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 a 1998 documentation. It tests common household appliances to see if they...
This circuit uses a triac to detect the level of a reservoir, and a sensitive SCR connected to the...
We found this circuit in an English publication from 1980. The transistor can be either the BC548...