The printer's paper motor runs on 32V. To get the signal from that I'm going to use an optoisolator. But the motor can run both ways and so I need to protect the LEDs inside the optoisolator from the huge reverse voltage.
Now I'm not absolutely certain this is the way, but I went with protecting them using another diode plus adding a big resistor to take care of the reverse current still leaking through.
And this is the resulting circuit. Seems to work fine so far. The optoisolator I'm using is HCPL 2631.
Unfortunately I can't report success yet. I get paper jam errors no matter what I do at the moment. Have to investigate more. Perhaps the stepper is too jumpy. Or too fast. Or too slow. I think I need to get another printer of this kind to get a solid reference and make a comparison.
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