This simple circuit tells you whether your Interior plant requires water or not. When the soil dry up, the Red LED lights to inform you that, the plant needs water. So this saves the life of your valuable interior plants.
Circuit is too simple. IC1 is the commonly available Op Amp uA 741. It is used as a Comparator. Its Inverting input (Pin2) is connected to a voltage divider R2,R3 that gives half supply voltage (4.5V) to it. The Non Inverting input (Pin3) of IC is connected to one of the probe. The other probe gets 9 volts directly from the battery. The comparator gives high output when its Non inverting input gets higher voltage than the Inverting input. Probes are two nails inserted in the soil with a gap of 2 cm. So when the soil is moist with water, 9 volt passes from probe 1 to probe 2 through the moist soil. So pin 3 of IC gets higher voltage and its output remains high.
Pin 7 Vcc
Pin 4 Ground
Pin 6 Output
Pin 2 Inverting Input
Pin 3 Non Inverting Input
T1 is a PNP transistor, so it remains off when its base gets high voltage from the comparator. LED also remains off. When the soil dry up, the electrical path between the probes break and pin3 of IC becomes low. As a result, output of IC1 turns low and T1 conducts to light the LED.