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.
This circuit is from Popular Electronics from the 90's. The circuit discharges the battery with a...
We found this time base circuit in a timer project from the August 1972 Radio Electronics...
This sonic switch, vox or sonic relay uses a Darlington of the time that has a gain of 10,000. The...