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 can generate signals from 100 kHz to 50 MHz depending on the crystal and the...
The integrated circuits used in this configuration are not common today. The circuit is from the...
We found this circuit with 5.3V zener in a Philips publication from the 60s. The op amp could be a...