Voltage Dividers

The circuit of Fig. 1 can be easily modified to measure a variable resistance, as shown in Fig. 2. Because the voltage drop across the resistor will vary continuously over a certain range, we will need to use an analog input pin to read the voltage values.

Figure 2: Basic circuits to measure a variable resistance.
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The circuits of Fig. 2 are referred to as voltage dividers, since the 5V voltage drop from the voltage source to the ground pin is distributed across the two resistors, depending on their values.
If the resistance of the variable resistors is set to 0 $\Omega$ , the voltage level at the analog pin on the left circuit will be +5V, while the voltage level in the circuit on the right will be 0V.
If the resistance of the variable resistors can be set to infinity (essentially an open switch), the voltage level at the analog pin on the left circuit will be 0V, while the voltage level in the circuit on the right will be +5V.
In most cases, variable resistors have a limited range of resistance and thus, the range of voltages that can be registered at the analog pins will be smaller than the full 5V.
For example, if we use a variable resistor with a resistance range between 0 to 10K $\Omega$ , the range of voltages seen by the analog pin for the circuit on the left will be 5V to 2.5V, while that on the right will be 0 to 2.5V.
On the other hand, if our variable resistor only has a resistance range between 100-200 $\Omega$ , the range of voltages seen by the analog pin for the circuit on the left will only be 4.95V to 4.9V, while that on the right will be 0.05V to 0.1V.
In order to expand the range of variable voltages seen at the analog pins, the value of the fixed resistor in the circuit can be changed. That said, the circuit design should avoid the possibility of very high current levels or short circuits.
We'll see that a large number of sensors have variable resistances and can be connected using the configurations shown above.
It is often helpful to have a multimeter and/or an oscilloscope available to evaluate electrical circuits.