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 a 1980s documentation, this amplifier uses an integrated circuit of the era. The RLC...
This interesting experimental and didactic configuration was found in a 1977 Radio Shack...
This circuit for a frequency of 40 kHz was found in an old Electronic Design. There were no...