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[b] Document feed sensor (DFD)
The document feed sensor is a reflection type sensor, and the LED and the photo transistor form a pair. Infrared light emitted from the LED is
reflected by the mirror on the opposite side, and the reflected light enters the photo transistor to increase the photo current in the photo transistor,
detecting "No document."
On the other hand, if there is an document between the LED and the mirror, there is no reflection from the mirror. Therefore the photo current does
not increase and the document is detected.
This circuit is also provided with the automatic adjustment function.
The LED cathode is connected to the voltage-current conversion circuit composed of the operation amp (IC13), Q3, and R94. The current value is
controlled with the D-A output (analog voltage output) from the CPU. That is, the operation is made so that the CPU D-A output value (IC11-68 pin)
is equal to IC13 5 pin input voltage (the voltage drop of LED current by R93).
When, therefore, the D-A output value is changed, the current value is also changed.
On the other hand, the photo current of the photo transistor is converted into a voltage value by emitter resistor R66, and is inputted to IC9 6 pin and
the CPU 56 pin through the noise filter composed of R101and C14.
R79, R81, R46, and IC9 form a voltage comparator, which compares the input voltage from the sensor with the threshold voltage (about 2V)
generated by dividing +24V with R79 and R81.
When the sensor input voltage exceeds the threshold voltage, the output of IC9 1pin turns LOW, being inputted to the CPU 73 pin as "No
document" signal.
The CPU 56 pin is an A-D input pin, which converts an analog voltage into a digital value inside the CPU. Since the sensitivity of a sensor generally
varies, it is automatically adjusted with the sensitivity at "No document" as the reference voltage. That is, the sensor voltage at "No document" is
A-D inputted to change the D-A output voltage, varying the LED current (LED light intensity) and controlling by the CPU so that the sensor voltage is
the specified constant level.
The D-A output value at that time is unique to every machine, and is stored in the EEPROM (IC3).
PH110M
Original feed
sensor DFD
TLN119B
173979-3
+5V
CN21-1
CN3-4
+5v
DFD
CN21-2
CN3-6
DFDLED
DFDLED
CN21-3
DFD
CN3-5
Original feed sensor input circuit
+5V
IC13-2
Q4
R87
1
2SC2712
10K
µ PC358G2
AN+24V
R46
R79
100K
22K
IC9-2
7
1
R11
6
µ PC339G2
10K
C14
R66
R81
4.7K
2.0K
0.10µ F
AGND
AGND
10 – 3
CPU
TP12
(IC11)
5
68
ANO1
C6
6
1000PF
R93
100
+5V
R54
TP12
4.7K
73
P22/INTP1
TP10
56
P70/ANI0
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