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.
We found this circuit in a 1968 Electronics World. The potentiometer should be set to trigger the SCR...
We found this circuit in a documentation from the 90s. It lets frequencies around 1 kHz pass,...
This wired intercom circuit was found in a March 1965 Radio Electronics. The circuit uses two...