We found this circuit in the 1964 RCA transistors manual. The sensor may be a common LDR and the transistor BC558. The relay has an operating voltage according to the power supply.
The sensor for this Intersil circuit (1983 databook) is a common silicon transistor. The trimpot...
This circuit is from a 1980 publication. The circuit has high gain with weak signals and lower gain...
The integrated circuit used in this 1987 project is not very common these days. However, if found...