We found this circuit in a magazine 73 for radio amateurs from the 80's. The circuit makes use of an epochal component which is programmed according to the table beside the diagram.
This circuit detects the presence of a pilot flame in heating systems by the presence of infrared...
In the figure we have a circuit that uses unusual components, being indicated for low frequency...
Found in older Signetics documentation, this circuit generates signals from 30 Hz to 25 kHz....