The ADC801 is not an easy component to obtain today, but the configuration is worth reference. The circuit is from National Semiconductor's Linear Applications Handbook.
This simple alarm was found in a 1982 documentation, but uses components that are still common,...
Found in a 1969’s Electronic Design, this circuit operates in a narrow range of voltages....
This experimental circuit was found in a technical publication from 1976. The diode can be of...