Variable resister/fuel level sensor - fuel gauge - change resistance range

Thread Starter

nicktruman

Joined Feb 4, 2019
26
Hi Everyone
I am really hoping that there is a solution to this issue..
I have made a car using bits of this and bits of that and it all great except...

My fuel gauge (built into my dash cluster) expects the fuel level sender to have a resistance range from 50 to 350 ohms; however my fuel level sensor has a resistance range from 0 to 100 ohm.

How can I connect the 2 and get the gauge to read approximately right? You can buy "interfaces" that will connect any gauge to any sensor but they are expensive, I also can't change the sensor as it part of the fuel cell.

Thank you
 

Alec_t

Joined Sep 17, 2013
11,414
Here you go. I've updated the circuit to cater for your resistance values :-

Adapter50_350.PNG

Edit:
Note that this adapter is for a conventional 'old'school' analogue gauge, not a fancy modern digital one.
 
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Alec_t

Joined Sep 17, 2013
11,414
Is the +v 12 volts?
Anything in the range 5V-12V simulates ok.
It appears Alec_T's circuit used 70 ohms.
I tried 50 and 100 Ohms too. They all give the same result.
The gauge uses a stepper motor,
Ah. So that implies some digital goings-on inside the gauge. My circuit assumes an interior resistance (coil or heater element), so I don't know if it will work with your gauge. It may well do, since the gauge is designed to work with conventional simple resistive fuel level senders.
 
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Thread Starter

nicktruman

Joined Feb 4, 2019
26
Anything in the range 5V-12V simulates ok.

I tried 50 and 100 Ohms too. They all give the same result.

Ah. So that implies some digital goings-on inside the gauge. My circuit assumes an interior resistance (coil or heater element), so I don't know if it will work with your gauge. It may well do, since the gauge is designed to work with conventional simple resistive fuel level senders.
Thank you is something like this easy to make? I have a pcb kit but I’ve never used it
 

Alec_t

Joined Sep 17, 2013
11,414
It could be made on a small piece of strip-board (e.g. Veroboard). The pinout details of the opamp are in its datasheet. Through-hole versions of the IC and the other components would be preferable if you have little circuit assembly experience.
 

crutschow

Joined Mar 14, 2008
25,126
Thanks (even if I don't know what Yer Yar means :)).
Interesting circuit.
I tweaked the values of R1, R2, and R3 to get closer to the desired value for the emulated sensor resistance (Remulated) of 50-350 ohms for the 0-100 ohms actual sensor resistance range (which probably is not that critical).

Edit: The plot shows the Remulated resistance (vertical scale) versus the sensor resistance (horizontal scale) for two values of Rgauge (10Ω and 50Ω). As you can see, the two Remulated resistance values are superimposed on each other, indicating the gauge resistance has no noticeable effect on the Remulated resistance (as desired).

1592673948194.png
 
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dl324

Joined Mar 30, 2015
10,987
My fuel gauge (built into my dash cluster) expects the fuel level sender to have a resistance range from 50 to 350 ohms; however my fuel level sensor has a resistance range from 0 to 100 ohm.
Doing something like this isn't as easy as you might think. The original sender might not be linear due to the nature of the fuel tank construction.

Do this mind exercise. If the tank was like a pyramid (base down), having the float at the halfway mark wouldn't mean the tank was half full. Now consider the pyramid standing on its tip. Same situation, but in the first, you had more than half of the fuel left; in the latter, you have less than half left.

Or the sender could be linear, but there's a look up table to map position to remaining volume.
 
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