This circuit was obtained from an 80's EDN. The components used are still common. The circuit can be used as a shield and the frequency is approximately 775 Hz.
This one-transistor alarm can be used with much more sensors than the two shown by the figure....
This is one of many possible configurations for this type of oscillator that generates sinusoidal...
The circuit in the Multisim Blue simulation sources a regulated voltage to loads up to 50 mA. The...