This circuit triggers a relay when a light beam that hits the phototransistor is cut off. The circuit is from Electronics Now and the supply voltage depends on the relay voltage.
This is the power supply indicated for the previous circuit (CB18905E) providing a voltage of 24 V...
This simple 1-transistor reflex receiver was published in Radio Electronics magazine in September 1959....
In this Radio Electronics from May 1958 we have the now common use of the varicap in a frequency...