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 traditional configuration found in a Radio Shack document can use 2N2646. More keys can be...
This is the circuit of a turntable from the 1970s. We found it in a repair article in the December...
Using the CA3018 released at the time, this circuit was found in the August 1970 Radio Electronics...