Circuits - Toshiba e-STUDIO520 Service Manual

16.11 Circuits

1) Input circuits for read sensor (S69), large original exit sensor (S68), small original reverse sensor
(S66) and original length detection sensor (S63)
Since the read sensor (S69), large original exit sensor (S68), small original reverse sensor (S66)
and original length detection sensor (S63) have almost the same circuitries, only the input circuit for
the read sensor (S69) is described as followed.
This circuit consists of the voltage-current conversion circuit, which adjusts the amount of light of the
infrared LED, and the voltage comparison circuit, which compares the voltage output from a pho-
totransistor (PTr) with a reference voltage and converts the output voltage into digital signals.
The read sensor (S69) is a mirror reflection sensor, and consists of a pair of emission elements
(infrared LEDs) and a light receiving element (PTr). Both elements have a light axis running in the
same direction.
There is a mirror on the extended light axis of these elements. When there is no original between the
elements and the mirror, infrared light emitted from LED is reflected by PTr in high reflectance. If
there is an original, light emission to the mirror and reflected light from the mirror are blocked by the
original, and the reflected light entering PTr becomes extremely small.
When a large amount of light is entering PTr (this means there is no original) the optical current flow-
ing in the PTr is increased, and the voltage of IC6-8 pin becomes higher due to a drop in voltage
caused by R109. IC6 is a comparator which compares the reference voltage input to 9 pin (non-
reverse input terminal) with the signal voltage of 8 pin (reverse input terminal). If the voltage of 8 pin
is higher, the output level (IC6-14 pin) is "L". On the contrary, when the amount of the incoming light
is small (this means there is an original), the voltage of 14 pin is low and the voltage of IC6-14 pin
becomes "H". R56, C56 and C57 are noise filters. This circuit has an automatic sensitivity adjust-
ment function to control the unevenness of the sensor sensitivity. Uneven sensor sensitivity is cor-
rected by controlling the machine to have a constant PTr voltage when there is no original. The
intensity of the infrared light emitted from the LEDs is changed to adjust the PTr voltage.
When the sensor sensitivity adjustment mode is selected, the PTr output voltage (analog value) is
measured by the A-D input terminals of CPU. The output voltage of the D-A converter is changed to
keep the PTr output voltage to a certain level, and the LED current is adjusted by a voltage-current
conversion circuit consisting of IC11, R65, Q6, etc. Each machine; has a different D-A output voltage
value, which is stored in the volatile memory.
e-STUDIO520/600/720/850 REVERSING AUTOMATIC DOCUMENT FEEDER (RADF)
Fig.16-94
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March 2005 © TOSHIBA TEC