This circuit is from a 1979 publication. The sensor consists of plates or screens that, when moistened, allow current to pass. The transistor can be the BC548 and the high impedance speaker can be replaced by a low impedance speaker if an output transformer is used. C2 which in the diagram is indicated as 100pF, in reality is 100uF. The operational amplifier admits equivalent and the circuit can be powered by battery or source.
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