Sharp SF-2052 Serviceanleitung Seite 222

3. Operations
A. Sensor input circuit
[a] Original timing sensor (DTD)
The original timing sensor is a reflection type sensor, and the LED and the photo transistor are integrated into one. 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 original."
On the other hand, if there is an original between the LED and the mirror, there is no reflection from the mirror. Therefore the photo current does not
increase and the original 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 (IC11-67 pin)
is equal to IC13 2 pin input voltage (the voltage drop of LED current by R94).
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 load resistor R95, and is inputted to IC9 4 pin and
the CPU 57 pin through the noise filter composed of R10 and C19.
R78, R80, R45, 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 R78 and R80.
When the sensor input voltage exceeds the threshold voltage, the output of IC9 2pin turns LOW, being inputted to the CPU 72 pin as "No original"
signal.
The CPU 57 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 original" as the reference voltage. That is, the sensor voltage at "No original" 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 timing
sensor DTD
TLN119B
1-173981-0
173979-3
3
+5V +5v
CN20-3 CN3-1
2
DTD CN20-2 CN3-3 DTDLED
1
DTDLED
CN20-1
CN3-2 DTD
Original timing sensor input circuit
+5V
1
3
Q3
2SC2712
AN+24V
R78
100K
22K
5
R10
2
4
10K C19
R95 R80
3.0K
2.0K
0.10µ F
AGND
AGND
12 – 19
IC13-1
3
R86
1
C5
2
10K
1000PF
R94
µ PC358G2
100
+5V
R45
IC9-1
2
µ PC339G2
CPU
TP7 (IC11)
67
ANO0
TP8
R53
4.7K
72
P21/INTP0
TP11
57
P71/ANI1