This Intersil circuit was found in a 1980s documentation. Operational are of the time, but current equivalents can be experienced. The graph with the circuit performance is given next to the diagram.
This fast-acting circuit uses any phototransistor as a sensor. The circuit drives a 30 V relay, but...
This circuit is from 1995 documentation. Integrated circuits are used which can still be found...
This circuit was found in a Popular Electronics from the 90s. The circuit uses common components...