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 circuit was found in a Radio Electronics magazine from November 1992. The components are from...
This circuit was found in a 1969 Mullard publication. The transistor may be the modern equivalent...
This simple expandable circuit was obtained from a Popular Electronics from the 80's. The supply...