The circuit is from an Electronics Hobbyst Handbook from the 90s, being valid for panels of the time, with lower performance. The circuit supplies 2.4V with 125mA current.
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....