This is a simple hobby circuit that will make youngsters familiar with electronic circuits and components.
It’s possible to monitor moisture condition of soil in a tub with electronic circuit to decide whether more water is needed or not.
The variable resistor of this circuit makes forward bias of the transistor, which turns the LED on. But if the moisture from soil is in desired level, little current conduction through the probes will prevent forward biasing of the transistor. So it will turn off the LED.
The variable resistor has to be adjusted (sensitivity control) before its operation. You can also try other NPN transistor for this circuit. 3V to 4.5 V power supply is enough.
First appeared on electronics-as-hobby.blogspot.com (2011)