Found in an EDN from the 1980s, this circuit uses a double 555. The circuit activates a relay that controls a horn or siren. Next to the diagram we have the timings.
The oscillation of the relay causes the applied audio signal to deform and start to resemble the...
This circuit, for frequencies up to 100 kHz, was found in American documentation from 1970. The...
We found this traditional configuration of a megaphone with a coal microphone in an American...