This circuit was obtained in a Popular Electronics of the 90s. The circuit uses integrated from the time, uncommon today. Note that the speaker has a higher impedance than ordinary speakers.
This circuit is from a 1979 publication. The sensor consists of plates or screens that, when...
The circuit is from a Radio Electronics from August 1989. The cells used at that time did not have...
The automatic modulated siren circuit shown in the figure really produces an interesting sound...