Trying to adapt a fuel gage circuit

Scopeman

Hi All.. Scratching my head here! does anyone have a circuit solution to a project I am working on?

I have a WWII Fuel gauge out of a B-17 Bomber that will work on 12v DC, if I put 128K ohms inline the gauge reads "full" my Jeep fuel tank is 17 ohms full and 80 ohms empty. trying to create a circuit that will tie the meter into the tank sensor and read "E" when the tank is empty and "F" when its full! any ideas?

Thanks,
Kendall ☺

daveg360

Probably going to need some data points inbetween full and empty to work that one out :)

Perhaps something like this:

Jeep B-17
0% 80 ??
10% ?? ??
20% ?? ??
30% ?? ??
40% ?? ??
50% ?? ??
60% ?? ??
70% ?? ??
80% ?? ??
90% ?? ??
100% 17 128k

Scopeman

Thanks daveg360,
just cant think of a way to take one changing variable and creating another one!

SpeedBird

Hello,

You could do this with a very cheap microcontroller and a digital potentiometer.

A simple circuit could measure the output of the potential divider made with the Jeep fuel tank sensor and R1 (in the diagram).

The microcontroller could then calculate the resistance and output the appropriate resistance via the digital potentiometer which will be presented to the B17 gauge.

This is just an idea. If you would like some more specific information send me a message.


Nik

Attached Images

Fuel_Gauge.gif (23.2 KB, 13 views)

BenchTop

An op amp could solve this.
With the inverting configuration, you need one pot to set offset - that's the baseline- and one pot to set gain - that's the range. That way you get perfect adjustment of the limits.
LM386 is nice.
http://www.national.com/mpf/LM/LM386.html#Overview
http://www.national.com/ds/LM/LM386.pdf

Scopeman

Thanks BenchTop, I will give that a try! Sounds pretty simple.


Kendall☺

Scopeman

Benchtop,..New to this but is there somewhere I can find a detail application breakdown the LM386 or may I ask for some type of schematic from you?

Thanks,
Kendall

Scopeman

Thanks SpeedBird,
I just got your message!!

Scopeman

Hi SpeedBird.. any information you can offer would be greatly appreciated!

Thanks,
Kendall

BenchTop

There is a googleplex of sites with op amp tutorials for getting a basic understanding.
Each company produces application notes for their devices showing lots of circuits.

The example inverting op amp configuration, as shown, uses your sensor as an input resistor.
R2 is a feedback resistor that sets what voltage is needed on the output to counteract the input current to make the - input of the op amp balance the reference on the + input of the op amp.

If you test your output indicator (they are sometimes electromagnetic, sometimes electrothermal - it it's electrothermal it takes a while for a reading to become valid) you can find what voltage is needed to reach the upper limit.
You may not need to adjust offset at all.
You may need to add resistance in series with the sensor to keep currents low.
You just have to stick the probes in and find out.

Scopeman

Hello BenchTop, Thanks so much for your help and direction.. I will do some good reading their!

Kendall ☺

Scopeman

Hello Again BenchTop, .. been reading on a wide varity of applications this little guy can handle! Found a chip that's offered by Radio Shack -

TL082/TL082CP Wide Dual JFET Input Op Amp (8-Pin DIP)Catalog #276-1715

Do you think this will work ok?

Thanks,
Kendall

BenchTop

You can usually find data sheets here:
http://www.datasheetcatalog.com/data.../8/TL082.shtml

If your output indicator will work on about 20 mA or less, it looks like tl082 should work.
If you have an unused op amp, tie the inputs to something so it doesn't oscillate.

digikey, mouser, jameco = much better prices and selection than radio shack.

Scopeman

Thanks BeachTop, I'm familiar with digikey, I will give them a try. I should be ok on 20mA!

mugaliens

Scopeman, fuel guages are linear. The only piece of information missing in your OP is at what inline resistance does your B-17 fuel guage read empty?

Armed with that, a simple resistor bridge is the easiest solution.

Scopeman

Hi mugaliens, this meter reads empty 0 ohms and full at 128k ohms using 12v the sensor in my tank is around 17 ohms full and about 80 ohms empty with 12v.
just trying to keep this as simple as possible! your idea sounds good, do you have a diagram I can look at?

Thanks, ☺