This basic Schmitt trigger was found in an American publication from 1968. The circuit uses transistors from the time. Modern general-purpose types can be used.
This circuit is from a 1979 publication. The sensor consists of plates or screens that, when...
Found in an 80's documentation this circuit uses an integrated from the uncommon era today. Other...
This circuit was found in a Popular Electronics from the 90s. It uses a LASCR which is a photo-SCR...