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[DPRG] Sensor ideas

Subject: [DPRG] Sensor ideas
From: Sluggy! sluggy9912 at swbell.net
Date: Wed Jan 6 17:38:59 CST 2010

Greetings, All!

Long time, so seize. I have been a-lurking, though....

Whilst lurking, I am doing another EFI conversion, this time on an air cooled VW engine. http://msruns.com/viewtopic.php?f=88&t=22894

One of the problems I need to solve is operating the fuel gauge.

The instrument I have http://tinyurl.com/ykje3lc wants to read a resistance between 0 and 100 ohms corresponding to the level of fuel in the tank. Since the tank is basically a hand fabricated 1 foot cube, there is not really a factory tank sensor that is actually made for it, at least not a cheap one. I am investigating salvage sensors, but it would be nice if I could use the one that is already on my fuel pump.

The fuel pump I'm using is an in-tank unit that was salvaged from a Kawasaki motorcycle, a model that is equipped with a fuel gauge. The stock sensor, however, is basically a variable capacitance sensor. At this writing, I don't know what the range is, other than either empty or full is approximately double the capacitance of the other state. I need to convert that capacitance into a variable resistance. There is a chance that a variable voltage may work; I will have to investigate that separately.

I'm contemplating a circuit that converts capacitance into either a pulsewidth or a frequency, then use that value to adjust a digital potentiometer like these http://www.maxim-ic.com/products/digi_pot/

Unfortunately, it appears that 10Kohm is the smallest end-to-end value and 64 steps is the smallest increment, so the smallest change I can affect is 156 ohms. That doesn't even consider that there is a minimum series "wiper" resistance of 100ohms.

By paralleling the digi-pot with a fixed resistor, I can get a piece of the range. 10K + 100 = 99.01 ohms, which is good. but minimum is 256 + 100 = 71.91, giving me a workable range of roughly the 1/4 tank.

Anyway, does anyone have any ideas or suggestions for this problem?



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