This circuit was found in a Popular Electronics from the 90's. The components used are common and instead of the buzzer we can use an LED or a relay.
This 12 V symmetrical power supply provides a current of the order of 100 mA according to the...
We found this circuit in the 1994 National Semiconductor Linear Applications Handbook. The circuit...
Found in a 1980s American documentation, this circuit is based on the common CMOS series...