Canon NP6251 Service Manual page 145

The RAM on the DC controller PCB keeps count of copies being made; when it has
detected the hopper toner level detection signal (
the presence of toner, thereby cleaning the counter and turning off the Add Toner
message on the control panel.
Reference:
The inside of both the developing assembly and the hopper assembly is each
equipped with a piezoelectric oscillator.
In the absence of toner, the oscillator oscillates at several kHz, and the output of
the sensor goes '0'. In the presence of toner, on the other hand, the weight of the
toner stops the oscillation, and the output of the sensor goes '1'.
Caution:
If the developing assembly is not set inside the copier, do not install the developing
assembly locking assembly. Otherwise, the toner collecting in the path from the
hopper to the developing assembly can pour out because of the vibrations caused
by inspection/repair work.
COPYRIGHT © 1997 CANON INC.
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AMP
Integration
circuit
Piezoelectric oscillator
Figure 4-203
CANON NP6350/NP6251 REV. 0 JULY 1997 PRINTED IN JAPAN (IMPRIME AU JAPON)
CHAPTER 4 IMAGE FORMATION SYSTEM
=1) for 1 sec or more, it will assume
TEP
Output
TEP
4-31