# How to measure resistance for component of circuit

Hello,

I have sensor and indicator which display level of fuel . Sensor change own resistance depending fuel level. So I need connect my Arduino to this circuit for measurement resistance of this sensor for future data processing.

I know about voltage divider , so I can measure resistance of sensor as standalone component (resistor) - but I can't understand how I can measure resistance of sensor without breaking car circuit -> sensor -> indicator .

Will be thankful for any advice's .

PS: I'm programmer with 7 years of experience so I'm thinking in very speciffic manner - it's not compatible with circuitry:)

You know about voltage divider: Vin -- resistor1--Vout--resistor2--Ground.

Vout = (Vin x R2)/(R1 + R2)

Measure Vout using an analog input pin. Vout = 0.00488 x analog reading (from 0 to 1023)

Then you will know Vin, Vout, and one of the resistors, Solve the equation to find the unknown Resistor value.

Vout = (Vin x R2)/(R1 + R2) Vout(R1+R2) = Vin x R2 VoutR1 + VoutR2 = VinR2 VoutR1 = VinR2-VoutR2 VoutR1 =(Vin-Vout)R2 R1 = (Vin-Vout)R2/Vout or VoutR1/(Vin-Vout)= R2

Hi,
The typical car fuel gauge has a variable resistor right in the fuel, connected to a visible gauge on the dashboard.

Typical “sender” in the tank varies something like FULL=15 ohms EMPTY=80 ohms.

The Gauge or “indicator” is (maybe) 20 ohms or so. (Anyone have better data??) Powered from a voltage regulator of some type…

The current is a maximum of about .15 amps as I recall.

So if you measure the voltage from ground to the fuel tank sender connection, it will change from Empty to Full over some range. It will not go to Zero however.

MAYBE the voltage will vary from something like Full= 1 volt Empty = 6 volts You need to test this with a voltmeter so you know what to do.

So, if you use a voltage divider that will bring this range into 0 to 5 volts Arduino can read it.

THEN you need to do the math to invert the sense of the data and scale it as you wish.

Exactly what vehicle is this?? Maybe you can find the information online…

Thank you for complete explanation - I'ts look like exactly what I need :) Thank you very much !

CrossRoads: Vin -- resistor1--Vout--resistor2--Ground. Vout = (Vin x R2)/(R1 + R2)

Vout = (Vin x R2)/(R1 + R2) Vout(R1+R2) = Vin x R2 VoutR1 + VoutR2 = VinR2 VoutR1 = VinR2-VoutR2 VoutR1 =(Vin-Vout)R2 R1 = (Vin-Vout)R2/Vout

Hi,
Thank you for your comment - it can useful for me.
I have very old and scary car - LADA 2106 (1985 y.o. made in USSR This car only good because price - \$2000 , but I like it.
Yes you right - I google about resistance values and we have :

Full 315-340 Ohms
half 100-135 Ohms
Empty < 7 Ohms.  Thank you .

terryking228:
Hi,
The typical car fuel gauge has a variable resistor right in the fuel, connected to a visible gauge on the dashboard.