This conventional relay light detector circuit uses a 741 as a voltage comparator. The sensitivity adjustment is made in R1. The supply voltage depends on the relay used.
This 1969 Electronics World power supply has 5 V to power digital circuits and a 170 V output for...
This configuration, which appears in the manual of other manufacturers, is an excellent 1.5 V...
This receiver was obtained in a documentation from the 90's and is of the super-regenerative type....