This rain, flood or leak sensor was found in a 90s Popular Electronics. All the components used are common today. The transistor can be the BC548.
This circuit uses unijunction transistors and drives a small speaker through an output...
This circuit, found in a publication from the 1980s, can have its output voltage determined by the...
The operational amplifier for this circuit, found in a 1979 publication, may be the 741....