I've tinkered with these gauges before.  Though I can't remember much of=20
the specific data on them, I'd approach it this way...

Are you sure the range is a few ohms to 25ohms?  That's really low,=20
unless they're "active"/electronic gauges (vs. very common electrical,=20
or newer stepper-motor-needle gauges).  Common values for fuel-level are=20
0-90 ohm, 0-180, 240-33, etc, though much older cars can stray from=20
this.  Temp senders usually range from a few hundred ohms to a few=20
k-ohms.  What brand are these gauges, or what car are they from?

You might want to measure to be sure.  Put resistors of different values=20
on the sensor line, and figure out what resistance gets you at either=20
end of the scale.

Measure the current with these two resistors in place.  Now, you can=20
determine if a digital pot can supply the required current.

I'd expect pwm to cause the buzzing, but are you filtering the pwm? =20
Fuel level and water temp are slow functions, so you should be able to=20
add a significant filter.

Cheers,
-Neil.



On 5/11/2012 5:55 PM, Cristiano Cesaretto wrote:
> Hello.
>
> I am building my racing simulator and I want to use a real fuel level and
> water temperature gauges.
> I would like your opinion about  which way to drive these gauges is the
> best. Sorry if I say somethig silly but my knowledge is limited.
>
> So the gauges have 3 wires: 12V, ground and the signal.
>
> http://dl.dropbox.com/u/3544050/gauge1.GIF
>
> the signal connects to the sender that converts the fuel level or water
> temperature in a variable resistance. the resistance varies from few Ohm
> until 20-25 Ohm.
>
> my doubts regard how to generate the variable resistance.
> I have three options in mind:
>
> 1) using a digital pot. the smallest pot I can find is 1KOhm so resolutio=
n
> could be a problem. I think I can solve putting a resistor in parallel.
> 2) I read I can use a mosfet as a variable resistor.I have never done tha=
t
> but I will investigate it
> 3) I have replaced the variable resistor with a mosfet and applied a pwm
> signal to the gate. it works but the gauge because noisy, it makes a soun=
d
> like BUZZZ...
> also on the drain I have voltage spikes up to 120V. I suppose because the=
re
> is something inductive in the gauge...
>
>
> Does anybody have suggestions or consideration to make?
>
> Thank you
> Cristiano

--=20
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