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 was obtained in the publication 101 Electronics Projects of 1979. The circuit can be...
Found in Radio Electronics from the 90's, this circuit employs a 3140 operational amplifier, still...
This simple power supply is not regulated and the output current depends on the transformer. It...