Printronix L5000 Series Maintenance Manual page 265

Toner empty sensor circuit
This sensor directly outputs the oscillation output of the piezoelectric vibrator
element, and determines whether or not toner exists by its P–P output voltage.
However, as this output voltage cannot be directly put into the logic circuit, it is
converted into a signal level with the following rectification circuit and comparator.
Electrical and Wire Data 6–27