A very simple and useful project of soil moisture circuit. The circuit is using only four components and a 3volt battery. The circuit will detect if there is water in the soil of any plant and light up the LED.
The resistor R2 is a potentiometer to adjust the level on which you want to make the LED on. This circuit will work on soil but you can also use it to test or detect moisture in other substances.
Q1 = BC547
R1 = 49R (ohm)
R2 = 50K (Variable resistor)