This OF-1 LO oscillator was found in a 1980s American documentation. The transistor can be any general-purpose and C1 / C2 are determined by the frequency.
This circuit triggers a relay when a light beam that hits the phototransistor is cut off. The...
This flip-flop controlled by an LDR-type photocell was found in Radio Electroinics magazine from...
This circuit is from a publication from the 60s. The circuit, with minor changes, can be assembled with...