The sensor of this circuit found in Popular Electronics can be formed by a piece of fabric between two bare wire screens. When wet, the fabric will pass the current between the screens, triggering the alarm.
Found in National Semiconductor documentation from the 90's, this circuit has an output current of...
With this circuit it is possible to raise the characteristics of a transistor by checking its gain...
The circuit presented is from the Linear Applications Handbook of the National Semiconductor of...