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 circuit, from a 1973 documentation, can be made with BC549 transistors. The frequency depends on...
The transistors in this 80's documentation project may be BC548, the most modern equivalents of...
This circuit was described in an article in the February 1993 Radio Electronics magazine. The...