The circuit is using only one BC547 transistor that is working as a switch. The sensitivity of the circuit can be adjusted with 100K variable resistor. The operating voltage of the circuit is 12 volt DC but you can also operate it with 9 volt DC. For operating with 9V simply remove the D5 and D6 diodes and connect the 680 Ohm resistor near LED 4 directly with D4.
Note: Try to use all LEDs from a same manufacturer, same type and color because different manufacturers have different brightness LEDs and using them in the circuit will not show accurate results. I have used four green transparent LEDs and they worked very well.
Here is a very simple and useful project of a moisture level LED Meter circuit that can be used as a sensor or detector to measure humidity or moisture level in plants, soil, wood, wall, floor etc. The interesting part of the circuit is four LEDs which will light up according to the moisture level and show low,