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 appears elsewhere on this site, but from different sources. Using the LM3900, we find this...
This interesting source or isolation transformer also allows to vary the output voltage by...
In this circuit the CA3140, an operational amplifier, is used as a comparator with the zener diode...