what I still can't figure out is, if I use a voltage divider for the variable resistor between 2-90 ohm, than how can I divide 2 ohm, without loading the circuit?
The change in resistor value must be detected in the 'main' unit by a change in the current flowing in the resistor, [usually done by measuring the change in voltage across the resistor], OR the current flow thru a resistor in the main unit.
If you add a resistor in series with 'sensor' resistor you will most likely affect the calibration of the main unit.
if I use 0,05 ohm, is it appropriate, as I don't want the car's speedometer to show wrong results in the driver's analog display... (showing 40 mph while going with 70 mph)
If that's possible, than I will use 0,05 ohm as a shunt resistor and measure the voltage across it for current, and another voltage measurement is needed for the 2-90 ohm varying resistor.
Than, the only question that I still can't resolve is:
how can I measure the voltage on 2-90 ohm resistor, if the voltage span is above 5 V...
if I need to divide the voltage, how should I make the division, so that the original car speedometer is not effected?
thanks for your patience..