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.
The configuration already explored in our Circuit Bench section is from the National...
This interesting manual circuit uses an SCR to switch a load on and off via pulses. The circuit was...
The circuit shown was found in a Japan publication from the 1960s. The transistors can be of any...