This is a very cheap analogue board containing a comparator and and a pot to give both an analogue and digital (threshold) output.

Various sensor configurations can be found which include an LDR, NTC thermistor, and two resistive water/moisture detectors.

The water/moisture detectors utilise a 2 pin header which can allow us to reuse the comparator board for any particular device we have available. For example, the picture shows a flat style 10K thermistor along with the moisture detector kit.

Due to the circuit's simplicity, the digital output tends to oscillate when the two inputs to the comparator have a similar magnitude.