This traditional signal injector configuration was obtained in a 1977 amateur radio manual. The frequency is given by the capacitors and the supply can be made with 1.5 or 3 V.
This oscillator circuit is from a Radio Electronics magazine from September 1960. The frequency depends...
This circuit uses a piezoelectric tweeter to emit a powerful continuous sound above the audible...
Found in an English documentation from the 90's, this circuit makes slow logical pulses audible,...