Found in a 1964 Electronic Design, this circuit uses a common LDR and transistors from that time. The relay is a high sensitivity type.
This circuit uses unijunction transistors and drives a small speaker through an output...
This circuit, found in a publication from the 1980s, can have its output voltage determined by the...
The operational amplifier for this circuit, found in a 1979 publication, may be the 741....