This circuit was obtained from an American documentation from the 90's. The transistor can be a low noise general purpose like the BC549.
We found this circuit in English documentation for PCs from 1996. The ZN448 is a very uncommon...
This photo-relay uses a Darlington stage to increase sensivity. Any photo-transistor can be used....
A new version of the previous circuit triggered by the negative transition of the input signal is...