We found this circuit in an American 1980s documentation. The circuit uses a piezoelectric transducer unusual in our day. The transistor is general purpose type.
This alarm, from a 1996 magazine, can be fitted with two BC548s in the original configuration...
Using only one general purpose transistor this circuit is powered by batteries. The lamp of the...
Can be used as a shield, this old American documentation circuit offers digitally selected...