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.
We found this circuit in an Electronic Engineering of the 90s. The circuit can be implemented with any...
I found this circuit in an article in a 2003 publication. The circuit is an example of how to...
We found this circuit in a 1975 publication. The power supply must be symmetrical, but other...