This circuit was found in documentation from the 1970s. However, it can be implemented with modern transistors such as 2N2222 for Q1 and BF494 for Q2. The crystal depends on the desired frequency.
Suggested by an American technical documentation, this circuit is polarized in order to provide an...
This modulated siren was found in the December 1974 issue of Radio Electronics magazine. A higher...
This circuit with 5 times gain was found in European documentation from the 1980s. The input and...