In the original publication of the 90s a 556 consisting of a double 555 is suggested. The circuit drives a speaker with good power. The power transistor shall be fitted with a heat sink.
This simple oscillator with two general-purpose transistors has its frequency given by capacitor...
This flood or humidity alarm was found in an English publication in 1991. It can still be fitted...
We found this interesting configuration in a 1976 technical documentation. Equivalent diodes can...