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.
In the figure we have the way to make the connection of voltage regulators in series to obtain a...
This circuit was obtained from HAM Radio magazine in an 80's issue. The FETs are types of the time...
This mixer uses only four passive component and can be used with high impedance sources. The input and...