I once acquired a Macintosh Plus in a pretty good condition locally, but neither with a mouse nor keyboard. A keyboard show up some weeks later, but I struggled to find a compatible mouse locally in Switzerland.
From my undergrad student time, I somehow recalled that early optical mouse sensors actually had quadrature outputs, simulating the behavior of mechnical mice with a shutter wheel. That should fit!
The project was triggered after digging out the mouse, a DB-9 connector and an old Ethernet cable from an E-Waste container at work. (here already assembled together)
Inside, the ADNS-2051 optical mouse chip directly outputs quadrature signals as required by a Macintosh. No conversion using a microcontroller or any other circuitry is needed.
Beautiful, ADNS-2051 actually directly outputs quadrature signals:
So it's enough to solder X1/X2/Y1/Y2 lines to the chip itself (XA/XB/YA/YB). Two blue wires tapped into ground, Orange one to +5V and Orange-White to the button tactile switch.
Ready to use it with my faithful Plus :)
Aaaand it works!