Hello,

I'm wanting to design an adapter box for the fuel level sensor in my racecar. I've stripped the factory wiring and dash and am installing all new Motec gear. I need to convert the fuel level signal to 0-5V to suit the analog inputs of the ECU. I was planning on building a small voltage divider circuit and installing this in to a small enclosure, then potting the enclosure with 2 flying leads with DTM connectors.

The fuel level sender is a float arm attached to a variable resistor. The resistance at full is 4 ohms, and the resistor at empty is 120 ohms. Therefore, I have worked out that I need a fixed resistor of 170 ohms to achieve a pretty close to 0-5V, if I use a regulated 12V supply.

Here's where I need some help. How would I incorporate a regulator to the circuit to ensure the voltage is fixed at 12V when the vehicle is charging, and how do I add filtering to the box in the event of sloshing moving the arm around - I want it to be slow acting, just as the factory fuel level gauge is.

Any help much appreciated!

 

@ericgibbs or @bertus , Can you please move this back to where I posted it? I don't think this section will get much traffic, and the application is kind of irrelevant - The core question is how to slow down the action of the voltage divider circuit.

Thanks

 

Hi DofW,
I will move to General Electronics Forum, see if that improves the responses.
The original Thread you posted too, was Old! , so thats why I Created your own Thread.
E
BTW: it often quiet this time of the day, due to Global Time zones...

 

Hi DofW,
I will move to General Electronics Forum, see if that improves the responses.
The original Thread you posted too, was Old! , so thats why I Created your own Thread.
E
BTW: it often quiet this time of the day, due to Global Time zones...

All good, sorry I'm new here I can't even see a link to get to the automotive section anywhere on the site, and there are only 3 threads in there so figured it's not a popular subforum

 

I would regulate it at 5V or 9V, damping can be provided by resistor and capacitor filtering..

 

Lots of dropouts, inductive kicks and noise in automotive power... it’s not trivial.

 

See

 

 

If you're going to regulate the voltage then since the system was designed for 5 volts - why not regulate it at 5 volts? Unless I misread part of your question, which sometimes I do.

 

Re : #8 -- i've multiple measured the battery - but this might be a problem as the low internal resistance and some internal parallel capacity makes the 12V PbSO4 a near ideal spike suppressor . . . anyway

interesting video not what i was looking for + what i was looking for (does not follow your spikes level ??)

 

See

 

Thanks all, actually the Motec dash I connect to has a regulated 5V and regulated 8V sensor supply available to use, so I could get away without the regulator circuit and use one of those sensor supplies.

 

See
View attachment 194597

Hi mate, what did you add here at the start of this circuit? Would it be better to convert to external regulated 8v supply and change R1 to 72 Ohms?

 

what did you add here at the start of this circuit?

The D2 , L1 , C3 , C4 , U1 in the attachment 194597 form a high performance "noise" filter , which is relatively simple compared to any alternative noise suppression scheme

 

The D2 , L1 , C3 , C4 , U1 in the attachment 194597 form a high performance "noise" filter , which is relatively simple compared to any alternative noise suppression scheme

Right, so if I converted over to a regulated 8V supply, assuming no noise suppresion is required, those components wouldn't be required then. Would the rest of the circuit work as intended with the only change required being to swap the 120ohm resistor for a 72 ohm resistor?

 

The - U1 - SMCJ15A appears to be a 15V TVS diode /// so the circuit is regulated by a constant current source - that depends on the characteristic reverse conduction curve of the BZX84C5V6 zener . . . so if you reduce the R1 or R4 you also may need to reduce the R3 for it to be enable to provide enough current to zener D1 for the circuit to remain stable ← ← as "always" -- it's a principial solution -- needs testing and fine tuning -- with the actual components used +
/!\ due unpredictable factors it may be and not sufficient for operation in target environment and conditions /!\
you always have to stay alert and be aware what is happening (nothing works just because "we" thought it would)

you need to be aware of the circuit possible behaviour while testing + able to cancel supply currents at "flash" or use thermistors , fuses and other precaution methods !!!!!
in purpose not to destroy anything else the circuit connects to (it's difficult but possible to fry the sensor pot inside the fuel tank leading to expolsion . . . unlikely because the gauge is likely built to disclude such)

so although the TVS (a voltage spike and overvoltage supressor) and inductor (a half wave rectifier with current spike suppressor) here won't be "that" system critical -- it's good to consider their role & possible role "outside the box" . . . it also wont hurt if your gauge reading is a bit more stable than otherwise