Table of Contents
Advertisement
9.6.9
Ambient Light
9.6.10 Ambient Temperature
The ambient light circuit is illustrated in Figure 9-28.
LIGHT SENSOR
VCC
R68
2.2M
Figure 9-28: Ambient Light Circuit
Diode D8 is a photodiode that is used to measure ambient light. Q8, R68, and
R136 provide current gain for D8's photocurrent. The amplified photocurrent
flowing through R136 creates a voltage drop, which is measured by the CPU.
The CPU continually monitors the light source output at AMB_LIGHT (TP72).
Under low-ambient light conditions, the CPU automatically switches on the
display backlight.
The ambient temperature circuit is illustrated in Figure 9-29.
TEMP SENSOR
VCC
Figure 9-29: Ambient Temperature Circuit
U5 is a precision-temperature sensor. It outputs (PR_TEMP) a voltage
proportional to the ambient temperature, which is 10 mV per degree centigrade.
For example, at a room temperature of 25 °C, the U5 output would be 250 mV.
U5 is used to compensate for ambient temperatures associated with thermal
printers.
VCC
D8
VTB8442B
Q8
MMBTA13L
R136
33.2K
U5
1
VCC
PR_TEMP
2
TEMP
3
GND
LM35
Technical Supplement
TP72
C63
.01UF
Ambient
Light
C115
0.1uF
9-27
Advertisement
Table of Contents
