This circuit was found in the September 1971 Radio Electronics magazine. The buzzer can be replaced by a relay according to the supply voltage. R1 adjusts the sensitivity.
This circuit, which appears at various points in this section and on the website with minor...
Motorola suggests this circuit in an 80's documentation. The circuit uses an inverter to produce a...
This circuit uses a 7812 and another regulator integrated circuit that is not very common...