This circuit produces a 300 Hz signal from a 10V input. The circuit is from a 60's documentation, making use of components from the time.
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....