Variable resistor may be used in the place of the fixed resistor R. It will enable you to adjust the output voltage Vo for the given resistance of the sensor. For example you can use a variable resistor to set the exact brightness level which makes an IC change state.
The variable resistor value should be the larger than the fixed resistor value it replaces. For finer control you can use a fixed resistor in the series with the variable resistor. For example if a 10kΩ fixed resistor is suitable you could replace it with a fixed 4.7kΩ resistor in series with a 10kΩ variable resistor, allowing you to adjust the resistance from 4.7k to 14.7kΩ.
Take Care Using Variable Resistors With Transistors
If you are planning to use the variable resistor connected between the +Vs supply and the base of a transistor you must include the fixed resistor in series with the variable resistor. This is to prevent excessive base current destroying the transistor when the variable resistor is reduced to zero. For further details see the Transistor Circuits page.
The sensor and variable resistor can be swapped over if necessary