This circuit is from a 1964 Electronic Design. The sensor is an LDR and the transistors are general-purpose types. The circuit triggers when the light that falls on the sensor is cut off.
Settings like this can be found in this section and on this website, but from different sources of...
This circuit with TTL output was found in a 1968 EEE. The transistor can be of general use. The...
Found in a 1980s manufacturer documentation, this circuit can be used as an optical shield. The...