This 1967 Philips circuit recognizes a light signal (visible or infrared) modulated by tone. The circuit makes use of components of the time that are equivalent to the current BCs.
In the figure we have a circuit suggested by National Semiconductor, using an operational...
For experiments with ionization chambers and Geiger valves this circuit generates high voltage from...
We found this circuit in a 1979 documentation, but it can be changed to use more modern...