so a full tank gives maximum voltage and all LEDs lit.

So you could use an inverting amp configuration with an offset to give maximum voltage with a 10Ω sender resistance.
 
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AllVol

New Member
Here is my suggestion:

A 741 is a piss-poor excuse for an opamp. It was obsolete 30 years ago.

It cannot difference two voltages close to it own negative supply pin. It can only pull its output within about 2V of either its positive supply pin or it negative supply pin, meaning that when running on 6V, the output can only swing from 2V to 4V.
The 741 also has an aversion to working properly when used in a single power supply as in OP's schematic. It was designed to be used dual supply, and operates properly under those conditions.

While it can be rigged in single supply configuration, it then only works reliably with an AC or pulsed DC input, hence the absolute requirement for caps on both input and output with a single supply.
 

MikeMl

Well-Known Member
Most Helpful Member
MikeMl, the circuit you show gives a increase in output for an increase in resistance, for a maximum output voltage when the tank is empty (75Ω). I believe the opposite would be preferable with the LM3914 so a full tank gives maximum voltage and all LEDs lit.

So you could use an inverting amp configuration with an offset to give maximum voltage with a 10Ω sender resistance.
Good suggestion. I reconfigured it. Since the LM358 comes two per 8 pin dip, I used the other half to make a slosh filter per this previous post. Output voltage vs sender resistance and AC gain show in the attached plots.
 

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slowted

New Member
Thanks both

Thanks to both of you for your input. I won't pretend to understand the finer details but will try the different op-amp in the circuit as described. I'll let you know how I get on.
 

slowted

New Member
It works

All,

Well done and thanks. I substituted the LM358 in place of the LM741 and it worked straight off. Will now try to get my head round your slosh filter...
 

MikeMl

Well-Known Member
Most Helpful Member
Google "Sallen-Key Low-Pass Active Filter". This one will damp out the normal sloshing of fuel in the fuel tank, and read out the "average" level instead of constantly running the bar graph up and down as the vehicle is driven.
 

marcbarker

New Member
[sarcastic mode] :) If you have a gas-guzzler car, you need to make sure the filter time constant isn't too long, or you'll not see the gauge dropping fast enough!
 

k_sarath_1988

New Member
cool study

Here is my suggestion:

A 741 is a piss-poor excuse for an opamp. It was obsolete 30 years ago.

It cannot difference two voltages close to it own negative supply pin. It can only pull its output within about 2V of either its positive supply pin or it negative supply pin, meaning that when running on 6V, the output can only swing from 2V to 4V.

In my version, I'm using a LM358 which is cheap, plentiful (and I have lots of them in my parts bin, and I have a trusted Spice model)

Note how I split the Sender pull-up resistor into two resistors, and used that as the signal to be amplified.

Note also that you will likely have to trim the Offset. See the variable resistor in the simulation.

The Plot is versus the Sender Resistance at various values of the TRIM resistor. If the gain is not suitable, then vary the feedback resistor.
i wanted to know the name of the software you used....plz tell me...
 

MikeMl

Well-Known Member
Most Helpful Member
i wanted to know the name of the software you used....plz tell me...
LTSpice. See Linear.com/downloads. Also join the LTSpice Users Group on Yahoo.com
 

caslor

Member
Hi

I need some explanation to the schematic in order to avoid any mistakes .




1) The 7806 is used to drop voltage from 12v to )6 volt in order to avoid faulse readings when we have dropped voltage in our battery ?

2) R4 is the floater - sender Resistor ?

3) This one has to do most with theory... Why we place the R3 resistance there.. between the + of our battery and in sender output that gives -
what wil be wrong if we didn't place that?

thanks
 

MikeMl

Well-Known Member
Most Helpful Member
Hi ...
1) The 7806 is used to drop voltage from 12v to )6 volt in order to avoid faulse readings when we have dropped voltage in our battery ?
Yes. It can be any way of reducing and regulating the circuit voltage to 6.00V. By changing some resistors around the opamp, it could work at 5V, too. My favorite regulator for something like this if an LM7806 is hard to find, would be an LM431 shunt reg, or an LM317 adjustable series reg.

2) R4 is the floater - sender Resistor ?
Yes. The circuit can be reconfigured for various sender resistances.

3) This one has to do most with theory... Why we place the R3 resistance there.. between the + of our battery and in sender output that gives -
what wil be wrong if we didn't place that?
R3 and R4 make a voltage divider, where R4 varies. The voltage created at the junction of R3 and R4 is signal that is offset/amplified/filtered/displayed by the following circuitry.
 
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