This circuit is from a Radio Electronics from February 1959. The circuit has a speaker output and good sensitivity. Power is supplied from a 9 V battery.
Found in documentation from the 90s, this circuit is based on operational amplifiers of the time....
This oscillator, which can be used as a signal injector, was found in a Popular Electronics from the 90s....
This traditional oscillator uses two complementary transistors to produce low frequency signals....