This circuit was found in an Electronics World of 1969. The components of the time. The operational can be replaced by modern equivalent type.
This photo-relay uses a Darlington stage to increase sensivity. Any photo-transistor can be used....
The second order filter shown in the figure is suggested by Texas Instruments, based on the TL081...
This circuit was adapted from a 1995 Electronics Now magazine. He shows how to implement a type of...