Making a non-linear resistance look linear

Hmm. Looking at some of the hair-raising suggestions being proposed for an analogue solution, you might have a point :-)

Pete

Sometimes high technology ain't the best solution. Plot a non-linear meter scale and paste it on the meter face.

t.

ther.

The gauges you are talking about use quite a bit of current, having a sensor and gauge circuit resistance of 100-300 Ohms power to ground. A micro based circuit should use less current, especially if used with a voltmeter type gauge. For a micro look at PicAxe, $20 will get you running with an easy to use Basic interpreter.

Westach makes gauges that can be adjusted for zero and span. Also they offer fluid level sensors, from Centroid Products, that can be creatively bent to help linearize the output. Centroid offers both current and voltage output sensors with digital or analog displays. Both companies are worth calling about your application.

Calibrating a sensor with exposed resistive element in water vs fuel can affect you readings since the water is most likely conductive from contamination.

A couple of quad op amps and some diodes, a reference voltage and a network of (adjusted) resistors might be 'simple' in principle, but it scales in complexity with the number of 'breakpoints' required in the curve. Easier, to just re-print the scale behind the meter needle.

As everybody else has said, digitise, linearise and use some kind of digital to analog converter to drive the meter.

If you must go analog, there are any number of ways of getting a monotonic (always slopes the same way) non-linear relationship between input and output.

Here is an old patent that is probably a massive ovekill for your application

National Semiconductor's Application Note 31

is a compendium of op amp circuits - it includes a "nonlinear operational amplifier with temperature compensated breakpoints" at the top of page 26.

-- Bill Sloman, Nijmegen

Late at night, by candle light, Pete Verdon penned this immortal opus:

I missed the OP, so I may be off target here. Most fuel gauges I've seen don't worry too much about absolute linearity. The actual readings seem to be "full", "empty" and "time to think about filling up". In-between you just give it the occasional glance to see if it's still OK.

- YD.

That's all I need, but the shape of the tank (wedge shaped in two dimensions) is such that the non-linearity is rather extreme.

Pete

The more extreme, the better table lookup is.

Pete Verdon wrote: > Is it possible to

Thanks for all the suggestions, but it looks like the actual solution will be simpler still. Apparently it only costs the tank manufacturers about £10 extra to get a custom sender made by their supplier, whose resistance at each point is calibrated to match the shape of the tank. Clearly that's the way to go - it hadn't occurred to me that it would even be possible.

Pete