Suggested by Maxim in documentation from the 90s, this circuit has its response curve given next to the diagram as well as the formulas for the component calculations.
Current drain with a 6 V supply is only 6 µA. Frequency is given by the capacitors, and output...
This circuit for old telephones was found in July 1985 Radio Electronics magazine. It activates an alarm...
Using a capacitor as a sensor, this circuit was found in the 1989 Radio Electronics Experimenter magazine....