This humidity sensor consists of 2 copper conductors that are located at small distance from each other. The relay switches as soon as the moisture makes a connection that is more or less electrically conductive between the 2 electrodes.
How does the humidity detector works?
When the electrical resistance between the 2 sensors drops below a certain value then the Schmitt trigger (T1 and T2) switches. The RS N1/N2 bistable multivibrator is flipped through C1 so that in point B we have a low voltage and so T3 will close the relay.
The relay is opened when the 10K resistor is connected to point A, not to point B as shown in the schematic. You can use other sensors like LDR or NTC instead of the copper ones so you can use this circuit for detecting light or temperature.
Schematic of the Humidity Sensor Circuit
