In Chapter 5, Connecting Your Raspberry Pi to the Physical World, we learned how to use the ADS1115 ADC breakout module, and that analog input is all about measuring a voltage from within a predefined range, which, for our purposes, is between 0 volts and 3.3 volts. While in digital I/O, we'd say 0 volts measured on a pin means low and 3.3 means high, in analog I/O, there are no concepts of high or low in this regard.
Many simple analog components and sensors operate on the principle that their resistance changes in accordance with what they measure. For example, a light dependent resistor, or LDR, changes its resistance in proportion to the light it detects. However, analog input is all about measuring voltage. To turn a varying resistance into a varying voltage, we use a voltage divider circuit.
