Sharp R-1850 Service Manual Download Page 29

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R-1850
R-1851

Key strobe signal.
Signal applied to touch-key section. A pulse signal is input to AN1 and P27 - P24
terminal while one of G2 line keys on key matrix is touched.

P00

OUT

Segment data signal.

Signal similar to P55.

Key strobe signal.
Signal applied to touch-key section. A pulse signal is input to AN1 and P27 - P24
terminal while one of G1 line keys on key matrix is touched.

57-59

P37-P35

OUT

Segment data signal.

Signal similar to P55.

60-64

P34-P30

OUT

Used for initial balancing of the bridge circuit (absolute humidity sensor).

Pin No.

Signal

I/O

Description

ABSOLUTE HUMIDITY SENSOR CIRCUIT

(1) Structure of Absolute Humidity Sensor

The absolute humidity sensor includes two thermistors
as shown in the illustration. One thermistor is housed in
the closed glass tube filled with dry air while another in
the open glass tube. Each sensor is provided with the
protective cover made of metal with holes to be protected
from the external airflow.

Sensing part

(Closed glass tube)

Sensing part

(Open glass tube)

(2) Operational Principle of Absolute Humidity Sensor

The figure below shows the basic structure of an absolute
humidity sensor. A bridge circuit is formed by two
thermistors and two resistors (R1 and R2).
The output of the bridge circuit is to be amplified by the
operational amplifier.
Each thermistor is supplied with a current to keep it
heated at about 150˚C (302˚F), the resultant heat is
dissipated in the air and if the two thermistors are placed
in different humidity conditions they show different
degrees of heat conductivity leading to a potential
difference between them causing an output voltage from
the bridge circuit, the intensity of which is increased as
the absolute humidity of the air increases. Since the
output is very minute, it is amplified by the operational
amplifier.

Operational
amplifier

Output
voltage

S : Thermistor
open vessel

C : Thermistor
closed vessel

Absolute humidity (g/m )

Output voltage

Absolute humidity vs,
output voltage characterist

(3) Detector Circuit of Absolute Humidity Sensor Circuit

This detector circuit is used to detect the output voltage
of the absolute humidity circuit to allow the LSI to control
sensor cooking of the unit. When the unit is set in the
sensor cooking mode, a 16 seconds clearing cycle
occurs. Then the detector circuit starts to function and
the LSI observes the initial voltage available at its AN6
terminal.

With this voltage given, the switches SW1 to SW5 in the
LSI are turned on in such a way as to change the
resistance values in parallel with R50-1. Changing the
resistance values results in that there is the same
potential at both F-3 terminal of the absolute humidity
sensor and AN7 terminal of the LSI. The voltage of AN6
terminal will indicate about -2.5V. This initial balancing
is set up about 16 seconds after the unit is put in the
Sensor Cooking mode. As the sensor cooking proceeds,
the food is heated to generate moisture by which the
resistance balance of the bridge circuit is deviated to
increase the voltage available at AN6 terminal of the LSI.
Then the LSI observes that voltage at AN6 terminal and
compares it with its initial value, and when the comparison
rate reaches the preset value (fixed for each menu to be
cooked), the LSI causes the unit to stop sensor cooking;
thereafter, the unit goes in the next operation
automatically.
After 16 seconds of operation in the sensor cook mode,
the LSI detects the initial voltage at AN6. If the absolute
humidity sensor is open or shorted, the circuit can not be
balanced for initial voltage comparisons. This will result
in the word "error" on the display and the cooking
operation is stopped.