Found in a 1970s RCA documentation, this circuit generates signals according to the component values. A table is given next to the diagram.
This sensitive circuit was found in the June 1995 Electronics Now magazine. R1 can be from 10k to 1M...
Obviously, it is a much more didactic setup than the one with great practical effect, but the idea...
This circuit is used in magnetic tape recorders, having been found in documentation in the 1960s....