This circuit was found in specialized microwave documentation from the 1960s. The circuit makes use of a varicap, and the transistor is a microwave type.
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....