This circuit for receivers was found in an American documentation from the 90's. The circuit operates at up to 500 MHz. The integrated circuit used is uncommon nowadays.
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....