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.
With the change of the coil, the circuit can operate at frequencies between 30 MHz and 150 MHz....
The frequency of this Pierce oscillator, found in 1969 documentation, depends on the crystal. The...
This simple circuit can be used to power a simple communication system using ordinary telephones....