This RF preamplifier for the 450 MHz band was found in an amateur radio documentation from the 1980s. The transistor supports an equivalent operating at the recommended frequency.
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...