Sharp AR-5132 Service Manual page 66

[c] Reverse sensor (RDD)
The reverse 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 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 circuit composed of R107, R108, and Q17. A high level or a low level input is passed to the base of Q17 to
vary the LED light quantity.
The base of Q17 is connected to the CPU 49 pin. When the CPU outputs a low level signal, Q17 is turned off and all forward current in the LED
flows through R108.
On the contrary, when the CPU outputs a high level signal, Q17 is turned on and the forward current in the LED flows through R108 and R107 in
parallel. That is, the forward current in the LED is doubled, increasing the light quantity.
On the other hand, the output voltage of the photo transistor is inputted through the noise filter composed of R32 and C20 to IC9 10 pin.
IC9 and R48 form a voltage comparator, which compares the sensor output voltage inputted to 10 pin and the threshold voltage inputted to 11 pin.
When the sensor output voltage is lower than the threshold voltage, the output at IC9 13 pin turns HIGH and the "document present" signal is
inputted to the CPU (IC11) 75 pin.
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 reduced into 1/3 by R74 and R63 and inputted to the CPU 58 pin.
At that time, the base of Q17 is at LOW level and the light quantity of the LED is kept low. The CPU 58 pin is an A-D input pin, which allows
conversion of an analog voltage into a digital voltage in the CPU. When the sensor output voltage inputted to the CPU is in the range of 1V ∼ 4.5V,
the gate array outputs from 18 pin the PWM signal of the duty corresponding to the voltage inputted to the CPU 58 pin.
The PWM signal is inputted to the integral circuit composed of R31 and C46 and converted into an analog voltage to be inputted to IC14 3 pin.
The converted analog output is the same as the sensor output voltage inputted to the CPU.
C22, R7, R6, and IC14 form a non-reverse amplifier, which amplifies the analog voltage inputted to IC14 3 pin to be double and outputs from 1 pin.
The output voltage is inputted to IC9 11pin as the threshold voltage, and compared with the sensor output voltage. That is, the threshold value is set
to 2/3 of the HIGH level of each sensor output. When the sensor output voltage inputted to the CPU 58 pin is lower than 1V, the gate array 18 pin
outputs a HIGH level signal to increase light quantity of the LED. If the output voltage is increased to the range of 1V ∼ 4.5V by this, the threshold
value setting similar to the above is performed.
If the sensor output voltage remains lower than 1V even though the light quantity of the LED is increased, it is judged as a sensor error.
If the sensor output voltage inputted to the CPU 58 pin is greater than 4.5V, the threshold value cannot be set and it is judged as a sensor error.
The set threshold voltage and the logic of CPU 49 pin when setting are unique to each machine and stored in the EEPROM (IC3).
173979-3
AN+24V
RDD
PH110M
Reverse
RDDLED
sensor
RDD
TLN119B
+5V
CN36-3 CN8-1 +5v
CN36-2 RDD
CN8-2
RDDLED
CN36-1 CN8-3
R108
2.2K
AGND
2
3
R6
10K
AGND
Reverse sensor input circuit
TP50
R74
3.0K
R107
2.2K R32
10K AGND
Q17
FA1L3M
AGND
C22
C20
0.10µ F
0.010µ F
R7
AGND
10K
TP47
IC14-1
R72
1
4.7K
µ PC358G2
R31
4.7K
C46
0.10µ F
AGND
10 – 4
+5V
D4
TP16
SB02-03Q
R70 R64
C15
10K 10K
R63
1.5K
0.10µ F
AGND
+5V
R23 TP14
4.7K
IC9-4
10
13
11
µ PC339G2
R48
100K
CPU
(IC11)
58
P72/ANI2
49
P12
75
P24/INTP3
G-A
(IC7)
18
PWM5