Nellcor NPB-40 Service Manual page 36

Technical Supplement
S-4
The NPB-40 is powered by four AA-size replaceable alkaline batteries. A dc
voltage in the range of 3.6 to 6 Vdc is provided over the life of these batteries.
When the voltage from these batteries drops to a level too low to operate the
NPB-40, the unit will shut itself off. At that time, the batteries must be
replaced.
The front panel keypad contains four keys that provide discrete signals that
are monitored by the microprocessor on the CPU PCB. Refer to Figure S4-5
for a schematic diagram of the front panel keypad.
3.6-6 Vdc
Batteries
On/off key
Front
panel
keypad
Function
keys
External
serial
interface
Figure S3-1: NPB-40 Block Diagram
CPU PCB
Power
Raw power
control
and PCB
power
Sensor
analog
Battery
Power
signal
voltage
down
and
sense
control
CPU
Printer
signal
LCD
enable
LCD
data
External
output
control
Beeper
External
signal
output
port
Backlight
control
LCD PCB
Power
condition
and PCB
power
Patient
Analog
sensor
sensor
input
circuits
Printer
IR LED
drive
LCD
Beeper
LCD
backlight