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 a 1977 factory documentation. The operational amplifiers can be 741 and...
This circuit with regenerative action was found in the June 1986 issue of Radio Electronics...
Equivalent FETs can be used according to the frequency of the crystal which can be up to a few...