This circuit was obtained in a Popular Electronics of the 90s. The circuit uses integrated from the time, uncommon today. Note that the speaker has a higher impedance than ordinary speakers.
We find this circuit in an old documentation, but it can be made using modern transistors. The circuit...
We found this circuit in a spy book from the 90s. Transistors admit equivalents and the coil can...
The circuit shown was found in an American documentation from 1964. The circuit uses transistors of...