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.
This powerful source was obtained in an American documentation of the 80s. Some components are...
This simple transistor tester was found in a Japanese magazine in 1967. It tests general purpose...
This transistorized astable for LEDs was found in the January 1992 issue of Radio Electronics...