# Multiply resistance

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#### TRIX

##### New Member
Hi
I'm building a custom motorbike. I'm using a digital speedometer that includes a digital fuel gauge. Rather than using this to indicate the fuel level, I want to use it to indicate coolant temperature.

My coolant temperature sensor has a resistance that reduces with temperature; about 150 ohms at 50 degC reducing to 30 ohms at 100 degC. (I gather the original, analogue temp gauge operates on the change in current due to the change in resistance?)

The digital fuel gauge reads almost empty if I ground the wire through a 450 ohm resistor and it reads almost full if I ground it through a 90 ohm resistor.

If I could somehow multiply the resistance of my coolant sensor by 3, I can use the fuel gauge as a temp gauge (i.e. empty would be "just warmed up", full would mean "too hot" and the mid position would be normal running temperature).

Is there a way could I "multiply" the resistance of the coolant sensor, so that the fuel level range could be used to indicate coolant temp?

I'm a mechanical engineer with some electrical experience but not enough to figure this out, but capable of building a circuit if someone can tell me how! Any ideas greatly appreciated.

#### dknguyen

##### Well-Known Member
Is the resistance change of the coolant sensor linear? And does the fuel gauge respond linearly with resistance? Most resistance output temperature sensors have non-linear output.

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#### MikeMl

##### Well-Known Member
The only hope is if you are willing to (first) plot the gauge reading vs resistance of the sender. You will need to power the gauge from whatever voltage it would see in the (running) bike. You will need to provide a table or graph of gauge reading with at least 10 (more-or-less) equally spaced resistance values between 450Ω and 90Ω. That will show how non-linear that function is... At a given input voltage, check to see how much the gauge reading changes as the voltage is varied.

(Second), you will need to provide a similar table or graph of sender resistance values between cold and hot. That will show how non-linear that function is, and we can only hope that it is the reciprocal of the first function...

By doing a curve-fit (using Excel) through those two sets of points, it may be possible to suggest some kind of circuit...

#### alec_t

##### Well-Known Member
Here's how you could provide a 3x resistance multiplier :-

This should work whether V+ is 5V or 12V.

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