This circuit with the 555, was obtained in a 90s American documentation. The signal must be applied to a good amplifier. The sound is similar to that of a police siren.
We found this circuit in an old publication for radio amateurs from 1975. It is intended for...
This circuit converts pulse frequency to current, which is indicated on an analog instrument. The...
This indicator was found in a 1974 publication, but it can be fitted with the BC548 transistor or...