Toshiba 4560, Service Manual

Page: 310 / 369

4560/4570 RADF 18 - 52 Nov. 1997 © TOSHIBA
18.11 Explanation of Circuits
18.11.1 Input circuit for the aligning sensor and exit sensor
Since the aligning sensor is the same input circuit as the exit sensor, the explanation for the exit-sensor
input circuit is omitted. Details for the aligning sensor are given below.
This is an input circuit for the aligning sensor. It consists of a voltage-current conversion circuit for adjust-
ing the light amount of infrared LEDs and a voltage comparison circuit which compares the output volt-
age of phototransistors (PT1) with the reference voltage for converting the voltage to digital signals.
A mirror-reflective type sensor, the aligning sensor consists of a pair of light-emitting elements (infrared
LEDs) whose optical axis is in the same direction as that for the light-receiving elements (PT1). On the
axis extended from each element, a mirror is provided. When there is no original between each element,
the ultrared light beam emitted from the LED is reflected at PT1 with a high reflectance. On the other
hand, when an original is present, the light beam to the mirror and the reflected light from the mirror are
shielded, causing the ultrared light entering into PT1 to become extremely small.
When much light enters into PT1 (when no original is present), optical current flowing through PT1 in-
creases, causing the voltage at IC14-pin 6 to increase due to voltage drop by R52. IC14 is an op amp and
comprises a voltage comparator circuit. It compares the reference voltage applied to pin 5 (non-inverting
input terminal) and the signal voltage at pin 6 (inverting input terminal). When the voltage at pin 6 is
higher, the output (IC14-pin 7) becomes L level. Conversely, when the light entering into PT1 is small (an
original is present), the voltage at IC14-pin 6 becomes lower, resulting in IC14-pin 7 becoming H level.
R39 and C24 comprise a noise filter.
In addition, in order to restrict variance in the sensitivity of sensors, this circuit is equipped with an auto-
matic sensitivity adjustment function. This function, by maintaining the PT1 voltage at a fixed value for all
machines when no original is present, eliminates variance in the sensitivity of sensors occurring due to
difference in machines. The PT1 voltage is adjusted by varying the intensity of the ultrared light emitted
from the LEDs.
For automatic sensitivity adjustment, when the adjustment mode for sensor sensitivity is selected, the
PT1 output voltage (analog value) is measured at the A-D input terminal of CPU. So that the PT1 output
voltage becomes a preset voltage, the D-A output voltage is varied and using the voltage-current conver-
sion circuit consisting of IC1, Q6, R78, etc., LED current is adjusted. This D-A output voltage is a value
unique to each machine and is memorized in EEPROM (IC7).