I need a circuit that can convert from a 0-5V signal to a signal suitable for driving a car fuel gauge for an EV project to show battery state of charge. Assistance with designing the circuit would be so appreciated.

The 0-5V is produced by my battery management system to represent battery state of charge, 0V = 0%, 5V = 100%. The original fuel level sender unit has a resistance of 15 Ohms for full and 409 Ohms for empty. It seems this equates to me needing to produce around 5mA for empty (seems zero current would also work) up to around 138mA for full.

As I see it I could generate this signal conversion using an opamp but I need help please with the details.

Many thanks

 

Welcome to AAC!
What is your battery voltage when fully charged?

 

The original fuel level sender unit has a resistance of 15 Ohms for full and 409 Ohms for empty. It seems this equates to me needing to produce around 5mA for empty (seems zero current would also work) up to around 138mA for full.

Depends upon the resistance of the gauge.
Can you measure that?

 

If you have a power source giving about 15 V, then an OP Amp with a power boost should be able to drive the meter with an input signal of 0 to + 5 V. Thumbnail shows a fuel gauge used as a display with an input to D1326 of 0 to + 10 v, then OP Amp. would need to supply a gain of around 2.

 

Suggest you read below, Related Forum Posts, How to drive 12V gauge from 5V MCU.

 

Thank you Bernard for the input. To provide some clarify, this is a new Toyota Yaris, I think the cluster computer is measuring the current and then relaying a signal to the guage - so it is not a resistance type guage.

I measure 2.1V on the signal wire (down to ground) when the original sender unit is disconnected. I read 1,08V when the original sender unit is connected and positioned at 100% (full). And I read 0,08V when positioned at 0% (empty).

I have the original 12V battery running the car systems that I can use as a supply.

The 0-5V analogue SoC signal can sink or source up to 10mA.

I read the other thread that sugests a NPN Transistor. It makes sense in principle, but exactly how must the circuit look? I will have Vbb as the 0-5V signal connected onto the Base, and Vcc as the 12V supply connected to the Collector. Each with the same ground on the Emitter. As far as I can see I need a Rb resistor and a Rc resistor. How do I determine the resistor value of these two resistors? I suppose I should use pots for both of them to enable fine tuning, correct? So I need to know what range pots to use.

I see that the BC549 is rated for 100mA collecor to emitter. This might not be enough to register FULL on the guage, so how do I design the circuit to allow for more current?

Looking forward to your help

 

To provide some clarify, this is a new Toyota Yaris, I think the cluster computer is measuring the current and then relaying a signal to the guage - so it is not a resistance type guage.

I thought this thread related to an EV project?
Are you saying you are re-purposing a gauge taken from a new Yaris?
Is the gauge a traditional cross-coil type or is it one with a stepper-motor drive?

 

I thought this thread related to an EV project?
Are you saying you are re-purposing a gauge taken from a new Yaris?
Is the gauge a traditional cross-coil type or is it one with a stepper-motor drive?

It is a EV conversion of a Toyota Yaris and I need to drive the OEM guage from my 0-5V SoC signal. I don't know the guage type actually.

 

By my calculations, if the sender resistance is 15Ω on 'full', then the 1.08V across it implies a gauge (or metering system) current of 72mA.
We don't know the gauge type, its supply voltage or its resistance, so all we have to go by is that current. It would be simple to design a circuit which converts a 0-5V input signal into a 0-72mA current sink, but whether or not the instrument cluster would be happy with that is uncertain.

 

By my calculations, if the sender resistance is 15Ω on 'full', then the 1.08V across it implies a gauge (or metering system) current of 72mA.
We don't know the gauge type, its supply voltage or its resistance, so all we have to go by is that current. It would be simple to design a circuit which converts a 0-5V input signal into a 0-72mA current sink, but whether or not the instrument cluster would be happy with that is uncertain.

Hi Alec, yes 75mA would be the right ball park I reckon, using the 1V and 15 Ohms. I think it will work. How do I determine the details of the circuit?

 

We don't know the gauge type,

I'd be willing to bet it's a stepper motor gauge, most new cars use them. If the gauge pointer makes a full sweep at start up and the goes to a level indication that's a pretty good reason to expect it to be a stepper gauge.

 

Here's a suggested 0-75mA-ish (adjustable) current sink which takes in the 0-5V signal and drives the metering system. It comes with no guarantee, in view of the unknowns. Use it at you peril .

R1,D1,C1 are intended to suppress any positive or negative load-dump spikes on the +12V rail.

 

Here's a suggested 0-75mA-ish (adjustable) current sink which takes in the 0-5V signal and drives the metering system. It comes with no guarantee, in view of the unknowns. Use it at you peril .
View attachment 144587
R1,D1,C1 are intended to suppress any positive or negative load-dump spikes on the +12V rail.

Dear Alec_t, thank you very much for the cuircuit design I will build this and test it. Please tell me, what software are you using to design the circuits and does it caluclate the resistor values for you automatically?

 

I use LTspice to draw schematics and simulate the circuit operation. The software doesn't design the circuit or choose component values; that's down to the user.