This very simple circuit uses a phototransistor and a very sensitive relay (reed type) being used over short distances with a lens system. The circuit is from a 1967 Japanese publication.
This alarm circuit is from a 1990 Electronics Handbook. Transistor Q1 can be a BD135 and the relay...
This trigger circuit was found in old European documentation, but it can be implemented with...
This circuit is from the SGS Integrated Circuits Manual of 1982. The integrated circuit used is...