This nonlinear gain circuit was found in a 1965 Electronic Design. The configuration can be used as a basis for modern designs with equivalent transistors.
This circuit, from a 1973 publication, locks the relay when the light that falls on the LDR is cut...
We found this circuit in a Popular Electronics of the 90s. Its purpose is to detect when it locks...
This circuit is from 1987 when there were only analog TVs and stereo sound was still a novelty for this...