This module allows you measure moisture via analog output pins and it provides a digital output when a threshold of moisture is exceeded.
The module is based on the LM393 op amp.
It includes the electronics module and a printed circuit board that “collects” the rain drops. As rain drops are collected on the circuit board, they create paths of parallel resistance that are measured via the op amp.
** Many of these books may be available from your local library. Check it out!
An interesting link found among my daily reading