Found in a 1980s documentation, this circuit uses transistors from that time. Modern equivalents can be used. The alarm is a piezoelectric buzzer with na included oscillator.
This is a Japanese commercial circuit found in a 1967 documentation. The transistors are from the...
This circuit was found in a Radio Electronics from October 1952. The circuit equalizes the sounds...
This circuit is from an old American publication, but it can be easily implemented with modern...