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 circuit was found in an Electronic Design from the 90s. The shapes of the signals obtained as result...
This circuit came out in an English publication from 1973, but it can be easily assembled using a...
This circuit is suggested by Motorola. The integrated circuit used is not easy to obtain. The...