Found in an American publication from the 1980s, this circuit can be found in commercial sound equipment of the time. The list with the component values is given next to the diagram.
This circuit beeps when the sensor is triggered by the opening of the freezer door or other...
Operational amplifiers used in this 80's circuit is not very common today, but the setting can be...
Unlike the previous detector circuit, this includes the inverter not requiring a high voltage...