This circuit was obtained from a General Electric publication from the 1960s. The transistors are from the time and the circuit produces a 1 kHz signal with 1 V input.
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....