Chapter 9

1. The DHT22 is a more accurate sensor, and it is capable of sensing a greater range of temperatures and humanities. 
2. The external pull-up resistor is optional because our Raspberry Pi can use its internal embedded pull-up resistor.

3. An LDR is a light-sensitive resistor. When used as part of a voltage-divider circuit, we turn the varying resistance into a varying voltage. This voltage can then be detected by an analog-to-digital converter such as the ADS1115, which is connected to your Raspberry Pi.
4. Try varying the resistance of the fixed resistor in the voltage-divider circuit. Try higher-value resistances to make the LDR more sensitive in darker conditions. Try lower-resistance values to make the LDR more sensitive to brighter conditions.
5. No two LDRs are identical when it comes to the resistances they measure. If you swap out an LDR in a circuit, re-calibrate the code just to be sure.
6. Water conducts electricity. It acts as a resistor between the two probe wires...