we have used one transistor but in this circuit we have used two transistors due to which the circuit has become very sensitive.

The circuit is using two 2N4401 transistors and few other components. Both the transistors are working as switches. When the two probes shown in the circuit will detect any moisture in a substance then the resistance between the two probes will decrease due to which the transistor Q1 will become switch ON. When the transistor Q1 will switch ON it will provide required voltage (0.7 volts) at the base of the transistor Q2 making it switch ON too and activates any load connected to it.

The 50K variable resistor is used to adjust the sensitivity of the circuit. In the place of relay you can also connect any other load like LED, Buzzer etc. The operating voltage of the circuit is 9 to 12 volt but it can also be operated with 5 or 6 volts but make sure to use the same voltage relay also.
 

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Here is a project of a versatile and very sensitive moisture sensor circuit. The circuit can be used to detect moisture in any substance like plants, soil, wood, wall, floor, irrigation areas etc. In the previous circuits of moisture sensors like plant soil moisture tester circuit or moisture sensor alarm,
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Sensitive Moisture Sensor Circuit Diagram