The method of using a sensor and a gauge is simple. The Gauge is a current meter, usually in milliamps, (2 milliamps
full scale for some). The gauge will measure the current as the sender changes resistance and shunts to ground. The
voltage and current supplied to the fuel sender is 1 ma and just a few millivolts. Fairly safe, at those levels.

To measure digitally, supply a 1 milliamp constant current source, to the sender. (use a 7805 with a series resister of
1% @ 5000 ohms to the sender.) Then measure the Milli-voltage with an A/D. Create a look up table or formula to convert
to pressure, temp, or fuel capacity. Don't forget to add an averaging scheme to make up for the D'Arsenval movement of
the mechanical meter. You know with digital numbers jumping around all over the place.

Older vehicles used a temperature sensitive mechanical contact system. The higher the voltage the faster the contacts
would open and close. in short a PWM.

Gordon Varney
www.iamnee.com


>
>
> Hi all....
>
> I've just taken delivery of a fuel tank sensor and display for the car I'm
> building next week :)
> However, the sensor has got me thinking.... Many moons ago, there was a
> long thread on this list about sensing fuel levels in containers. I seem
> to remember optical solutions where popular. Most people shyed away from
> anything involving electricity within the fuel tank. So when I took receipt
> of my sensor I was rather surprised to find an open potentiometer- a float
> arm actuates the wiper (all this under petrol or in the vapours). This leads
> to believe me that
> 1) the company that produces this is heavily insured :)
> 2) I may have pre-warmed petrol and/or a free afterburner function
> 3) the meter limits the current going down the sensor.
>
> I tested the resistance and it was fairly linear 0-300 ohms. The meter appears
> to be moving coil/iron but it's sealed so I can't peak inside. Any ideas
> on how the internals of the meter work? I'm also curious how they stop the
> petrol guage from being adversely affected by the car battery voltage (9-15v)....
>
>
> I'd probably be tempted to use the R as part of a RC oscillator circuit
> going into a F/V converter. Hopefully that way I'd limit the current through
> the potentiometer. I'm not sure what heat dissapation I'd be happy with
> in my petrol tank though!! If I was going to invent it from afresh I'd use
> a platic bowden cable off the float arm and have it actuate an external
> potentiometer.... But any ideas how they do it in the real world??
>
> Ben
>
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