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.
With this circuit it is possible to monitor the charge and discharge of a battery. The circuit is...
Found in 90s Radio Electronics, this shield can control high voltage loads in the power grid....
The duration of the output pulse in this circuit depends on C. This capacitor can have values in...