This diagram was found in the November 1972 issue of Radio Electronics magazine in an article describing benchtop equipment. The transistors are of the time and the frequencies are given by the coils.
The oscillator in figure 1 can be used in alarm, call or warning systems, producing a sound of...
This circuit is from a 1973 documentation. In it we have the way of using a FET as an input of...
This circuit is for traditional old-style telephone lines found in an American 1980s...