Working of the circuit simple, when the two probes shown in the circuit will detect any moisture in a substance then the resistance between these two probes decreases hence the transistor become switch ON and activates the relay. The circuit can also be operated with 12 volt DC but make sure to use the 12 volt relay also.
This simple moisture sensor switch circuit can be used to switch ON any equipment on the certain level of moisture content in any substance like soil, wood etc. The circuit is very simple and using only few components. The 300K variable resistor is used to adjust the circuit to switch on the relay on the certain