This circuit protects the speaker from dangerous direct currents that could damage it. The circuit is from an Electronics Experimenter Handbook 1993 edition.
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...