Need help with a circuit to alter fuel sender to match new gauge

Thread Starter

Beflyguy

Joined Jul 12, 2018
4
Hi. I'm new here and looking forward to getting some help and maybe helping in my area of expertise. I've been in the automotive business for 42 years and have done a lot of work with auto fuel and electrical systems, particularly Subaru. I'm very familiar with electrical, not so much with electronics.

I looking for help with a retrofitting a gauge cluster to match a different fuel tank sender. The original sender had a resistance range of 2.5-94 ohms pretty linear. The new sending unit has a resistance range of 35-275 ohms, also linear. The gauge reads with the new sender but obviously is not accurate.

Thanks in advance
Brian
 

crutschow

Joined Mar 14, 2008
23,352
Here's a circuit by Alec_t, one of the forum members.
It was originally designed for different conversion resistance values.
I'm not sure I completely understand it but I modified it so it seems to be close to the conversion resistances you want.

The graph X-axis is you new "ActualSender" resistance, and the Y-axis is the calculated converted resistance to the gauge.

Perhaps Alec can chime in and help with that, if it needs some further tweaking.

Edit: Had to change the op amp to a Rail-Rail type because the LM324 has marginal common-mode range for this application.
Also changed Q1 to a power transistor since it's maximum power dissipation is near 5W. It will thus need to be on a heatsink, something around 10°C/W or better.

upload_2018-7-13_8-59-51.png
 
Last edited:

Thread Starter

Beflyguy

Joined Jul 12, 2018
4
Thanks for the responses. I should have said that we will be building 50-100 of these so simplicity and compactness are very important. The circuit will be integrated into an existing board used to change the way the cluster connects to the car. The board will no doubt need to be enlarged.

DSC01281t.jpg
 

shortbus

Joined Sep 30, 2009
6,984
Thanks for the responses. I should have said that we will be building 50-100 of these so simplicity and compactness are very important. The circuit will be integrated into an existing board used to change the way the cluster connects to the car. The board will no doubt need to be enlarged.

View attachment 156136
You do know that some of the older cars used the temp sender gauge internally as a kind of voltage "regulator" for the other gauges?
 

Alec_t

Joined Sep 17, 2013
10,367
Perhaps Alec can chime in and help with that, if it needs some further tweaking.
Looks ok to me, Crutschow, and thanks for your contribution. I hadn't determined power consumption and heatsinking requirements.
My circuit was intended to work with old-school gauges having various resistances, but it would be nice to know what the actual resistance is. If it's higher than my guessed 'rg' value, then perhaps the R1 and R2 values could be increased (to reduce power consumption) without unduly affecting the circuit's resistance conversion linearity.
Note that at the low resistance end of the sender there is a simulated resistance offset error (because the sender resistance doesn't drop to zero), but for a practical fuel gauge this may not be significant. I can't think of a simple analogue way of overcoming this error.
simplicity and compactness are very important
In that case a digital approach would likely be better, using a MCU with A/D input and PWM output ...... probably the approach commercial adapters would use.
 

crutschow

Joined Mar 14, 2008
23,352
Beflyguy said:
simplicity and compactness are very important


In that case a digital approach would likely be better, using a MCU with A/D input and PWM output ...... probably the approach commercial adapters would use.
I agree with Alec on both points.
The question then is, are you up to doing the programming for an MCU approach?
 

Thread Starter

Beflyguy

Joined Jul 12, 2018
4
I have thought about the MCU approach, particularly if it could do matching of multiple gauges. I have contacted the guy that makes Metermatch and Tachmatch to see if his products would work. Thanks for the link to those.

An MCU would be like an Arduino?
 

DickCappels

Joined Aug 21, 2008
5,890
I have thought about the MCU approach, particularly if it could do matching of multiple gauges. I have contacted the guy that makes Metermatch and Tachmatch to see if his products would work. Thanks for the link to those.

An MCU would be like an Arduino?
Yes. An Arduino is an MCU and the distinguishing feature is the programming environment and programming language. AVR controllers which make up the majority of Arduino controllers can also be programmed in C, assembly language, AttoBasic, Forth, and other languages.
 
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