This alarm with automatic shutoff was found in the April 1975 Radio Electronics magazine. The activation time is given by C1 and R3. The alarm uses series and parallel sensors.
Using an operational amplifier, this circuit was found in the March 1996 issue of Electronics Now...
This simple transistor gain test acts as an oscillator for the transistor under test. The circuit is...
Found in a 1980 documentation, this circuit drives a transducer (X1) when the car battery has its...