# multiply resistance??

Discussion in 'General Electronics Discussion' started by trix2, Dec 12, 2017.

1. ### trix2

1
0
Dec 12, 2017
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.

2. ### Bluejets

4,419
938
Oct 5, 2014
Arduino would be one way using the map function where one would take your sensor and add another resistor to give a voltage divider. This varying voltage value would be monitored and then fed into an analog input. Mapping involves taking one range of inputs (min-max)and converting into a different range of outputs, in this instance perhaps a varying pwm signal to drive the meter. It is also possible to invert the readings if required.
It would involve an amount of effort but the programming is fairly easy if you are even just a small amount familiar with Arduino.

I've no doubt others may be able to supply a different arrangement using op amps, too long ago since I worked/played with them.

In the meantime it may be of assistance if you can work out the resistance/ full scale current of your meter movement. There are various ways of doing this , many shown on the web. See if I can find the one I used a couple of weeks ago on a vu meter. Also, to get even a bit more indepth, there are ways to make the movement expanded scale. i.e. so it will read from say 50 to 100 in place of 0 to 100. Perhaps put that aside for later.

Last edited: Dec 12, 2017

4,419
938
Oct 5, 2014