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 found in an ancient American documentation. The transformer is the type used in...
We found this circuit in a Poptronics of the 90s. The circuit operates from 100 kHz to 30 MHz and...
The circuit shown is from the SGS manual of linear integrated circuits of 1982. The L78xx is...