We found this circuit in the September 1971 issue of Radio Electronics magazine. The sensor is an LDR and R1 adjusts the sensitivity. A relay with the appropriate voltage can be used instead of a buzzer.
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...